17) Aorta; 18) forming gut tube; 19) coelom; 20) blood vessels in the yolk sac wall.
The serous membrane is formed by extra-embryonic portion of the parietal layer of mesoderm (inside) and the ectoderm (outside). The serous membrane grows around the whole embryo and lines the shell. The function of the serous membrane is a respiratory exchange.
The extra-embryonic splanchopleure (entoderm + visceral layer of mesoderm) covers the yolk mass as the wall of yolk sac. During the body fold formation, the yolk sac becomes separated from the primitive gut. Yolk is transformed into soluble form under influence of enzymes, produced by endodermal cells. The dense net of blood vessels, developing in the wall of the yolk sac, transport nutrients derived from the soluble yolk, to the embryo. So, the function of the yolk sac is assimilation of the yolk.
The allantois arises as a diverticulum of the ventral wall of the gut' caudal part, and its wall is composed of the entoderm (inside) and visceral layer of mesoderm (outside) (Fig.23). The allantois expands, almost filling the extra-embryonic coelom, and its outer wall lines the serous membrane. Blood vessels in the wall of the expending allantois vascularize the overlying serous membrane. Respiratory exchanges occur between the vascularized membranes and the external air via the porous shell. The cavity of allantois serves as a receptacle for foetal urine.
Fig.23. Formation of extraembryonic organs in Birds
EARLY STAGES OF HUMAN EMBRYOGENESIS
Egg cell. In eutherian mammals' ova yolk has been secondary reduced to the oligolecithal form because their prenatal development occurs in a uterus. Thus, the human ovum is classified as the secondary oligolecithal, isolecithal type of ova.
THE FIRST WEEK of LIFE. FERTILIZATION to IMPLANTATION PERIOD
Cleavage. Cleavage of the zygote occurs as the fertilized ovum moves passively toward the uterus. The cleavage type is:
the holoblastic (complete),
unequal (blastomeres are different in size),
asynchronous (blastomeres are divided at different time)
The first cleavage division is meridian with the respect to the animal-vegetative axis and results in two blastomeres of unequal size. The larger cell divides next, giving a three-cell stage. Thereafter the divisions are irregular with the formation of a mulberry-shaped mass of blastomeres - the morula of 16-32 cells (Fig.24). Some of the cells are smaller and light (they'll give rise to trophoblast), another cells are larger and dark (they give rise to embryoblast). The morula is surrounded by the zona pellucida. This occurs during the 3-4 days taken for passage through the uterine tube.
Fig.24.
During next 3-4 days, while free in the uterine cavity, the morula develops into a blastocyst. The cells of the morula are highly active and undergo the changes of shape and relative position. While still within the zona pellucida, fluid-filled spaces appear between the centrally placed cells of the morula; these space soon coalesce to form a blastocystic cavity. The origin of the fluid has been ascribed to endosmosis of uterine fluid, to secretion by the blastomeres, or to central cellular degeneration. The resulting blastocyst has an external wall of primary trophoblast within which is an eccentrically-place inner cell mass, or embryoblast, bulged into the cavity. The primary trophoblast is the first extra-embryonic membrane. Cells of the trophoblast are connected by the tight junctions, within the embryoblast cells are interact by gap junctions (nexuses). Certain formative cells of the embryoblast will form the body of the embryo while all remaining blastocystic cells will form extra-embryonic membranes. The area of contact between the embryoblast and the overlying polar trophoblast defines the embryonic pole of the blastocyst. The blastocyst remains in the lumen of the uterus for 2 or 3 days and comes into contact with the surface of the endometrium, immersed in the secretion of the endometrial glands. The day 5, preimplantation human embryo contains 200 to 250 cells, only 30 to 34 of which are inner cell mass cells.
On the 6 or 7 day after fertilization the zona pellucida disappears, allowing cells of the trophoblast, which have the capacity to invade the mucosa, to come into direct contact with the endometrium. Immediately thereafter, the cells of the trophoblast begin to multiply, ensuring, with the help of the endometrium, the nourishment of the embryo.
Implantation (Fig.25) involves penetration through the uterine epithelium.
