Information

Does Amphibian embryo's blastocoel become a primitive yolk sac without yolk?

Does Amphibian embryo's blastocoel become a primitive yolk sac without yolk?


We are searching data for your request:

Forums and discussions:
Manuals and reference books:
Data from registers:
Wait the end of the search in all databases.
Upon completion, a link will appear to access the found materials.

The mammalian blastocoel becomes yolk sac without yolk according to my professor. I have not found any evidence that such a thing happens in amphibians like frog.

I need to be able to compare and contrast cleavage and formation of blastula in amphibians and in humans. My thoughts raised another question. My friend says that blastula exists only when blastocoel exists.

2. Is the thing without blastocoel but with yolk sac the blastula?


Amphibians are produced externally so they do not get proteins and ATP from the mother during their maturation and differentiation. This means that they must have some way to get those things to protect themselves and survive.

The way that they get these things is yolk. Amphibians start to have yolk-filled endoderm during blastulation, [page 9, Gilbert, Developmental Biology].

I am considering the frog here as an example of amphibian. To know that things need energy to survive and that they must get it somewhere, you can deduce this solution. So nutrition and external "hatching" or development outside of female is the key here.

The overall answer to the question: The organism must have a yolk sac, since it has yolk.

The second question seems to be an idealized one. Without blastocoele but with yolk-sac suggests me that the blastocoel has developed already to yolk-sac. If that is the case, then I would call the thing gastrula, since for instance some species' ectoderm develops into yolk-sac. I assume that the question refers with "yolk-sac" to complete yolk-sac, not to the developing one during gastrulation.

If it is about developing yolk-sac, I would say that the cell is still blastula but is developing to gastrula. The yolk sac is not ready until gastrulation is complete.


History - Who discovered what set of factors is responsible for visibility of celestial bodies on Earth's daytime sky?

Visibility of a celestial body on the sky during the day depends on a number of factors - if I recall correctly: the body's magnitude, Seeing, Sky brightness, and probably some others I have missed.

Who was the first to discover/compile the set of variables responsible for determining, whether a celestial body remains visible with naked eye in certain (daylight) conditions?


Chapter 28 - Embryology and Teratology

Rodents, specifically the rat, have been used extensively to study the development of the mammalian embryo. The rat is one of the species of choice for the regulatory assessment of developmental toxicology. This chapter discusses the normal embryology of the rat and the methods used in experimental teratology. Fertilization in rats takes place in the ampullae of the oviducts. Fertilization may be considered complete with the condensation of the chromosomes in the male and female pronuclei and the coming together of the two groups of chromosomes to form a single chromosome group. Implantation requires strictly timed hormonal conditioning, consisting of continuous preparation of the endometrium by progesterone for at least 48 hours, and then a brief intervention of minute amounts of estrogen on the fourth day. The period of gastrulation covers the period of development of the conceptus from a bilaminar germ disc of epiblast and hypoblast that arises from the inner cell mass through the formation of the primitive streak to the trilaminar ectoderm/mesoderm endoderm of the embryo. The most common treatment period to evaluate teratogenicity requires exposure from implantation to closure of the hard palate or from gestation day 6 through 17 in the rat. Studies to evaluate effects on embryo-fetal development are required in two species: one rodent and one non-rodent, usually the rat and the rabbit. The international conference of harmonization (ICH) guidelines state reasons for using rats as the predominant rodent species are practicality, comparability with other results obtained in this species.


Contents

In the human embryo, the earliest stages of the formation of the amnion have not been observed in the youngest embryo which has been studied the amnion was already present as a closed sac, and appears in the inner cell-mass as a cavity. This cavity is roofed in by a single stratum of flattened, ectodermal cells, the amniotic ectoderm, and its floor consists of the prismatic ectoderm of the embryonic disk—the continuity between the roof and floor being established at the margin of the embryonic disk. Outside the amniotic ectoderm is a thin layer of mesoderm, which is continuous with that of the somatopleure and is connected by the body-stalk with the mesodermal lining of the chorion.

When first formed, the amnion is in contact with the body of the embryo, but about the fourth or fifth week amniotic fluid (also called liquor amnii) begins to accumulate within it. This fluid increases in quantity and causes the amnion to expand and ultimately to adhere to the inner surface of the chorion, so that the extra-embryonic part of the coelom is obliterated. The amniotic fluid increases in quantity up to the sixth or seventh month of pregnancy, after which it diminishes somewhat at the end of pregnancy it amounts to about 1 liter.

The amniotic fluid allows the free movements of the fetus during the later stages of pregnancy, and also protects it by diminishing the risk of injury from without. It contains less than two percent solids, consisting of urea and other extractives, inorganic salts, a small amount of protein, and frequently a trace of sugar. That some of the liquor amnii is swallowed by the fetus is proved by the fact that epidermal debris and hairs have been found among the contents of the fetal alimentary canal.

Extra-amniotic pregnancy is a rare condition that results from a rupture of the amnion, leading to development of the fetus within the extraembryonic coelom. [2]

In reptiles, birds, and many mammals the amnion is developed in the following manner:

At the point of constriction where the primitive digestive tube of the embryo joins the yolk sac a reflection or folding upward of the somatopleure takes place.

This, the amniotic fold, first makes its appearance at the cephalic extremity, and subsequently at the caudal end and sides of the embryo, and gradually rising more and more, its different parts meet and fuse over the dorsal aspect of the embryo, and enclose a cavity, the amniotic cavity. This kind of amnion is known as pleuroamnion (formed by folding), as opposed to schyzoamnion (formed by delamination).

After the fusion of the edges of the amniotic fold, the two layers of the fold become completely separated, the inner forming the amnion, the outer the false amnion or serosa.

The space between the amnion and the serosa constitutes the extra-embryonic celom, and for a time communicates with the embryonic celom.

Cats and dogs are born inside of the amnion it is cut open by the mother and eaten.

In elephants, "The amnios is continued from the base of the umbilical cord upon the allantois, which is of considerable size, and is so interposed between the chorion and amnios, as to prevent any part of the amnios attaining the inner surface of the placenta. The amnios consists of two layers:one is the granular layer, continued upon the inner or foetal surface of the allantois, and thence upon the umbilical cord the other is the smooth outer layer, continued upon the outer or chorional surface of the allantois, and thence upon the inner surface of the chorion." [3] : 348

The amniotic membrane is used as a biological dressing to heal incurable wounds. For this purpose, the placenta in cesarean delivery is collected and under aseptic conditions, the amniotic membrane is separated and packaged and sold commercially. In valid commercial products to prevent transmission of viral infections such as HIV, hepatitis and similar diseases the donor 's blood (mother) tested . Products usually pass the sterility and endotoxin test in accordance with the rules of the Food and Drug Administration of the country of manufacture.

Placenta with attached fetal membranes (ruptured at the margin at the left in the image), which consists of the amnion (inner layer) and chorion (outer layer)

Surface view of embryo of Hylobates concolor.

Human embryo—length, 2 mm. Dorsal view, with the amnion laid open. X 30.

Section through the embryo.

Diagram of a transverse section, showing the mode of formation of the amnion in the chick.

Model of human embryo 1.3 mm. long.

Sectional plan of the gravid uterus in the third and fourth month.

Scheme of placental circulation.

Human embryo of about fourteen days, with yolk-sac.

Opened uterus with cat fetus in midgestation: 1 umbilicus, 2 amniotic sac (chorion and amnion), 3 allantois, 4 yolk sac, 5 developing marginal hematoma, 6 maternal part of placenta (endometrium)

This article incorporates text in the public domain from page 56 of the 20th edition of Gray's Anatomy (1918)


Contents

Fertilization Edit

Fertilization takes place when the spermatozoon has successfully entered the ovum and the two sets of genetic material carried by the gametes fuse together, resulting in the zygote (a single diploid cell). This usually takes place in the ampulla of one of the fallopian tubes. The zygote contains the combined genetic material carried by both the male and female gametes which consists of the 23 chromosomes from the nucleus of the ovum and the 23 chromosomes from the nucleus of the sperm. The 46 chromosomes undergo changes prior to the mitotic division which leads to the formation of the embryo having two cells.

