Book - Human Embryology and Morphology 8
|Embryology - 21 Oct 2020 Expand to Translate|
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Chapter VIII. The Manner in which a Connection is Established between the Foetus and Uterus
Implantation of the Ovum
The blastodermic vesicle is represented in Fig. 69. In its course down the Fallopian tube the somatopleure, which forms its outer wall, becomes differentiated into four areas, shown diagram matically in Fig. 74, by a species of folding, already described and shown in transverse section in Fig. 72 and in longitudinal section in Fig. 75. The embryo becomes enclosed in a double capsule of somatopleure — the outer the prechorion and the inner the amnion. But, as is shown in Fig. 75, the amnion in man is peculiar in that it forms practically no tail fold; the head fold grows backwards like a hood until it comes in contact with the caudal pole of the embryo. When the uterus is reached a third or decidual coat is added by the mucous membrane of the uterus (Fig. 71).
Fig. 74. Showing what becomes of the Somatopleure of the Blastodermic Vesicle.
The serotinal part of the decidua marks the place where the placenta will be formed. The implantation of the ovum in the decidua is in the posterior wall of the uterus in over 60% of cases ; hence the placenta is developed there. It sometimes happens that implantation occurs near the internal os of the uterus, and in such cases the placenta is developed over the os (placenta praevia), a form liable to give rise to a sudden haemorrhage from the uterus during pregnancy.
When the amnion and prechorion are formed from the human ovum that part of the somatopleure lying behind the neural groove, and in which the primitive streak appeared (see Figs. 68, 74 and 75), joins the embryo to the chorion. His named it the body-stalk. Through it the allantois — a hollow protrusion from the hind gut (Fig. 75) — grows out to the pre-chorion Afterwards the body-stalk, with the containd stalk of the allantois, becomes the umbilical cord. The developmental changes which occur in the body-stalk are of the greatest practical importance (p. 99).
Fig. 75. Showing the growth backwards of the Somatopleuric Head Fold in the human ovum of 15 days to form the Amnion and Prechorion. The "bodystalk " part of the Somatopleure keeps the embryo attached to the Prechorion. (After Graf Spee.)
The Chorion and Allantois
At this early stage a diverticulum has arisen from the hind gut below the caudal part of the embryo (Fig. 75). This ventral evagination of the hind gut, which grows along the body-stalk, carrying the splanchnopleure with it, forms the allantois. The allantoic bud, which is hollow only at its basal part, spreads out on the inner surface of the prechorion, forming a lining to it. The prechorion or false chorion, with the addition of the allantois, forms the chorion, or true chorion. The posterior ends of the two primitive dorsal aortae, which terminate until now on the yolk sac, are carried out with the allantoic bud and distributed within the villi of the chorion. The posterior ends of the two dorsal aortae thus become the hypo-gastric and umbilical arteries (Young and Eobinson). The veins which return the blood become the umbilical veins. At first there are two of them, and they return the allantoic blood direct to the sinus venosus, afterwards to the ducts of Cuvier, and thus to the heart (Fig. 190, p. 232). In this way the foetal circulation is set up. The heart pumps the blood into the chorionic villi ; these are imbedded in the maternal decidual covering; the blood is returned by the umbilical veins (Fig. 76). At this stage, and indeed until the end of the 6th week, the membranes, with the embryo within them, can easily be detached from the decidual nest in the uterus, and then appear as a villous vesicle, the chorionic vesicle, about the size of a pigeon's egg. The allantois never forms a free vesicle in man nor in the higher primates. It occurs as a vesicle in other mammals, birds and reptiles, in which it has a double function : 1st, to form a respiratory medium, as is also the case in man ; 2nd, to form a reeeptaculum for the secretion of the kidney.
Formation of the Placenta
The condition of the membranes in the 3rd month (Fig. 76) differs from that of the 1st month (Fig. 75) by the formation of the placenta. In the first month the chorion is uniformly covered by shaggy villi, which project into the decidua and draw sustenance for the embryo therefrom. This is the permanent condition in low primates (Lemurs). In man the chorionic villi which project within the decidua serotina hypertrophy, while those within the decidua reflexa atrophy, and in this way the discoidal placenta of man is formed. In lower primates (Monkeys) there are two discs (bi-discoidal), and this form occasionally occurs in man.
Fig. 76. Showing the arrangement of the Amnion, Chorion, and Decidua in the 3rd month and the Formation of the Placenta.
The elements entering into the formation of the placenta are diagrammatically shown in Fig. 77.
They are :
- The decidua serotina, formed from the mucous membrane of the uterus.
- The prechorion, from the somatopleure.
- The allantois from the splanchnopleure.
- The amnion from the somatopleure.
The amnion is a thin, transparent membrane easily stripped off from the inner surface of the placenta. The serotinal area of the true chorion has become hypertrophied ; each villus branches again and again until it resembles, in the complexity of its ramifications, a miniature beech tree. In these villi the umbilical arteries of the foetus break up into capillaries, which in turn end in the venules of the foetal umbilical vein. The villi project within great blood spaces formed in the decidua serotina (Fig. 76). The ovarian and uterine arteries end in these blood sinuses, and the ovarian and uterine veins begin in them.