Fig.25
Implantation steps are (Fig.26):
Apposition: of blastocyst to endometrial epithelium; attachment occurs at the embryonic pole, probably around 6 days; attachment usually occurs between the surface openings of the uterine glands.
Adherence: via cell adhesion molecules
Formation of Syncytiotrophoblast: Fusion of cytotrophoblast cells results in giant multinucleated symplast that will surround complete embryo
Penetration: syncytiotrophoblast is invasive and works way into uterine tissue ultimately making contact with maternal blood vessels
Decidual Reaction: uterine tissue responds to embryonic invasion by setting up an immunological barrier, the decidua (because the fetus has a different genetic makeup than the mother). The stromal cells enlarge and become pale as glycogen and lipid droplets collect in the cytoplasm.
Fig.26.
THE SECOND WEEK of LIFE. BILAMINAR PERIOD.
This type of interstitial implantation occurs in humans and a few other mammals. The process starts around the 7-th day, and on about the 9-th day the embryo is totally submerged in the endometrium, from which it will receive protection and nourishment during the pregnancy. Within implantation cavity, the blastocyst is in fluid medium consisting of extravasated blood, uterine milk and the cytolytic products which follow the breakdown of surface and glandular epithelia, decidual cells and vascular endothelium. The syncytiotrophoblast engulfs this material - thus, histiotrophic type of the embryo' nutrition occurs during the first 2 weeks.
During implantation, the trophoblast differentiates into 2 layers: the syncytiotrophoblast (or, symplastotrophoblast) and the cytotrophoblast (Fig.26). The cytotrophoblast consists of an irregular layer of mononucleated ovoid cells. The syncytiotrophoblast is a multinucleated external layer (symplast) arises from the fusion of mononucleated cytotrophoblast cells. So, the cytotrophoblast expands mitotically into the syncytiotrophoblast to form primary chorionic villi (composed of trophoblast only). [Cells from these villi can be removed for early genetic testing at some risk to the fetus (chorionic villus sampling)]
The syncytiotrophoblast superficial cytoplasm contains vesicles covered by smooth membranes. The syncytiotrophoblast has active invasive properties and erodes maternal epithelia, stroma and blood vessels to form extra-cytoplasmic cavities. These cavities increase in size and communicate with one another, resulting in a spongy structure. Thus, lacunae are formed, lined with syncytiotrophoblast. The lytic activity of the syncytiotrophoblast causes the rupture of both arterial and venous maternal blood vessels, with overflow of blood into these lacunar spaces. Blood flows from the arterial vessels to the lacunae and from there to the veins. As the conceptus enlarges, histiotrophic nutrition diminishes and haemotrophic nutrition begins.
Gastrulation. In mammals gastrulation occurs by ways of combination of delamination and immigration. The first phase of gastrulation – the delamination coincides in time with the process of implantation and results in formation of the bilaminar disk-shaped mass of the cells – germ disk:
the upper layer of the disk called the epiblast consists of high columnar cells;
the lower layer called the hypoblast consists of cuboidal cells.
The epiblast subsequently gives rise to all 3 germ layers of the embryo.
The hypoblast does not take part in the formation of the embryo body proper and is later displaced to extra-embryonic regions
Fig.27.
As implantation proceeds, Epiblast cells cavitate to form the amniotic vesicle; Hypoblast cells migrate and will form the yolk sac. In humans the yolk sac contains no yolk but is important for the transfer of nutrients between the fetus and mother. The cells forming the amniotic wall are called the amnioblasts or amniotic epithelium. The wall of the yolk sac is called the yolk extra-embryonic endoderm. The epiblast forms the floor of the amniotic cavity, whereas the hypoblast represents the roof of the yolk sac.
The loosely arranged cells called the extra-embryonic mesoderm (mesoblast) differentiate and surround the amnion and the yolk sac stabilizing their wall. Extraembryonic somatic mesoderm lines the cytotrophoblast and covers the amnion. Extraembryonic somatic mesoderm also forms the connecting stalk. Extraembryonic visceral mesoderm covers the yolk sac.