Successful fertilization is enabled by three processes, which also act as controls to ensure species-specificity. The first is that of chemotaxis which directs the movement of the sperm towards the ovum. Secondly there is an adhesive compatibility between the sperm and the egg. With the sperm adhered to the ovum, the third process of acrosomal reaction takes place the front part of the spermatozoan head is capped by an acrosome which contains digestive enzymes to break down the zona pellucida and allow its entry. [3] The entry of the sperm causes calcium to be released which blocks entry to other sperm cells. A parallel reaction takes place in the ovum called the zona reaction. This sees the release of cortical granules that release enzymes which digest sperm receptor proteins, thus preventing polyspermy. The granules also fuse with the plasma membrane and modify the zona pellucida in such a way as to prevent further sperm entry.

Cleavage Edit

The beginning of the cleavage process is marked when the zygote divides through mitosis into two cells. This mitosis continues and the first two cells divide into four cells, then into eight cells and so on. Each division takes from 12 to 24 hours. The zygote is large compared to any other cell and undergoes cleavage without any overall increase in size. This means that with each successive subdivision, the ratio of nuclear to cytoplasmic material increases. [4] Initially the dividing cells, called blastomeres (blastos Greek for sprout), are undifferentiated and aggregated into a sphere enclosed within the membrane of glycoproteins (termed the zona pellucida) of the ovum. When eight blastomeres have formed they begin to develop gap junctions, enabling them to develop in an integrated way and co-ordinate their response to physiological signals and environmental cues. [5]

When the cells number around sixteen the solid sphere of cells within the zona pellucida is referred to as a morula [6] At this stage the cells start to bind firmly together in a process called compaction, and cleavage continues as cellular differentiation.

Blastulation Edit

Cleavage itself is the first stage in blastulation, the process of forming the blastocyst. Cells differentiate into an outer layer of cells (collectively called the trophoblast) and an inner cell mass. With further compaction the individual outer blastomeres, the trophoblasts, become indistinguishable. They are still enclosed within the zona pellucida. This compaction serves to make the structure watertight, containing the fluid that the cells will later secrete. The inner mass of cells differentiate to become embryoblasts and polarise at one end. They close together and form gap junctions, which facilitate cellular communication. This polarisation leaves a cavity, the blastocoel, creating a structure that is now termed the blastocyst. (In animals other than mammals, this is called the blastula.) The trophoblasts secrete fluid into the blastocoel. The resulting increase in size of the blastocyst causes it to hatch through the zona pellucida, which then disintegrates. [7] [4]

The inner cell mass will give rise to the pre-embryo, [8] the amnion, yolk sac and allantois, while the fetal part of the placenta will form from the outer trophoblast layer. The embryo plus its membranes is called the conceptus, and by this stage the conceptus has reached the uterus. The zona pellucida ultimately disappears completely, and the now exposed cells of the trophoblast allow the blastocyst to attach itself to the endometrium, where it will implant. The formation of the hypoblast and epiblast, which are the two main layers of the bilaminar germ disc, occurs at the beginning of the second week. [9] Either the embryoblast or the trophoblast will turn into two sub-layers. [10] The inner cells will turn into the hypoblast layer, which will surround the other layer, called the epiblast, and these layers will form the embryonic disc that will develop into the embryo. [9] [10] The trophoblast will also develop two sub-layers: the cytotrophoblast, which is in front of the syncytiotrophoblast, which in turn lies within the endometrium. [9] Next, another layer called the exocoelomic membrane or Heuser’s membrane will appear and surround the cytotrophoblast, as well as the primitive yolk sac. [10] The syncytiotrophoblast will grow and will enter a phase called lacunar stage, in which some vacuoles will appear and be filled by blood in the following days. [9] [10] The development of the yolk sac starts with the hypoblastic flat cells that form the exocoelomic membrane, which will coat the inner part of the cytotrophoblast to form the primitive yolk sac. An erosion of the endothelial lining of the maternal capillaries by the syncytiotrophoblastic cells of the sinusoids will form where the blood will begin to penetrate and flow through the trophoblast to give rise to the uteroplacental circulation. [11] [12] Subsequently new cells derived from yolk sac will be established between trophoblast and exocelomic membrane and will give rise to extra-embryonic mesoderm, which will form the chorionic cavity. [10]

At the end of the second week of development, some cells of the trophoblast penetrate and form rounded columns into the syncytiotrophoblast. These columns are known as primary villi. At the same time, other migrating cells form into the exocelomic cavity a new cavity named the secondary or definitive yolk sac, smaller than the primitive yolk sac. [10] [11]

Implantation Edit

After ovulation, the endometrial lining becomes transformed into a secretory lining in preparation of accepting the embryo. It becomes thickened, with its secretory glands becoming elongated, and is increasingly vascular. This lining of the uterine cavity (or womb) is now known as the decidua, and it produces a great number of large decidual cells in its increased interglandular tissue. The blastomeres in the blastocyst are arranged into an outer layer called the trophoblast. The trophoblast then differentiates into an inner layer, the cytotrophoblast, and an outer layer, the syncytiotrophoblast. The cytotrophoblast contains cuboidal epithelial cells and is the source of dividing cells, and the syncytiotrophoblast is a syncytial layer without cell boundaries.

The syncytiotrophoblast implants the blastocyst in the decidual epithelium by projections of chorionic villi, forming the embryonic part of the placenta. The placenta develops once the blastocyst is implanted, connecting the embryo to the uterine wall. The decidua here is termed the decidua basalis it lies between the blastocyst and the myometrium and forms the maternal part of the placenta. The implantation is assisted by hydrolytic enzymes that erode the epithelium. The syncytiotrophoblast also produces human chorionic gonadotropin, a hormone that stimulates the release of progesterone from the corpus luteum. Progesterone enriches the uterus with a thick lining of blood vessels and capillaries so that it can oxygenate and sustain the developing embryo. The uterus liberates sugar from stored glycogen from its cells to nourish the embryo. [13] The villi begin to branch and contain blood vessels of the embryo. Other villi, called terminal or free villi, exchange nutrients. The embryo is joined to the trophoblastic shell by a narrow connecting stalk that develops into the umbilical cord to attach the placenta to the embryo. [10] [14] Arteries in the decidua are remodelled to increase the maternal blood flow into the intervillous spaces of the placenta, allowing gas exchange and the transfer of nutrients to the embryo. Waste products from the embryo will diffuse across the placenta.

As the syncytiotrophoblast starts to penetrate the uterine wall, the inner cell mass (embryoblast) also develops. The inner cell mass is the source of embryonic stem cells, which are pluripotent and can develop into any one of the three germ layer cells, and which have the potency to give rise to all the tissues and organs.

Embryonic disc Edit

The embryoblast forms an embryonic disc, which is a bilaminar disc of two layers, an upper layer called the epiblast (primitive ectoderm) and a lower layer called the hypoblast (primitive endoderm). The disc is stretched between what will become the amniotic cavity and the yolk sac. The epiblast is adjacent to the trophoblast and made of columnar cells the hypoblast is closest to the blastocyst cavity and made of cuboidal cells. The epiblast migrates away from the trophoblast downwards, forming the amniotic cavity, the lining of which is formed from amnioblasts developed from the epiblast. The hypoblast is pushed down and forms the yolk sac (exocoelomic cavity) lining. Some hypoblast cells migrate along the inner cytotrophoblast lining of the blastocoel, secreting an extracellular matrix along the way. These hypoblast cells and extracellular matrix are called Heuser's membrane (or the exocoelomic membrane), and they cover the blastocoel to form the yolk sac (or exocoelomic cavity). Cells of the hypoblast migrate along the outer edges of this reticulum and form the extraembryonic mesoderm this disrupts the extraembryonic reticulum. Soon pockets form in the reticulum, which ultimately coalesce to form the chorionic cavity (extraembryonic coelom).