The blood sinuses are formed :
- By the distension of uterine venules in the decidua serotina.
- Possibly by the dilatation of uterine glands.
Fig. 77. Diagrammatic section to show the Elements which enter into the formation of the Placenta.
At full time all the membranes of blastodermic origin come away in the after-birth; also the decidua, except a thin, deep layer next the uterine muscle, which contains the deepest parts of the uterine glands. From this layer the mucous membrane of the uterus is regenerated (Fig. 77).
Formation of the Umbilical Cord and Umbilicus
The body-stalk, the basis of the cord, is that piece of the embryonic somatopleure situated between the chorion and neural groove (Figs. 74 and 75). The outgrowth of the allantois into the bodystalk adds to it the elements of the splanchnopleure. A transverse section of the body-stalk shows within it the same elementary structures as are seen in a transverse section of the embryo. The cord must be regarded as a real part of the embryo. The umbilicus, which marks the point of attachment of the cord, is situated in the adult on the ventral surface, but before it was thrust into this position by the development of the caudal and perineal regions of the body, it represented the posterior termination of the embryo (Figs. 74 and 75). The amniotic somatopleure rises from the sides of the body-stalk and encloses it just as it rises from, and encloses, the embryo (Fig. 72), and, as is shown in the next paragraph, the lateral folds of the somatopleure unite in the ventral line of the body-stalk as in the ventral line of the belly (Fig. 78).
Fig. 78. Diagrammatic section showing the structures which go to form the Umbilical Cord. (After Wilhelm His (1831-1904))
Transverse Section of the Umbilical Cord (Fig. 78). A section of the cord shows :
- Two umbilical arteries (continuations of the primitive dorsal aortae).
- One umbilical vein, formed by the fusion of the two original veins.
- The cavity of the allantois formed from the hind gut. Within the cord its lumen becomes obliterated early.
- The vitelline duct, the stalk of the yolk sac, communicating with the intestine and yolk sac. It becomes obliterated in the 3rd month.
- Wharton's jelly, a primitive embryonic tissue composed of branching cells in a mucoid matrix.
- A covering of epiblast. The amnion is attached round the placental insertion of the cord.
Up to the end of the 4th week the embryo is closely united to the chorion by the short body-stalk (Fig. 75), but in the second month the cord elongates, and in the third month it measures about 12 cm. and about 40 cm. (16 inches) at birth.
Formation of the Umbilicus
In the adult the umbilicus marks the point where the umbilical cord was attached. It is the point at which the lateral somatopleuric plates fused and thus shut off the intra-embryonic coelom (peritoneal, pleural and pericardiac cavities) from the extra embryonic coelom which is enclosed between the amnion and chorion (Figs. 70 and 72). If these diagrams be examined the yolk sac will be seen to hang free within the great primitive coelom through the umbilicus which, at this stage, is nearly as extensive as the ventral surface of the embryo (Fig. 75). The embryo during the 3rd week is only from 3 to 5 mm. long (?? inch) ; while it grows the umbilicus increases at a slower rate. It thus comes about that when the embryo is an inch (25 mm.) long, the primitive umbilicus, not having kept pace with the body growth, remains comparatively small. The somatic layers gradually contract round the yolk sac and allantoic stalk, the umbilicus and umbilical cord being thus formed. At first the belly end of the umbilical cord is funnelshaped and a coil of intestine hangs within the umbilicus until the 3rd month of foetal life. After the third month the coil of intestine, probably owing to an increase in the capacity of the abdomen, retreats within the umbilicus, which then contracts round the umbilical vessels. It occasionally happens that the process of development at the umbilicus is arrested at the second month and the child is born with some of its abdominal contents within the umbilicus and upper part of the cord. This condition is known as congenital umbilical hernia. Sometimes, too, the stalk of the yolk sac — the vitello-intestinal canal — persists, giving rise to an umbilical faecal fistula. The cavity of the allantois may also remain open, leading to an umbilical urinary fistula. Fistulae at the umbilicus are, however, comparatively rare.
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Human Embryology and Morphology (1902): Development or the Face | The Nasal Cavities and Olfactory Structures | Development of the Pharynx and Neck | Development of the Organ of Hearing | Development and Morphology of the Teeth | The Skin and its Appendages | The Development of the Ovum of the Foetus from the Ovum of the Mother | The Manner in which a Connection is Established between the Foetus and Uterus | The Uro-genital System | Formation of the Pubo-femoral Region, Pelvic Floor and Fascia | The Spinal Column and Back | The Segmentation of the Body | The Cranium | Development of the Structures concerned in the Sense of Sight | The Brain and Spinal Cord | Development of the Circulatory System | The Respiratory System | The Organs of Digestion | The Body Wall, Ribs, and Sternum | The Limbs | Figures | Embryology History
Cite this page: Hill, M.A. (2020, October 21) Embryology Book - Human Embryology and Morphology 8. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Human_Embryology_and_Morphology_8
- © Dr Mark Hill 2020, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G