When the extraembryonic mesoderm grows into the primary villi, they become the secondary chorionic villi (Fig.28 ).
Fig.28. Chorionic villi
While these changes have been occurring, the loose fluid-supported reticulum of extra-embryonic mesoderm develops multiple small cavities, which soon coalesce to form an extensive extra-embryonic coelom (the chorionic cavity).
The embryo, its amnion, and the yolk sac are suspended in this cavity by the connecting (amniotic) stalk from the extra-embryonic mesoderm. The connecting stalk attaches the complex of the embryonic disk with its vesicles to the inner surface of the chorionic sac.
The amniotic stalk - is a pathway, prepared in advance, for subsequent growth of the embryo blood vessels toward the chorion. Later the amniotic stalk transforms in the funiculus umbiliculus, or umbilical cord.
The second week of the embryogenesis is often called as “the period of twos”, because
two embryonic layers – the epiblast and hypoblast – make up the embryonic disk;
two vesicles – the amnion and yolk sac – develop;
two layers of the trophoblast – the cytotrophoblast and syncytiotrophoblast – differentiate.
The distinctive feature of the human embryogenesis is the early development of the provisory organs:
the chorion (extra-embryonic mesoderm + trophoblast),
the amnion,
the yolk sac.
THE THIRD WEEK of LIFE. TRILAMINAR and EMBRYONIC SHIELD PERIOD
The second stage of gastrulation – immigration – occurs at the 14-th or 15th day of embryogenesis. In mammalian species, the morphological changes of the second stage of the gastrulation takes place only in the epiblast (hypoblast gives rise to the yolk sac only).
With the development of the extra-embryonic coelom, the extra-embryonic mesoderm has been divided into two zones - somatopleuric in contact with amniogenic cells, or with trophoblast, and splanchnopleuric in contact with entoderm. The early extra-embryonic membranes - amnion, chorion and yolk sac - can now be recognized (Fig.27).
By 14-15 days the second phase of gastrulation – immigration begins. In mammalian species, the morphological changes of the second stage of the gastrulation takes place only in the epiblast (hypoblast gives rise to the extra-embryonic endoderm only).
The primitive streak, the key structure of the second stage, appears in the epiblast. Epiblastic cells from the cranial end of the embryonic disk proliferate and migrate along the disc margins to its caudal end. Cellular currents converge at the disk caudal end, then turn toward the midline, and elongate back to the disk cranial end. The anterior portion of the primitive streak thickens to form the primitive knot (Hensen’s nodule). Concurrently, a narrow primitive groove develops in the primitive streak, which is continuous with a depression in the primitive knot known as the primitive pit. Since the primitive streak appears, it becomes possible to identify the craniocaudal axis of the embryo, its cranial and caudal ends, and its right and left sides.
The primitive streak is a source of the embryonic mesoderm and embryonic endoderm: epiblastic cells move medially towards the primitive streak and enter the primitive groove. They lost their attachment to the rest of the epiblastic cells and migrate inwardly between epiblast and hypoblast.
The early-migrating cells are those that replace hypoblastic cells to become the endoderm. The later-migrating cells begin to spread laterally, ventrally, and cranially to form the mesoderm. As soon as the primitive streak gives rise to the mesoderm and endoderm, the cells that remain in the epiblast are referred to as the embryonic ectoderm. Thus, the gastrulation is completed: the trilaminar embryo is formed.
The subsequent processes - neural tube formation, development of mesenchyme, differentiation of intra-embryonic mesoderm on somites (dermatome, myotome and sclerotome), intermediate mesoderm (nephrogonotome) and lateral plate mesoderm (parietal and visceral layers of splanchnotome, bounding coelom cavity) proceed in the same way as they occur in birds.
The complex of axial organs includes: notochord, neural tube, mesodermal somites.