The primitive streak, a linear band of cells formed by the migrating epiblast, appears, and this marks the beginning of gastrulation, which takes place around the seventeenth day (week 3) after fertilisation. The process of gastrulation reorganises the two-layer embryo into a three-layer embryo, and also gives the embryo its specific head-to-tail, and front-to-back orientation, by way of the primitive streak which establishes bilateral symmetry. A primitive node (or primitive knot) forms in front of the primitive streak which is the organiser of neurulation. A primitive pit forms as a depression in the centre of the primitive node which connects to the notochord which lies directly underneath. The node has arisen from epiblasts of the amniotic cavity floor, and it is this node that induces the formation of the neural plate which serves as the basis for the nervous system. The neural plate will form opposite the primitive streak from ectodermal tissue which thickens and flattens into the neural plate. The epiblast in that region moves down into the streak at the location of the primitive pit where the process called ingression, which leads to the formation of the mesoderm takes place. This ingression sees the cells from the epiblast move into the primitive streak in an epithelial-mesenchymal transition epithelial cells become mesenchymal stem cells, multipotent stromal cells that can differentiate into various cell types. The hypoblast is pushed out of the way and goes on to form the amnion. The epiblast keeps moving and forms a second layer, the mesoderm. The epiblast has now differentiated into the three germ layers of the embryo, so that the bilaminar disc is now a trilaminar disc, the gastrula.

The three germ layers are the ectoderm, mesoderm and endoderm, and are formed as three overlapping flat discs. It is from these three layers that all the structures and organs of the body will be derived through the processes of somitogenesis, histogenesis and organogenesis. [15] The embryonic endoderm is formed by invagination of epiblastic cells that migrate to the hypoblast, while the mesoderm is formed by the cells that develop between the epiblast and endoderm. In general, all germ layers will derive from the epiblast. [10] [14] The upper layer of ectoderm will give rise to the outermost layer of skin, central and peripheral nervous systems, eyes, inner ear, and many connective tissues. [16] The middle layer of mesoderm will give rise to the heart and the beginning of the circulatory system as well as the bones, muscles and kidneys. The inner layer of endoderm will serve as the starting point for the development of the lungs, intestine, thyroid, pancreas and bladder.

Following ingression, a blastopore develops where the cells have ingressed, in one side of the embryo and it deepens to become the archenteron, the first formative stage of the gut. As in all deuterostomes, the blastopore becomes the anus whilst the gut tunnels through the embryo to the other side where the opening becomes the mouth. With a functioning digestive tube, gastrulation is now completed and the next stage of neurulation can begin.

Following gastrulation, the ectoderm gives rise to epithelial and neural tissue, and the gastrula is now referred to as the neurula. The neural plate that has formed as a thickened plate from the ectoderm, continues to broaden and its ends start to fold upwards as neural folds. Neurulation refers to this folding process whereby the neural plate is transformed into the neural tube, and this takes place during the fourth week. They fold, along a shallow neural groove which has formed as a dividing median line in the neural plate. This deepens as the folds continue to gain height, when they will meet and close together at the neural crest. The cells that migrate through the most cranial part of the primitive line form the paraxial mesoderm, which will give rise to the somitomeres that in the process of somitogenesis will differentiate into somites that will form the sclerotomes, the syndetomes, [17] the myotomes and the dermatomes to form cartilage and bone, tendons, dermis (skin), and muscle. The intermediate mesoderm gives rise to the urogenital tract and consists of cells that migrate from the middle region of the primitive line. Other cells migrate through the caudal part of the primitive line and form the lateral mesoderm, and those cells migrating by the most caudal part contribute to the extraembryonic mesoderm. [10] [14]

The embryonic disc begins flat and round, but eventually elongates to have a wider cephalic part and narrow-shaped caudal end. [9] At the beginning, the primitive line extends in cephalic direction and 18 days after fertilization returns caudally until it disappears. In the cephalic portion, the germ layer shows specific differentiation at the beginning of the 4th week, while in the caudal portion it occurs at the end of the 4th week. [10] Cranial and caudal neuropores become progressively smaller until they close completely (by day 26) forming the neural tube. [18]

Organogenesis is the development of the organs that begins during the third to eighth week, and continues until birth. Sometimes full development, as in the lungs, continues after birth. Different organs take part in the development of the many organ systems of the body.

Blood Edit

Haematopoietic stem cells that give rise to all the blood cells develop from the mesoderm. The development of blood formation takes place in clusters of blood cells, known as blood islands, in the yolk sac. Blood islands develop outside the embryo, on the umbilical vesicle, allantois, connecting stalk, and chorion, from mesodermal hemangioblasts.

In the centre of a blood island, hemangioblasts form the haematopoietic stem cells that are the precursor to all types of blood cell. In the periphery of a blood island the hemangioblasts differentiate into angioblasts the precursors to the blood vessels. [19]

Heart and circulatory system Edit

The heart is the first functional organ to develop and starts to beat and pump blood at around 22 days. [20] Cardiac myoblasts and blood islands in the splanchnopleuric mesenchyme on each side of the neural plate, give rise to the cardiogenic region. [10] : 165 This is a horseshoe-shaped area near to the head of the embryo. By day 19, following cell signalling, two strands begin to form as tubes in this region, as a lumen develops within them. These two endocardial tubes grow and by day 21 have migrated towards each other and fused to form a single primitive heart tube, the tubular heart. This is enabled by the folding of the embryo which pushes the tubes into the thoracic cavity. [21]

Also at the same time that the endocardial tubes are forming, vasculogenesis (the development of the circulatory system) has begun. This starts on day 18 with cells in the splanchnopleuric mesoderm differentiating into angioblasts that develop into flattened endothelial cells. These join to form small vesicles called angiocysts which join up to form long vessels called angioblastic cords. These cords develop into a pervasive network of plexuses in the formation of the vascular network. This network grows by the additional budding and sprouting of new vessels in the process of angiogenesis. [21] Following vasculogenesis and the development of an early vasculature, a stage of vascular remodelling takes place.

The tubular heart quickly forms five distinct regions. From head to tail, these are the infundibulum, bulbus cordis, primitive ventricle, primitive atrium, and the sinus venosus. Initially, all venous blood flows into the sinus venosus, and is propelled from tail to head to the truncus arteriosus. This will divide to form the aorta and pulmonary artery the bulbus cordis will develop into the right (primitive) ventricle the primitive ventricle will form the left ventricle the primitive atrium will become the front parts of the left and right atria and their appendages, and the sinus venosus will develop into the posterior part of the right atrium, the sinoatrial node and the coronary sinus. [20]

Cardiac looping begins to shape the heart as one of the processes of morphogenesis, and this completes by the end of the fourth week. Programmed cell death (apoptosis) at the joining surfaces enables fusion to take place. [21] In the middle of the fourth week, the sinus venosus receives blood from the three major veins: the vitelline, the umbilical and the common cardinal veins.

During the first two months of development, the interatrial septum begins to form. This septum divides the primitive atrium into a right and a left atrium. Firstly it starts as a crescent-shaped piece of tissue which grows downwards as the septum primum. The crescent shape prevents the complete closure of the atria allowing blood to be shunted from the right to the left atrium through the opening known as the ostium primum. This closes with further development of the system but before it does, a second opening (the ostium secundum) begins to form in the upper atrium enabling the continued shunting of blood. [21]

A second septum (the septum secundum) begins to form to the right of the septum primum. This also leaves a small opening, the foramen ovale which is continuous with the previous opening of the ostium secundum. The septum primum is reduced to a small flap that acts as the valve of the foramen ovale and this remains until its closure at birth. Between the ventricles the septum inferius also forms which develops into the muscular interventricular septum. [21]

Digestive system Edit

The digestive system starts to develop from the third week and by the twelfth week, the organs have correctly positioned themselves.

Respiratory system Edit

The respiratory system develops from the lung bud, which appears in the ventral wall of the foregut about four weeks into development. The lung bud forms the trachea and two lateral growths known as the bronchial buds, which enlarge at the beginning of the fifth week to form the left and right main bronchi. These bronchi in turn form secondary (lobar) bronchi three on the right and two on the left (reflecting the number of lung lobes). Tertiary bronchi form from secondary bronchi.

While the internal lining of the larynx originates from the lung bud, its cartilages and muscles originate from the fourth and sixth pharyngeal arches. [22]

Urinary system Edit

Kidneys Edit

Three different kidney systems form in the developing embryo: the pronephros, the mesonephros and the metanephros. Only the metanephros develops into the permanent kidney. All three are derived from the intermediate mesoderm.