The notochord is the first to appear. The primitive pit extends into the primitive knot to form the notochordial canal. The cells migrating through this canal give rise to the notochord. The notochordial process looks like a cellular rod extending cranially from the primitive knot between the ectoderm and endoderm. The wing-like mesoderm is on each side of the notochordial process. The notochord forms a midline axis of the embryo. It is the structure around which the vertebral column forms. The notochord degenerates and disappears where it becomes surrounded by the vertebral bodies, but persists as the nucleus pulposus of the intervertebral disks. The notochord also induces the overlying ectoderm to form the neural plate, i.e., the embryonic induction of neurulation.
Neurulation is a process of the neural tube formation. As the notochord develops, the embryonic ectoderm over it (neuroectoderm) thickens to form the neural plate. On about the 18th day, the neural plate invaginates along the central axis to form the neural groove with neural folds on each side of it. By the end of the 3-rd week, the neural folds begin to move and close up with formation of the neural tube. The neural tube is then separated from the surface ectoderm that differentiates into the skin epidermis. The neural tube is the primordium of the CNS (the brain and the spinal cord). As the neural folds close, some ectodermal cells lying along and over each fold are not incorporated in the neural tube. They look like a cell mass between the neural tube and the covering ectoderm constituting the neural crest. The neural crest gives rise to the spinal ganglia and the ganglia of the autonomic nervous system, Schwann cells, the meningeal covering of the brain and the spinal cord (the pia mater and the arachnoid), pigment cells (melanocytes), adrenal gland medulla.
The mesoderm on each side of the notochord and neural tube thickens to form the longitudinal columns of the paraxial mesoderm. Each paraxial mesoderm is continuous laterally with the intermediate mesoderm, which gradually thins laterally to form the lateral mesoderm.
The paraxial mesoderm begins to divide into paired bodies called somites (32-34 in humans). This series of mesodermal tissue blocks is located on each side of the developing neural tube. Somites are subdivided into three regions: the myotome that gives rise to skeletal muscles; the dermotome that gives rise to the skin dermis; the sclerotome, from which bone and cartilaginous tissues arise.
The intermediate mesoderm differentiates into nephrogonotome that gives rise to the kidney and gonads.
Within the lateral mesoderm the space called the coelom appears dividing the lateral mesoderm into two layers: the parietal layer, the somatopleure; the visceral layer, the splanchnopleure. The coelom is then divided into the following body cavities: the pericardial, pleural, and peritoneal ones. The cells of the parietal and visceral layers give rise to the mesothelium lining these cavities. The splanchnopleure takes part in the development of the myocardium, epicardium, and the adrenal gland cortex.
Some mesodermal cells migrate and are disposed among the axial organs; they form the mesenchyme: embryonic connective tissue composing of a loose net of stellate cells embedded in gelatineous intercellular substance. The mesenchyme will give rise to the blood, all types of connective tissue, smooth muscle cells, blood vessels, microglial cells, and endocardium. Ectomesenchyme has similar properties to mesenchyme. The major difference is that ectomesenchyme is usually considered to arise from neural crest cells. Ectomesenchyme plays a critical role in the formation of the hard and soft tissues of the head and neck such as bones, muscles and most importantly the branchial arches. So, mesenchyme can have different embryonic origins (e.g., from lateral mesoderm or neural crest)
The embryonic endoderm develops into the epithelium of the gastrointestinal tract, the liver, the pancreas, the gallblader, the epithelial lining of the lungs and the urinary bladder.
Blood vessel development, i.e. angiogenesis, begins in the extraembryonic mesoderm of the yolk sac. Angiogenesis and hemopoiesis occur concurrently: the primitive blood cells differentiate from mesenchyme inside embryonic vessels. The primitive heart also arises from mesenchymal cells. It appears as paired endothelial channels called the endocardial heart tubes.
The mesenchymal cells inside the chorionic villi differentiate into blood vessels. The vessels then become connected with the embryonic vessels via the vessels in the connecting stalk. By the end of the third week, the embryonic blood begins to circulate through the capillaries of the chorionic villi. The villi containing the blood vessels are called the tertiary villi (Fig.).
Fetal circulation is entirely closed, confined to vessels within the chorionic villi. Maternal blood flow though the placenta is open. “Lakes” of maternal blood fill the intervillous space.
Early in this period, two longitudinal folds – the cranial and caudal folds – and two transverse folds appear. They convert the flat trilaminar embryonic disk into a C-shaped cylindrical embryo, and the body shape is thus established. During folding, the embryo body is separated from the yolk sac, and the primitive gut is formed. By 20-21 days the embryo detaches itself from extra-embryonic parts by the body folds formation (Fig.34-36).
Extra-embryonic membranes and provisory organs
Amnion. In human the amnion is not formed by folding as in birds, but cavitation of the inner cell mass. The bottom of the amniotic vesicle is the epiblast, the amniotic walls are formed by extra-embryonic ectoderm and extra-embryonic mesoderm. The amnion grows with the embryo and fetus development. The amniotic epithelium produces the amniotic fluid that creates watery environment for the fetus and provides its mechanical defence. By 7 weeks the amnion mesoderm comes in contact with the mesoderm of chorion. It exists and functions up to birth as part of the fetal bladder, or the amniochorionic membranes. Besides the amniotic epithelium passes onto the amniotic stalk, and makes contact with the epithelial covering of the embryo. The amnion wall grows and encloses the embryo by thin amniotic membrane (Fig.34,36). By 7 weeks the amnion mesoderm comes in contact with the mesoderm of chorion. Besides the amniotic epithelium passes onto the amniotic stalk, and makes contact with the epithelial covering of the embryo. The amniotic epithelium produces the amniotic fluid. Toward the end of pregnancy the fluid amounts to 1-1,5 liters. It suspends embryo in amniotic fluid that: protects the embryo (foetus) against mechanical injury/shock and adhesions (1), allows for fetal growth and movement (2), and helps to regulate fetal body temperature (3).
Amniotic fluid is produced by dialysis of maternal and fetal blood through blood vessels in the placenta. It contains 200 proteins and cells from embryo used for assessing status of mother & fetus, and in genetic analysis (amniocentesis: e.g., alpha-fetoprotein signals neural tube defects; the detection of sex or genetic defects, for example, Tay Sachs disease). The water content of amniotic fluid turns over every three hours.
Yolk sac. In mammals the yolk sac functions in the early stages of prenatal life. The yolk sac wall is formed by extra-embryonic endoderm and extra-embryonic mesoderm. It arises on the second week and takes part in the embryo nutrition during the period of histiotrophic nutrition. For a short time the yolk sac performs a haematopoeitic function (it gives rise to blood cells and vessels) and after 7-8 weeks it undergoes atrophy and allows development and movement. It remains in umbilical cord as a narrow tube conducting the embryo vessels to the placenta (Fig.36).
Allantois. The allantois grows as a diverticulum from the yolk sac. Its wall consists of extra-embryonic entoderm and visceral layer of mesoderm. Allantois grows into the amniotic stalk. The umbilical blood vessels develop in the wall of the allantois and they vascularize the chorionic part of the human placenta. On the second month allantois is reduced and together with reducing yolk sac it remains in the umbilical cord as a cord of cells (Fig.34, 36).
Chorion. After implantation of the embryo, the endometrium goes through profound changes and is called the decidua. The decidua can be divided into the decidua basalis, situated between the embryo and the myometrium; decidua capsularis, between the embryo and the lumen of the uterus; and the decidua parietalis, the remainder of the decidua (Fig.35). The trophoblast in contact with the decidua capsularis develops only to a slight extent, since its nutrition is deficient. Growth of the trophoblast in the part of the embryo facing the myometrium is ensured by the maternal blood, and its growth is rapid. From this part of the trophoblast, elongated projections, primary villi, are formed (Fig.33, 37b). Their main characteristic is their composition of only cytotrophoblasts and syncytiotrophoblast. During this stage of embryonic development, an extraembryonic mesenchyme appears before the intraembryonic mesenchyme and contributes to the formation of the placenta and the fetal membranes. The extraembryonic mesenchyme and the trophoblast form the chorion. On the side of the decidua basalis, the chorion develops very slightly (smooth chorion); on the side of decidua basalis, the chorion grows extensively and forms the chorion frondosum. The layers of the chorion (beginning at the surface) are the syncytiotrophoblast, cytotrophoblast, and extraembryonic mesenchyme.