Pronephros Edit

The pronephros derives from the intermediate mesoderm in the cervical region. It is not functional and degenerates before the end of the fourth week.

Mesonephros Edit

The mesonephros derives from intermediate mesoderm in the upper thoracic to upper lumbar segments. Excretory tubules are formed and enter the mesonephric duct, which ends in the cloaca. The mesonephric duct atrophies in females, but participate in development of the reproductive system in males.

Metanephros Edit

The metanephros appears in the fifth week of development. An outgrowth of the mesonephric duct, the ureteric bud, penetrates metanephric tissue to form the primitive renal pelvis, renal calyces and renal pyramids. The ureter is also formed.

Bladder and urethra Edit

Between the fourth and seventh weeks of development, the urorectal septum divides the cloaca into the urogenital sinus and the anal canal. The upper part of the urogenital sinus forms the bladder, while the lower part forms the urethra. [22]

Reproductive system Edit

Integumentary system Edit

The superficial layer of the skin, the epidermis, is derived from the ectoderm. The deeper layer, the dermis, is derived from mesenchyme.

The formation of the epidermis begins in the second month of development and it acquires its definitive arrangement at the end of the fourth month. The ectoderm divides to form a flat layer of cells on the surface known as the periderm. Further division forms the individual layers of the epidermis.

The mesenchyme that will form the dermis is derived from three sources:

  • The mesenchyme that forms the dermis in the limbs and body wall derives from the lateral plate mesoderm
  • The mesenchyme that forms the dermis in the back derives from paraxial mesoderm
  • The mesenchyme that forms the dermis in the face and neck derives from neural crest cells[22]

Nervous system Edit

Late in the fourth week, the superior part of the neural tube bends ventrally as the cephalic flexure at the level of the future midbrain—the mesencephalon. Above the mesencephalon is the prosencephalon (future forebrain) and beneath it is the rhombencephalon (future hindbrain).

Cranial neural crest cells migrate to the pharyngeal arches as neural stem cells, where they develop in the process of neurogenesis into neurons.

The optical vesicle (which eventually becomes the optic nerve, retina and iris) forms at the basal plate of the prosencephalon. The alar plate of the prosencephalon expands to form the cerebral hemispheres (the telencephalon) whilst its basal plate becomes the diencephalon. Finally, the optic vesicle grows to form an optic outgrowth.

Face and neck Edit

From the third to the eighth week the face and neck develop.

Ears Edit

The inner ear, middle ear and outer ear have distinct embryological origins.

Inner ear Edit

At about 22 days into development, the ectoderm on each side of the rhombencephalon thickens to form otic placodes. These placodes invaginate to form otic pits, and then otic vesicles. The otic vesicles then form ventral and dorsal components.

The ventral component forms the saccule and the cochlear duct. In the sixth week of development the cochlear duct emerges and penetrates the surrounding mesenchyme, travelling in a spiral shape until it forms 2.5 turns by the end of the eighth week. The saccule is the remaining part of the ventral component. It remains connected to the cochlear duct via the narrow ductus reuniens.

The dorsal component forms the utricle and semicircular canals.

Middle ear Edit

The tympanic cavity and eustachian tube are derived from the first pharyngeal pouch (a cavity lined by endoderm). The distal part of the cleft, the tubotympanic recess, widens to create the tympanic cavity. The proximal part of the cleft remains narrow and creates the eustachian tube.

The bones of the middle ear, the ossicles, derive from the cartilages of the pharyngeal arches. The malleus and incus derive from the cartilage of the first pharyngeal arch, whereas the stapes derives from the cartilage of the second pharyngeal arch.

Outer ear Edit

The external auditory meatus develops from the dorsal portion of the first pharyngeal cleft. Six auricular hillocks, which are mesenchymal proliferations at the dorsal aspects of the first and second pharyngeal arches, form the auricle of the ear. [22]


30.4 Aging

A number of characteristic aging symptoms are experienced by a majority or by a significant proportion of humans during their lifetimes.

  • Teenagers lose the young child’s ability to hear high-frequency sounds above 20 kHz.
  • Wrinkles develop mainly due to photoageing, particularly affecting sun-exposed areas (face).
  • After peaking in the mid-20s, female fertility declines.
  • After age 30 the mass of human body is decreased until 70 years and then shows damping oscillations.
  • Muscles have reduced capacity of responding to exercise or injury and loss of muscle mass and strength (sarcopenia) is common. VO2 max and maximum heart rate decline.
  • People over 35 years of age are at increasing risk for losing strength in the ciliary muscle which leads to difficulty focusing on close objects, or presbyopia. Most people experience presbyopia by age 45–50. The cause is lens hardening by decreasing levels of α-crystallin, a process which may be sped up by higher temperatures.
  • Around age 50, hair turns grey. Pattern hair loss by the age of 50 affects about 30–50% of males and a quarter of females.
  • Menopause typically occurs between 44 and 58 years of age.
  • In the 60–64 age cohort, the incidence of osteoarthritis rises to 53%. Only 20% however report disabling osteoarthritis at this age.
  • Almost half of people older than 75 have hearing loss (presbycusis) inhibiting spoken communication. Many vertebrates such as fish, birds and amphibians do not suffer presbycusis in old age as they are able to regenerate their cochlear sensory cells, whereas mammals including humans have genetically lost this ability.
  • By age 80, more than half of all Americans either have a cataract or have had cataract surgery.
  • Frailty, a syndrome of decreased strength, physical activity, physical performance and energy, affects 25% of those over 85.
  • Atherosclerosis is classified as an aging disease. It leads to cardiovascular disease (for example stroke and heart attack) which globally is the most common cause of death. Vessel aging causes vascular remodeling and loss of arterial elasticity and as a result causes the stiffness of the vasculature.
  • Recent evidence suggests that age-related risk of death plateaus after age 105. The maximum human lifespan is suggested to be 115 years. The oldest reliably recorded human was Jeanne Calment who died in 1997 at 122.

Dementia becomes more common with age. About 3% of people between the ages of 65 and 74, 19% between 75 and 84, and nearly half of those over 85 years of age have dementia. The spectrum ranges from mild cognitive impairment to the neurodegenerative diseases of Alzheimer’s disease, cerebrovascular disease, Parkinson’s disease and Lou Gehrig’s disease. Furthermore, many types of memory decline with aging, but not semantic memory or general knowledge such as vocabulary definitions, which typically increases or remains steady until late adulthood. Intelligence declines with age, though the rate varies depending on the type and may in fact remain steady throughout most of the lifespan, dropping suddenly only as people near the end of their lives. Individual variations in rate of cognitive decline may therefore be explained in terms of people having different lengths of life. There are changes to the brain: after 20 years of age there is a 10% reduction each decade in the total length of the brain’s myelinated axons.

Age can result in visual impairment, whereby non-verbal communication is reduced, which can lead to isolation and possible depression. Older adults, however, may not suffer depression as much as younger adults, and were paradoxically found to have improved mood despite declining physical health. Macular degeneration causes vision loss and increases with age, affecting nearly 12% of those above the age of 80. This degeneration is caused by systemic changes in the circulation of waste products and by growth of abnormal vessels around the retina.

A distinction can be made between “proximal aging” (age-based effects that come about because of factors in the recent past) and “distal aging” (age-based differences that can be traced to a cause in a person’s early life, such as childhood poliomyelitis).

Aging is among the greatest known risk factors for most human diseases. Of the roughly 150,000 people who die each day across the globe, about two thirds—100,000 per day—die from age-related causes. In industrialized nations, the proportion is higher, reaching 90%. In humans, aging represents the accumulation of changes in a human being over time and can encompass physical, psychological, and social changes. Reaction time, for example, may slow with age, while knowledge of world events and wisdom may expand.

The causes of aging are uncertain current theories are assigned to the damage concept, whereby the accumulation of damage (such as DNA oxidation) may cause biological systems to fail, or to the programmed aging concept, whereby internal processes (such as DNA methylation) may cause aging. Programmed aging should not be confused with programmed cell death (apoptosis).

At present, researchers are only just beginning to understand the biological basis of aging even in relatively simple and short-lived organisms such as yeast. Less still is known of mammalian aging, in part due to the much longer lives of even small mammals such as the mouse (around 3 years). A model organism for studying of aging is the nematode C. elegans. Thanks to its short lifespan of 2–3 weeks, our ability to easily perform genetic manipulations or to suppress gene activity with RNA interference, or other factors. Most known mutations and RNA interference targets that extend lifespan were first discovered in C. elegans.

The factors proposed to influence biological aging fall into two main categories, programmed and damage-related. Programmed factors follow a biological timetable, perhaps one that might be a continuation of the one that regulates childhood growth and development. This regulation would depend on changes in gene expression that affect the systems responsible for maintenance, repair and defense responses. Damage-related factors include internal and environmental assaults to living organisms that induce cumulative damage at various levels. A third, novel, concept is that aging is mediated by vicious cycles.

In a detailed review, Lopez-Otin and colleagues (2013), who discuss aging through the lens of the damage theory, propose nine metabolic “hallmarks” of aging in various organisms but especially mammals:

  • genomic instability (mutations accumulated in nuclear DNA, in mtDNA, and in the nuclear lamina)
  • telomere attrition (the authors note that artificial telomerase confers non-cancerous immortality to otherwise mortal cells)
  • epigenetic alterations (including DNA methylation patterns, post-translational modification of histones, and chromatin remodelling)
  • loss of proteostasis (protein folding and proteolysis)
  • deregulated nutrient sensing (relating to the Growth hormone/Insulin-like growth factor 1 signalling pathway, which is the most conserved aging-controlling pathway in evolution and among its targets are the FOXO3/Sirtuin transcription factors and the mTOR complexes, probably responsive to caloric restriction)
  • mitochondrial dysfunction (the authors point out however that a causal link between aging and increased mitochondrial production of reactive oxygen species is no longer supported by recent research)
  • cellular senescence (accumulation of no longer dividing cells in certain tissues, a process induced especially by p16INK4a/Rb and p19ARF/p53 to stop cancerous cells from proliferating)
  • stem cell exhaustion (in the authors’ view caused by damage factors such as those listed above)
  • altered intercellular communication (encompassing especially inflammation but possibly also other intercellular interactions)

There are three main metabolic pathways which can influence the rate of aging, discussed below:

  • the FOXO3/Sirtuin pathway, probably responsive to caloric restriction
  • the Growth hormone/Insulin-like growth factor 1 signalling pathway
  • the activity levels of the electron transport chain in mitochondria and (in plants) in chloroplasts.

It is likely that most of these pathways affect aging separately, because targeting them simultaneously leads to additive increases in lifespan.


P otential of BMP4 in E x V ivo E xpansion of HSC s

Successful ex vivo expansion of HSCs must amplify their numbers while preserving their critical functions of self-renewal, multilineage differentiation capacity, and the ability to repopulate a myeloablated host [ 168 ]. However, many of the cytokine cocktails used in ex vivo expansion cultures, although leading to expansion in cell number from primitive populations, appear to fail to expand or maintain HSCs with long-term reconstitution ability (LTRC). For example, four small clinical trials that have evaluated the effect of the inclusion of ex vivo–expanded CB cells in the donor population together with unmanipulated cells, do not appear to show any significant effects on engraftment kinetics [ 161 ]. The recent demonstration that factors such as Shh, Notch, mKirre, and Wnt3a have a role in HSC self-renewal (see above) suggests that the inclusion of such factors to expansion cultures may lead to enhanced maintenance or expansion of HSC ex vivo.

The HGF activity of BMP4 and its role in the development of HSCs suggest that it may also have an ability to modulate the properties of HSCs in ex vivo cytokine expansion cultures. Indeed, recent findings suggest that HSCs may respond to BMP4 ex vivo. For example, treatment of CB cell populations highly enriched for human HSCs (CD34 + CD38 − Lin − ) with a high dose of BMP4 (25 ng/ml) results in a modest increase in the length of time SRC can be maintained in serum-free media containing the hematopoietic cytokines SCF, fetal liver tyrosine kinase ligand-3 (Flt-3), granulocyte colony-stimulating factor (G-CSF), interleukin (IL)-3, and IL-6 [ 157 ]. However, the BMP4 effects were highly dependent on concentration because a lower dose of BMP4 in the culture resulted in complete differentiation of the starting CD34 + CD38 − Lin − population to CD34 + CD38 + cells by day 4, with resultant decreases in cell expansion, colony-forming cells, and a significant reduction in day-4 SRC. Other BMPs (BMP2 and BMP7) were also tested in these assays and found to have concentration-dependent effects (Table 1). While low doses of these BMPs did not affect day-4 SRC, higher doses significantly reduced the ability of the cytokine cocktail to maintain SRC to day 4. No net expansion of SRC was detected in the cultures with BMP4 at 25 ng/ml, suggesting that BMP4 may provide a survival signal for HSCs in this culture system.

Low concentrationa High concentrationb
BMP2 No effect on expansion Expansion ↓↓↓
CFC output ↓ CFC output ↓↓↓
Slight maintenance (25%) of SRC to day 6 Day-4 SRC ↓↓
BMP7 No effect on expansion Expansion ↓↓↓
CFC output ↓ CFC output ↓↓↓
No effect on day-4 SRC Day-4 SRC ↓↓
BMP4 Complete differentiation to 34 + 38 + Maintenance of 38 − cells
Expansion ↓↓ Expansion ↑
CFC output ↓ Maintenance (83%) of SRC to day 6
Day-4 SRC ↓
  • a BMP family members have different dose-dependent effects on CD34 + CD38 − Lin − cells maintained in serum-free culture (stem cell factor, fetal liver tyrosine kinase ligand-3, G-CSF, IL-3, and IL-6). Total cell numbers and CFC frequencies were determined on day 0 and day 3 relative to cells maintained in serum-free cultures without BMPs. SRC frequency was determined in mice 8 weeks after transplantation with CD34 + CD38 − Lin − cells cultured for 2, 4, and 6 days in serum-free media.
  • a a Low BMP concentration, 5 ng/ml.
  • b b High BMP concentration, 25 ng/ml for BMP4 and 50 ng/ml for BMP2 and BMP7.
  • d Abbreviations: CFC, colony-forming cell IL, interleukin SRC, severe combined immunodeficiency–repopulating cell.

Bhardwaj et al. [ 153 ] subsequently indicated that BMP4 and its inhibitor noggin form part of an Shh-dependent pathway involved in regulation of primitive hematopoietic cell fate. In combination with SCF, Flt-3, G-CSF, IL-3, and IL-6, Shh induces marked proliferation of CD34 + CD38 − cells and, most important, a moderate expansion of SRC in CD34 + CD38 − Lin − cells maintained in serum-free conditions [ 153 ]. Shh may achieve these outcomes in part by regulating the expression of BMP4 and noggin in CD34 + CD38 − Lin − cells. The proliferative effects of Shh appear to require BMP signaling, as cotreatment of cultures with exogenous Shh and noggin inhibits the Shh-dependent proliferation of CD34 + CD8 − cells. Bhardwaj et al. propose that Shh, acting in concert with the hematopoietic cytokines used in these cultures, regulates HSCs in part by determining the level of BMP4 protein available for signaling. Whether regulation of BMP signaling by Shh is important for SRC expansion was not addressed in this study, but, given the earlier finding that BMP4 alone cannot induce SRC expansion [ 157 ], it would appear that Shh affects HSC over and above the effects on the BMP4/noggin axis. These additional activities may relate to Shh effects on other TGF-βfamily members, or on independent signaling pathways, such as those activated by the Notch ligands [ 169 ], and Wnt family members [ 151 , 170 ], which are known to affect self-renewal.