When the mesenchyme invades the primary villi, it transforms them into secondary villi (Fig.33, 37c). At the beginning of the 3-th week within the villi, blood vessels are developed gradually and tertiary villi (Fig.37d) are formed. Later the hemocapillaries of the tertiary villi make connection with umbilical (allantoic) vessels from the connecting stalk of the embryo, establishing a circulation and thus allowing exchange of substances and gases between the fetal and maternal blood.
The chorion functions:
provides exchange between the embryo and the maternal organism;
secretes enzymes to erode the endometrium;
provides defense of the embryo, especially immune defense;
produces hormones.
[Human chorionic gonadotropin (hCG) is secreted by the syncytiotrophoblast in maternal blood and is then excreted with maternal urine. hCG maintains the corpus luteum and stimulates it to continue progesterone production. The detection of hCG in the woman’s urine is a simple, rapid, and a early test to detect pregnancy.]
Further, the chorion will take part in the placenta formation.
Placenta. The placenta is a temporary organ found only in eutherian mammals; it is the site of physiologic exchanges between the mother and the fetus. It consists of a fetal part - chorion - and a maternal part - decidua basalis (Fig.37a). The placenta is the only organ composed of cells derived from 2 different individuals.
A. Fetal Part. The fetal part of the placenta, the chorion, has a chorionic plate at the point where the chorionic villi arise. These villi consist of connective tissue core derived from the extraembryonic mesenchyme surrounded by the syncytiotrophoblast and the cytotrophoblast. The syncytiotrophoblast remains until the end of pregnancy, but the cytotrophoblast disappears gradually during the second half. The chorionic villi may be either free or anchored to the decidua basalis. Both villi have the same structure, but the free ones do not reach the decidua, while the anchored chorionic villi become embedded within the decidua basalis. The stem villous divides into many branches - and resembles a tree (Fig.38). The surfaces of the villi are bathed with blood from the lacunae of the basal decidua; it is here that the exchange of substances between fetal and maternal blood occurs.
B. Maternal Part. The maternal part of the placenta - the decidua basalis - supplies arterial blood to and receive venous blood from the lacunae situated between the secondary and terminal villi. The chorionic villi are submerged in maternal blood because the maternal blood vessels are open during implantation, but fetal blood and maternal blood do not mix. The fetal blood remains isolated by the structures that form the placental barrier (Fig.37e).:
the endothelium of the fetal capillaries and the basal lamina of these capillaries;
the mesenchyme in the interior of the villus;
the basal lamina of the trophoblast;
the cytotrophoblast;
the syncytiotrophoblast
The intervillous space of the chorion frondosum soon becomes incompletely divided into cotyledonary bays (Fig.37f). Each bay contains a major villous stem with its branches. As adjacent bays expand, tissue persists between them in form of septa, which project from the basal plate towards the chorionic plate.
At the end of a full-term pregnancy, the placenta has the shape of a disk. The umbilical cord usually arises at the center of the placenta and forms a connection between the fetal and placental circulation.
The placenta is an organ for physiological exchanges between foetus and mother. The placenta is permeable to several substances; it normally transfers oxygen, water, electrolytes, carbohydrates, lipids, proteins, vitamins, hormones, some antibodies, and some drugs from maternal blood to fetal blood. CO2, water, hormones, and residual products of metabolism are transferred from fetal blood to the maternal blood (Tabl. 4) Thus, the placenta carries out respiratory, trophic, excretory, selective-barrier functions and function of immunologic protection.
The placenta also functions as a complex endocrine gland and as a store for certain metabolites. The placenta produces such hormones as chorionic gonadotropin, chorionic thyrotropin, chorionic corticotropin, estrogens, progesterone, human placental lactogen. All these hormones are synthesized by the syncytiotrophoblast.