References

  1. ↑ Chandrashekar J, Hoon MA, Ryba NJ & Zuker CS. (2006). The receptors and cells for mammalian taste. Nature , 444, 288-94. PMID: 17108952DOI.
  2. ↑ Chen J, Jacox LA, Saldanha F & Sive H. (2017). Mouth development. Wiley Interdiscip Rev Dev Biol , 6, . PMID: 28514120DOI.
  3. ↑ Ueno S, Yamada S, Uwabe C, Männer J, Shiraki N & Takakuwa T. (2016). The Digestive Tract and Derived Primordia Differentiate by Following a Precise Timeline in Human Embryos Between Carnegie Stages 11 and 13. Anat Rec (Hoboken) , 299, 439-49. PMID: 26995337DOI.
  4. ↑ Guizetti B & Radlanski RJ. (1996). Development of the parotid gland and its closer neighboring structures in human embryos and fetuses of 19-67 mm CRL. Ann. Anat. , 178, 503-8. PMID: 9010565DOI.
  5. ↑ 5.05.1 Raghoebir L, Bakker ER, Mills JC, Swagemakers S, Kempen MB, Munck AB, Driegen S, Meijer D, Grosveld F, Tibboel D, Smits R & Rottier RJ. (2012). SOX2 redirects the developmental fate of the intestinal epithelium toward a premature gastric phenotype. J Mol Cell Biol , 4, 377-85. PMID: 22679103DOI.
  6. ↑ 6.06.1 Goto A, Sumiyama K, Kamioka Y, Nakasyo E, Ito K, Iwasaki M, Enomoto H & Matsuda M. (2013). GDNF and endothelin 3 regulate migration of enteric neural crest-derived cells via protein kinase A and Rac1. J. Neurosci. , 33, 4901-12. PMID: 23486961DOI.

Reviews

Chen J, Jacox LA, Saldanha F & Sive H. (2017). Mouth development. Wiley Interdiscip Rev Dev Biol , 6, . PMID: 28514120 DOI. PDF

Articles

Guizetti B & Radlanski RJ. (1996). Development of the submandibular gland and its closer neighboring structures in human embryos and fetuses of 19-67 mm CRL. Ann. Anat. , 178, 509-14. PMID: 9010566 DOI.

Guizetti B & Radlanski RJ. (1996). Development of the parotid gland and its closer neighboring structures in human embryos and fetuses of 19-67 mm CRL. Ann. Anat. , 178, 503-8. PMID: 9010565 DOI.

Online Textbooks

  • Developmental Biology (6th ed) Gilbert, Scott F. Sunderland (MA): Sinauer Associates, Inc. c2000. The Digestive Tube and Its Derivatives | Endodermal development of a human embryo
  • The Gastrointestinal Circulation Peter R. Kvietys. San Rafael (CA): Morgan & Claypool Publishers 2010. Table of Contents
  • Motor Function of the Pharynx, Esophagus, and its Sphincters. Mittal RK. San Rafael (CA): Morgan & Claypool Life Sciences 2011. Table of Contents
  • Search NLM Online Textbooks "gastrointestinal tract" : Developmental Biology | Endocrinology | Molecular Biology of the Cell | The Cell- A molecular Approach

Historic Textbooks

  • The Elements of Embryology by Foster, M., Balfour, F. M., Sedgwick, A., & Heape, W. (1883) The Alimentary Canal and its Appendages
  • Text-Book of the Embryology of Man and Mammals by Dr Oscar Hertwig (1892) The Organs of the Inner Germ-Layer The Alimentary Tube with its Appended Organs
  • Atlas of the Development of Man Volume 2 by Julius Kollmann (1907) Gastrointestinal
  • Text-Book of Embryology by Bailey, F.R. and Miller, A.M. (1921) Alimentary tube and organs

Search PubMed


Germinal stage

Fertilization

Fertilization takes place when the spermatozoon has successfully entered the ovum and the two sets of genetic material carried by the gametes fuse together, resulting in the zygote (a single diploid cell). This usually takes place in the ampulla of one of the fallopian tubes. The zygote contains the combined genetic material carried by both the male and female gametes which consists of the 23 chromosomes from the nucleus of the ovum and the 23 chromosomes from the nucleus of the sperm. The 46 chromosomes undergo changes prior to the mitotic division which leads to the formation of the embryo having two cells.

Successful fertilization is enabled by three processes, which also act as controls to ensure species-specificity. The first is that of chemotaxis which directs the movement of the sperm towards the ovum. Secondly there is an adhesive compatibility between the sperm and the egg. With the sperm adhered to the ovum, the third process of acrosomal reaction takes place the front part of the spermatozoan head is capped by an acrosome which contains digestive enzymes to break down the zona pellucida and allow its entry. The entry of the sperm causes calcium to be released which blocks entry to other sperm cells. A parallel reaction takes place in the ovum called the zona reaction. This sees the release of cortical granules that release enzymes which digest sperm receptor proteins, thus preventing polyspermy. The granules also fuse with the plasma membrane and modify the zona pellucida in such a way as to prevent further sperm entry.

Cleavage

The beginning of the cleavage process is marked when the zygote divides through mitosis into two cells. This mitosis continues and the first two cells divide into four cells, then into eight cells and so on. Each division takes from 12 to 24 hours. The zygote is large compared to any other cell and undergoes cleavage without any overall increase in size. This means that with each successive subdivision, the ratio of nuclear to cytoplasmic material increases. Initially the dividing cells, called blastomeres (blastos Greek for sprout), are undifferentiated and aggregated into a sphere enclosed within the membrane of glycoproteins (termed the zona pellucida) of the ovum. When eight blastomeres have formed they begin to develop gap junctions, enabling them to develop in an integrated way and co-ordinate their response to physiological signals and environmental cues.

When the cells number around sixteen the solid sphere of cells within the zona pellucida is referred to as a morula At this stage the cells start to bind firmly together in a process called compaction, and cleavage continues as cellular differentiation.

Blastulation

Cleavage itself is the first stage in blastulation, the process of forming the blastocyst. Cells differentiate into an outer layer of cells (collectively called the trophoblast) and an inner cell mass. With further compaction the individual outer blastomeres, the trophoblasts, become indistinguishable. They are still enclosed within the zona pellucida. This compaction serves to make the structure watertight, containing the fluid that the cells will later secrete. The inner mass of cells differentiate to become embryoblasts and polarise at one end. They close together and form gap junctions, which facilitate cellular communication. This polarisation leaves a cavity, the blastocoel, creating a structure that is now termed the blastocyst. (In animals other than mammals, this is called the blastula.) The trophoblasts secrete fluid into the blastocoel. The resulting increase in size of the blastocyst causes it to hatch through the zona pellucida, which then disintegrates.

The inner cell mass will give rise to the pre-embryo, the amnion, yolk sac and allantois, while the fetal part of the placenta will form from the outer trophoblast layer. The embryo plus its membranes is called the conceptus, and by this stage the conceptus has reached the uterus. The zona pellucida ultimately disappears completely, and the now exposed cells of the trophoblast allow the blastocyst to attach itself to the endometrium, where it will implant. The formation of the hypoblast and epiblast, which are the two main layers of the bilaminar germ disc, occurs at the beginning of the second week. Either the embryoblast or the trophoblast will turn into two sub-layers. The inner cells will turn into the hypoblast layer, which will surround the other layer, called the epiblast, and these layers will form the embryonic disc that will develop into the embryo. The trophoblast will also develop two sub-layers: the cytotrophoblast, which is in front of the syncytiotrophoblast, which in turn lies within the endometrium. Next, another layer called the exocoelomic membrane or Heuser’s membrane will appear and surround the cytotrophoblast, as well as the primitive yolk sac. The syncytiotrophoblast will grow and will enter a phase called lacunar stage, in which some vacuoles will appear and be filled by blood in the following days. The development of the yolk sac starts with the hypoblastic flat cells that form the exocoelomic membrane, which will coat the inner part of the cytotrophoblast to form the primitive yolk sac. An erosion of the endothelial lining of the maternal capillaries by the syncytiotrophoblastic cells of the sinusoids will form where the blood will begin to penetrate and flow through the trophoblast to give rise to the uteroplacental circulation. Subsequently new cells derived from yolk sac will be established between trophoblast and exocelomic membrane and will give rise to extra-embryonic mesoderm, which will form the chorionic cavity.

At the end of the second week of development, some cells of the trophoblast penetrate and form rounded columns into the syncytiotrophoblast. These columns are known as primary villi. At the same time, other migrating cells form into the exocelomic cavity a new cavity named the secondary or definitive yolk sac, smaller than the primitive yolk sac.

Implantation

After ovulation, the endometrial lining becomes transformed into a secretory lining in preparation of accepting the embryo. It becomes thickened, with its secretory glands becoming elongated, and is increasingly vascular. This lining of the uterine cavity (or womb) is now known as the decidua, and it produces a great number of large decidual cells in its increased interglandular tissue. The blastomeres in the blastocyst are arranged into an outer layer called Trophoblast.The trophoblast then differentiates into an inner layer, the cytotrophoblast, and an outer layer, the syncytiotrophoblast. The cytotrophoblast contains cuboidal epithelial cells and is the source of dividing cells, and the syncytiotrophoblast is a syncytial layer without cell boundaries.

The syncytiotrophoblast implants the blastocyst in the decidual epithelium by projections of chorionic villi, forming the embryonic part of the placenta. The placenta develops once the blastocyst is implanted, connecting the embryo to the uterine wall. The decidua here is termed the decidua basalis it lies between the blastocyst and the myometrium and forms the maternal part of the placenta. The implantation is assisted by hydrolytic enzymes that erode the epithelium. The syncytiotrophoblast also produces human chorionic gonadotropin, a hormone that stimulates the release of progesterone from the corpus luteum. Progesterone enriches the uterus with a thick lining of blood vessels and capillaries so that it can oxygenate and sustain the developing embryo. The uterus liberates sugar from stored glycogen from its cells to nourish the embryo. The villi begin to branch and contain blood vessels of the embryo. Other villi, called terminal or free villi, exchange nutrients. The embryo is joined to the trophoblastic shell by a narrow connecting stalk that develops into the umbilical cord to attach the placenta to the embryo. Arteries in the decidua are remodelled to increase the maternal blood flow into the intervillous spaces of the placenta, allowing gas exchange and the transfer of nutrients to the embryo. Waste products from the embryo will diffuse across the placenta.

As the syncytiotrophoblast starts to penetrate the uterine wall, the inner cell mass (embryoblast) also develops. The inner cell mass is the source of embryonic stem cells, which are pluripotent and can develop into any one of the three germ layer cells, and which have the potency to give rise to all the tissues and organs.

Embryonic disc

The embryoblast forms an embryonic disc, which is a bilaminar disc of two layers, an upper layer called the epiblast (primitive ectoderm) and a lower layer called the hypoblast (primitive endoderm). The disc is stretched between what will become the amniotic cavity and the yolk sac. The epiblast is adjacent to the trophoblast and made of columnar cells the hypoblast is closest to the blastocyst cavity and made of cuboidal cells. The epiblast migrates away from the trophoblast downwards, forming the amniotic cavity, the lining of which is formed from amnioblasts developed from the epiblast. The hypoblast is pushed down and forms the yolk sac (exocoelomic cavity) lining. Some hypoblast cells migrate along the inner cytotrophoblast lining of the blastocoel, secreting an extracellular matrix along the way. These hypoblast cells and extracellular matrix are called Heuser's membrane (or the exocoelomic membrane), and they cover the blastocoel to form the yolk sac (or exocoelomic cavity). Cells of the hypoblast migrate along the outer edges of this reticulum and form the extraembryonic mesoderm this disrupts the extraembryonic reticulum. Soon pockets form in the reticulum, which ultimately coalesce to form the chorionic cavity or extraembryonic coelom.


Abnormalities

  • Q39.0 Atresia of oesophagus without fistula Atresia of oesophagus NOS
  • Q39.1 Atresia of oesophagus with tracheo-oesophageal fistula Atresia of oesophagus with broncho-oesophageal fistula
  • Q39.2 Congenital tracheo-oesophageal fistula without atresia Congenital tracheo-oesophageal fistula NOS
  • Q39.3 Congenital stenosis and stricture of oesophagus
  • Q39.4 Oesophageal web
  • Q39.5 Congenital dilatation of oesophagus
  • Q39.6 Diverticulum of oesophagus Oesophageal pouch
  • Q39.8 Other congenital malformations of oesophagus Absent Congenital displacement Duplication (of) oesophagus
  • Q39.9 Congenital malformation of oesophagus, unspecified

Note ICD-10 is currently being updated to ICD-11 and will have new replacement coding.

Oesophageal Atresia with Tracheo-Oesophageal Fistula

(Q39.1 Atresia of oesophagus with tracheo-oesophageal fistula Atresia of oesophagus with broncho-oesophageal fistula, OA/TOF)

ICD-11 (beta) LB12.2 Atresia of oesophagus "Oesophageal atresia encompasses a group of congenital anomalies with an interruption in the continuity of the oesophagus, with or without persistent communication with the trachea. In 86% of cases there is a distal tracheooesophageal fistula, in 7% of cases there is no fistulous connection, while in 4% of cases there is a tracheooesophageal fistula without atresia. The remaining cases are made up of patients with OA with proximal, or both proximal and distal, tracheooesophageal fistula."


This abnormality has been shown to be associated with Tbx1 mutations that also include DiGeorge syndrome. ⎗]