Although the placental membrane is referred as placental barrier, harmful substances can cross it to affect the developing embryo (Table 5)
Table 5
Harmful Substances that cross the Placental Membrane
|
Poisonous gases |
Carbon monoxide |
|
Infectious agents |
Viruses (HIV, cytomegalovirus, rubella, Coxsackie, variola, varicella, measles, poliomyelitis) Bacteria (tuberculosis, Treponema) Protozoa (Toxoplasma) |
|
Drugs |
Cocaine, alcohol, caffeine, nicotine, warfarin, trimethadione, phenytoin, tetracycline, cancer chemotherapeutic agents, anesthetics, sedatives, analgesics |
|
Immunoglobulins |
Anti-Rh antibodies |
Placentae in mammals may be classified according to general shape, fine structure, intimacy of fusion between extraembryonic membranes and maternal tissue, source of foetal blood and whether maternal tissue is shed with the placenta and membranes. In these terms the human placenta would be described as diskoidal, labyrinthine, haemo-chorial, chorio-allantoic and deciduate.
In epithelio-chorial placenta (in the pig) the chorionic villi make contact with the epithelium of the uterine gland (Fig.). In desmo-chorial placenta (in ruminating animals) the chorionic villi contact with connective tissue of maternal endometrium. In vaso-chorial placenta (in predatory animals) chorionic villi erode the epithelium and connective tissue and contact with the endothelium of maternal blood vessels.
Table 4
|
Exchange across the placenta | |
|
Maternal blood to foetal blood (supply) |
Foetal Blood to maternal blood (excretion) |
|
|
Table 6
Differentiation of the Germ Layers
|
Germ Layer |
Adult Derivatives | |||
|
Ectoderm |
Surface ectoderm |
Epidermis and its derivatives (hairs, nails, sweat and sebaceous glands); Mammary glands Epithelial lining of the oral cavity structures, ameloblasts, salivary glands; Lens of eye; Adenohypophysis; Epithelial lining of external auditory meatus; utricle, semicircular ducts Olfactory placode; Epithelial lining of lower anal canal | ||
|
Neuroectoderm |
Nerve tube |
CNS (all neurons and glial cells); Retina; Pineal gland | ||
|
Nerve crest |
PNS end many other structures (table) | |||
|
Mesoderm |
Paraxial (SOMITES) |
Dermatome |
Skin dermis | |
|
Myotome |
Skeletal muscles of trunk, limbs, head and neck; extraocular muscles; intrinsic muscles of tongue | |||
|
sclerotome |
Vertebrae and ribs; cranial bone | |||
|
Intermediate (Nephrogonotome) |
Kidneys Reproductive system organs | |||
|
Lateral (splanchnotome) |
Serous membranes of body cavities (mesothelium) Carddiogenic mesoderm (splanchnic mesoderm) → heart Suprarenal cortex cells | |||
|
Mesenchyme |
Blood, lymph, connective tissue, smooth muscle cells, blood vessels, endocardium | |||
|
Endoderm |
Epithelial lining of the gastrointestinal tract, the liver, the pancreas, the gall bladder; epithelial lining of the respiratory system (trachea, bronchi, lungs), the thymus stroma, the thyroid and the parathyroid glands endocrine cells; epithelial lining of the urinary bladder, the female urethra and most of the male urethra | |||
DIAGRAM of CONSECUTIVE STAGES of HUMAN EMBRYOGENESIS
1- uterine mucosa; 2- epithelium; 3- connective tissue; 4- trophoblast;
5- embryoblast; 6- amniotic vesicle (ectoderm material); 7- entoderm material;
8- extraembryonic mesoderm; 9- extraembryonic mesenchyme;
10- differentiation of trophoblast; 11- yolk sac; 12-amniotic stalk;
13- chorion cavity; 14- lacunae; 15- symplastotrophoblast; 16- cytotrophoblast;
17- intraembryonic mesoderm; 18- primitive streak; 19- chorion frondosum;
20- smooth chorion; 21- amnion; 22- body fold; 23- yolk sac; 24-embryo body;
25- amniotic membrane; 26- allantois; 27- chorionic villi
Fig.1 Cleavage in mammals
1 – blastomere;
2 – fertilization membrane
Fig.2 Implantation. Differentiation of trophoblast.