Terms

  • allantois - An extraembryonic membrane, endoderm in origin extension from the early hindgut, then cloaca into the connecting stalk of placental animals, connected to the superior end of developing bladder. In reptiles and birds, acts as a reservoir for wastes and mediates gas exchange. In mammals is associated/incorporated with connecting stalk/placental cord fetal-maternal interface.
  • amnion - An extra-embryonic membrane, ectoderm and extraembryonic mesoderm in origin, also forms the innermost fetal membrane, that produces amniotic fluid. This fluid-filled sac initially lies above the trilaminar embryonic disc and with embryoic disc folding this sac is drawn ventrally to enclose (cover) the entire embryo, then fetus. The presence of this membrane led to the description of reptiles, bird, and mammals as amniotes.
  • amniotic fluid - The fluid that fills amniotic cavity totally encloses and cushions the embryo. Amniotic fluid enters both the gastrointestinal and respiratory tract following rupture of the buccopharyngeal membrane. The late fetus swallows amniotic fluid.
  • atresia - is an abnormal interruption of the tube lumen, the abnormality naming is based upon the anatomical location.
  • buccal - (Latin, bucca = cheek) A term used to relate to the mouth (oral cavity).
  • bile salts - Liver synthesized compounds derived from cholesterol that function postnatally in the small intestine to solubilize and absorb lipids, vitamins, and proteins. These compounds act as water-soluble amphipathic detergents. liver
  • buccopharyngeal membrane - (oral membrane) (Latin, bucca = cheek) A membrane which forms the external upper membrane limit (cranial end) of the early gastrointestinal tract. This membrane develops during gastrulation by ectoderm and endoderm without a middle (intervening) layer of mesoderm. The membrane lies at the floor of the ventral depression (stomodeum) where the oral cavity will open and will breakdown to form the initial "oral opening" of the gastrointestinal tract. The equivilent membrane at the lower end of the gastrointestinal tract is the cloacal membrane.
  • celiac artery - (celiac trunk) main blood supply to the foregut, excluding the pharynx, lower respiratory tract, and most of the oesophagus.
  • cholangiocytes - epithelial cells that line the intra- and extrahepatic ducts of the biliary tree. These cells modify the hepatocyte-derived bile, and are regulated by hormones, peptides, nucleotides, neurotransmitters, and other molecules. liver
  • cloaca - (cloacal cavity) The term describing the common cavity into which the intestinal, genital, and urinary tracts open in vertebrates. Located at the caudal end of the embryo it is located on the surface by the cloacal membrane. In many species this common cavity is later divided into a ventral urogenital region (urogenital sinus) and a dorsal gastrointestinal (rectal) region.
  • cloacal membrane - Forms the external lower membrane limit (caudal end) of the early gastrointestinal tract (GIT). This membrane is formed during gastrulation by ectoderm and endoderm without a middle (intervening) layer of mesoderm. The membrane breaks down to form the initial "anal opening" of the gastrointestinal tract.
  • coelomic cavity - (coelom) Term used to describe a space. There are extra-embryonic and intra-embryonic coeloms that form during vertebrate development. The single intra-embryonic coelom forms the 3 major body cavities: pleural cavity, pericardial cavity and peritoneal cavity.
  • crypt of Lieberkühn - (intestinal gland, intestinal crypt) intestinal villi epithelia extend down into the lamina propria where they form crypts that are the source of epithelial stem cells and immune function.
  • duplication - is an abnormal incomplete tube recanalization resulting in parallel lumens, this is really a specialized form of stenosis. (More? Image - small intestine duplication)
  • enteric nervous system - (ENS) neural crest in origin, both neurons and glia. Regulates gastrointestinal tract: motility, secretion and blood flow.
  • esophageal - (oesophageal)
  • esophageal atresia - (oesophageal atresia, atresia of oesophagus) group of congenital anomalies with an interruption in the continuity of the oesophagus, with or without persistent communication with the trachea. (More? gastrointestinal abnormalities |  ICD-11LB12.1 Atresia of oesophagusMedline Plus)
  • foregut - first embryonic division of gastrointestinal tract extending from the oral (buccopharyngeal) membrane and contributing oesophagus, stomach, duodenum (to bile duct opening), liver, biliary apparatus (hepatic ducts, gallbladder, and bile duct), and pancreas. The forgut blood supply is the celiac artery (trunk) excluding the pharynx, lower respiratory tract, and most of the oesophagus.
  • galactosemia - Metabolic abnormality where the simple sugar galactose (half of lactose, the sugar in milk) cannot be metabolised. People with galactosemia cannot tolerate any form of milk (human or animal). Detected by the Guthrie test.
  • gastric transposition - clinical term for postnatal surgery treatment for esophageal atresia involving esophageal replacement. Typically performed on neonates between day 1 to 4. (More? gastrointestinal abnormalities | PMID 28658159
  • gastrointestinal divisions - refers to the 3 embryonic divisions contributing the gastrointestinal tract: foregut, Midgut and hindgut.
  • gastrula - (Greek, gastrula = little stomach) A stage of an animal embryo in which the three germ layers (endoderm/mesoderm/ectoderm) have just formed. All of these germ layers have contributions to the gastrointestinal tract.
  • gastrulation - The process of differentiation forming a gastrula. Term means literally means "to form a gut" but is more in development, as this process converts the bilaminar embryo (epiblast/hypoblast) into the trilaminar embryo (endoderm/mesoderm/ectoderm) establishing the 3 germ layers that will form all the future tissues of the entire embryo. This process also establishes the the initial body axes. (More? gastrulation)
  • Guthrie test - (heel prick) A neonatal blood screening test developed by Dr Robert Guthrie (1916-95) for determining a range of metabolic disorders and infections in the neonate. (More? Guthrie test)
  • heterotaxia - (Greek heteros = different taxis = arrangement) is the right/left transposition of thoracic and/or abdominal organs.
  • hindgut - final embryonic division of gastrointestinal tract extending to the cloacal membrane and contributing part of the transverse colon (left half to one third), descending colon, sigmoid colon, rectum, part of anal canal (superior), urinary epithelium (bladder and most urethra). The hindgut blood supply is the inferior mesenteric artery.
  • inferior mesenteric artery - main blood supply to the hindgut
  • intestine - (bowel) part of the gastrointestinal tract (GIT) lying between the stomach and anus where absorption of nutrients and water occur. This region is further divided anatomically and functionally into the small intestine or bowel (duodenum, jejunum and ileum) and large intestine or bowel (cecum and colon).
  • intestinal perforation - gastrointestinal abnormality identified in neonates can be due to necrotizing enterocolitis, Hirschsprung’s disease or meconium ileus.
  • intraembryonic coelom - The "horseshoe-shaped" space (cavity) that forms initially in the third week of development in the lateral plate mesoderm that will eventually form the 3 main body cavities: pericardial, pleural, peritoneal. The intraembryonic coelom communicates transiently with the extraembryonic coelom.
  • meconium ileusintestine obstruction within the ileum due to abnormal meconium properties.
  • mesentery - connects gastrointestinal tract to the posterior body wall and is a double layer of visceral peritoneum.
  • mesothelium - The mesoderm derived epithelial covering of coelomic organs and also line their cavities.
  • Midgut - middle embryonic division of gastrointestinal tract contributing the small intestine (including duodenum distal bile duct opening), cecum, appendix, ascending colon, and part of the transverse colon (right half to two thirds). The midgut blood supply is the superior mesenteric artery.
  • neuralation - The general term used to describe the early formation of the nervous system. It is often used to describe the early events of differentiation of the central ectoderm region to form the neural plate, then neural groove, then neural tube. The nervous system includes the central nervous system (brain and spinal cord) from the neural tube and the peripheral nervous system (peripheral sensory and sympathetic ganglia) from neural crest. In humans, early neuralation begins in week 3 and continues through week 4.
  • neural crest - region of cells at the edge of the neural plate that migrates throughout the embryo and contributes to many different tissues. In the gastrointestinal tract it contributes mainly the enteric nervous system within the wall of the gut responsible for peristalsis and secretion.
  • peritoneal stomata - the main openings forming the pathways for drainage of intra-peritoneal fluid from the peritoneal cavity into the lymphatic system.
  • pharynx - uppermost end of gastrointestinal and respiratory tract, in the embryo beginning at the buccopharyngeal membrane and forms a major arched cavity within the phrayngeal arches.
  • recanalization - describes the process of a hollow structure becoming solid, then becoming hollow again. For example, this process occurs during GIT, auditory and renal system development.
  • retroperitoneal - (retroperitoneum) is the anatomical space (sometimes a potential space) in the abdominal cavity behind (retro) the peritoneum. Developmentally parts of the GIT become secondarily retroperitoneal (part of duodenum, ascending and descending colon, pancreas)
  • somitogenesis The process of segmentation of the paraxial mesoderm within the trilaminar embryo body to form pairs of somites, or balls of mesoderm. A somite is added either side of the notochord (axial mesoderm) to form a somite pair. The segmentation does not occur in the head region, and begins cranially (head end) and extends caudally (tailward) adding a somite pair at regular time intervals. The process is sequential and therefore used to stage the age of many different species embryos based upon the number visible somite pairs. In humans, the first somite pair appears at day 20 and adds caudally at 1 somite pair/4 hours (mouse 1 pair/90 min) until on average 44 pairs eventually form.
  • splanchnic mesoderm - Gastrointestinal tract (endoderm) associated mesoderm formed by the separation of the lateral plate mesoderm into two separate components by a cavity, the intraembryonic coelom. Splanchnic mesoderm is the embryonic origin of the gastrointestinal tract connective tissue, smooth muscle, blood vessels and contribute to organ development (pancreas, spleen, liver). The intraembryonic coelom will form the three major body cavities including the space surrounding the gut, the peritoneal cavity. The other half of the lateral plate mesoderm (somatic mesoderm) is associated with the ectoderm of the body wall.
  • stomodeum - (stomadeum, stomatodeum) A ventral surface depression on the early embryo head surrounding the buccopharyngeal membrane, which lies at the floor of this depression. This surface depression lies between the maxillary and mandibular components of the first pharyngeal arch.
  • stenosis - abnormal a narrowing of the tube lumen, the abnormality naming is based upon the anatomical location.
  • superior mesenteric artery - main blood supply to the Midgut.
  • viscera - the internal organs in the main cavities of the body, especially those in the abdomen, for example the Template:Intestines.
  • visceral peritoneum - covers the external surfaces of the intestinal tract and organs within the peritoneum. The other component (parietal peritoneum) lines the abdominal and pelvic cavity walls.
  • yolk sac - An extraembryonic membrane which is endoderm origin and covered with extraembryonic mesoderm. Yolk sac lies outside the embryo connected initially by a yolk stalk to the midgut with which it is continuous with. The endodermal lining is continuous with the endoderm of the gastrointestinal tract. The extra-embryonic mesoderm differentiates to form both blood and blood vessels of the vitelline system. In reptiles and birds, the yolk sac has a function associated with nutrition. In mammals the yolk sac acts as a source of primordial germ cells and blood cells. Note that in early development (week 2) a structure called the "primitive yolk sac" forms from hypoblast, this is an entirely different structure.
  • yolk stalk - (vitelline duct, omphalomesenteric duct, Latin, vitellus = yolk of an egg) The endodermal connection between the midgut and the yolk sac. See vitelline duct.


Watch the video: Frog embryo development (June 2022).