1 – trophoblast; 2 – embryoblast; 3 – epithelium of endometrium;
4 – connective tissue of endometrium; 5 – hypoblast; 6 – epiblast;
7 – roof of amniotic vesicle; 8 – amniotic cavity; 9 – blood vessels of endometrium;
10 – symplastotrophoblast; 11 – cytotrophoblast; 12 – mitosis in cytotrophoblast;
13 – extraembryonic mesoderm; 14 – fibrinoid;
15 – lacunae within symplastotrophoblast; 16 – exocoelomic cavity.
Fig.5 Extraembryonic organs in human
A – the third week;
B – the beginning of the forth week
1 – amnion wall;
2 – cardiac primordium;
3 – blood islets; 4 – oral membrane;
5 – cloacal membrane;
6 – chorionic plate;
7 – tertiary chorionic villi;
8 – chorionic blood vessels;
9 – allantois; 10 – primitive foregut;
11 – primitive midgut;
12 – primitive hindgut;
13 – yolk sac cavity;
14 – primitive umbilicus;
15 – amniotic cavity;
16 – extraembryonic cavity
(chorion’ cavity)
Fig.7. Changes in extraembryonic membranes and the embryo’ correlation in
the process of development
1 – amniotic vesicle; 1a – amniotic cavity;
2 – the embryo body;
3 – yolk sac;
4 – extraembryonic coelom (chorion cavity)
5 – primary chorionic villi;
6 – secondary chorionic villi
7 – connective stalk;
8 – tertiary chorionic villi;
9 – allantois;
10 – umbilical cord
Fig.8 HUMAN PLACENTA
A – General plan of placenta structure (arrows point blood circulation in one of lacunae);
B, C, D – consecutive stages of chorionic villi’ formation;
E – structure of placentar barrier in the first trimester of pregnancy;
F – structure of cotyledon
1 – amniotic epithelium; 2 – chorionic plate; 3 – stem villus; 4 – fibrinoid; 5 – reducing yolk sac;
6 – umbilical cord; 7 – septum between cotyledons; 8 – lacuna; 9 – spiral artery;
10 – decidua basalis; 11 – myometrium; 12 - symplastotrophoblast; 13 – cytotrophoblast;
14 – basal lamina of trophoblastic epithelium; 15 – hemocapillar of chorionic villus;
16 – fibroblast; 17 – mesoderm of secondary chorionic villus;
18 – blood vessel of tertiary chorionic villus.
Fig.10 Gastrulation and differentiation of the germ layers in human
A – the germ disk (15 days);
B – stage of the primitive streak;
C – migration of cells of mesoderm and notochord (17-18 days);
D – trilaminar embryo;
E – neurulation and differentiation of mesoderm (20-21 days)
1 – the primitive node; 2 – the primitive streak; 3 – extra-embryonic mesoderm;
4 – the amnion wall; 5 – the yolk sac wall; 6 – epiblast; 7 – hypoblast; 8 - cloacal membrane; 9 – prechordal plate; 10 epidermal ectoderm; 11 – neuroectoderm;
12 – intraembryonal mesoderm; 13 – entoderm; 14 – notochord; 15 – neural tube’ primordium; 16 – somite; 17 – intermediate mesoderm (nephrotomes);
18 – parietal mesoderm; 19 – visceral mesoderm
Table 7
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Derivatives of the nerve crest
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Sensory neurons of spinal ganglia and ganglia of cranial nerves; Neurons of the sympathetic and parasympathetic nerve system; Schwann cells and cells-satellites of ganglia; Neurohypophysis (posterior pituitary gland); Pia mater and arachnoid membrane Melanocytes; Cells of the carotoid body; Calcitonin-producing cells (parafollicular cells) of the thyroid gland; Chromaffine cells (the adrenal gland) Cartilage, bones, muscles and connective tissue of the face; Maxillary and mandibular processes; Sublingual arches and the 3-d throat arch; Odontoblasts; Corneal endothelium Dilator and sphincter pupillae muscles Ciliary muscle |