Book - Aids to Embryology (1948) 4
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Baxter JS. Aids to Embryology. (1948) 4th Edition, Bailliere, Tindall And Cox, London.
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Chapter IV Implantation and Placentation
The Implantation of the Ovum
The problem of the mechanism of implantation of the human ovum has presented numerous difficulties in the past, because there were described only a relatively small number of good specimens to illustrate the condition. The investigations of Wislocki and Streeter (1938) gave, for the first time, a connected account of implantation in a primate, and the technical methods devised for their study stimulated other workers, particularly Hertig and Rock to search for earlier stages in human development than had hitherto been known. At the present time our knowledge of the process of implantation in the human has been greatly extended by the recovery of normal human ova as young as seven and a half days conceptional age. The following account of implantation in the human is based on the work of Hertig and Rock (1944).
The human ovum is fertilized at the outer end of the uterine tube, and while segmentation takes place, the ovum passes to the uterine cavity, its journey occupying probably three days. Its transport is accomplished partly by the downward streaming flow of fluid directed by the ciliary action of the tubal epithelium and partly by peristaltic contractions of the tubal musculature. The ovum arrives in the uterine cavity at the morula stage. It was formerly thought that the human ovum remained free in the uterine cavity for the next six days (Teacher, 1926) being nourished by the secretion of the uterine glands (â€˜ uterine milk â€™). It is now known that the human ovum starts to implant about the seventh day since the youngest human embryo thus far studied (seven and a half days), is already superficially attached to the endometrium. It is then in the blastocyst stage ; the zona pellucida has disappeared. The embryonic pole of the trophoblast comes in contact with the epithelium, usually on the anterior or posterior wall of the uterus, and the epithelial cells are broken down by the trophoblastic cell secretion. That part of the trophoblast in contact becomes greatly thickened and shows two kinds of cells : ( a ) peripheral syn cytiotrophoblast cells, that is, a layer where cell boundaries are not distinct, which actively erodes and penetrates the endometrium ; and ( b ) cytotrophoblast cells forming a layer next to the cavity of the blastocyst. The syncytiotrophoblast cells proliferate rapidly, actually erode and digest the endometrial stroma, and soon they form a*thick zone of anastomosing strands with spaces or lacunae between them. The lacunae contain broken down endometrial cells and some maternal blood derived from eroded uterine vessels. While these events take place the developing blastocyst sinks into a cavity in the substance of the endometrium, the implantation cavity. The site of entry into the endometrium becomes obliterated, first by a plug of fibrinous material and later, by reepithelialization from the surrounding intact uterine cells. This mode of implantation is termed interstitial.
Continued activity of the cells of the syncytiotrophoblast results in further destruction of maternal tissue and opening up of blood vessels so that the lacunar spaces become progressively filled with maternal blood. A few days after implantation commences there is a slow circulation of maternal blood through the lacunae of the trbphoblast. Before this time the developing embryo depends for its nutrition upon the absorption of broken down endometrial stroma cells. Certain of these, termed decidual cells, contain much glycogen and fat and are prominent around the implantation cavity. This type of nutrition is called embryo trophic. When, however, maternal blood flows through the syncytial lacunae of the implantation cavity a haemotrophic form of nutrition for the embryo is established.
When the ovum has become implanted in the endometrium the destructive powers of the syncytiotroph oblast gradually diminish and beyond the zone of destruction there is reaction on the part of the maternal tissues to hinder the erosive activities of the growing blastocyst.
In the normal course of events the ovum becomes embedded near the fundus of the uterus, but in certain cases it wanders and implants itself in the lower part of the uterus near the cervix. When later the placenta is formed, it will partly or completely cover the internal os of the uterus, and the important clinical condition of placenta praevia results. Some times the ovum may be retarded in its journey towards the uterus and will attempt to embed itself in the tubal mucosa. The embryo develops for a short time but the condition usually terminates by the rupture of the tube, a grave surgical emergency.
During the process of implantation the developing embryo has come to lie in a little cavity in the endometrium which, altered somewhat in character, is now known as the decidua. Different names are applied to various regions of this : thus the decidua immediately deep to the implanted ovum is the decidua basalis, that covering the developing embryo is the decidua capsularis and the remainder which lines the uterine cavity is the decidua parietalis.
Formation of Chorionic Villi
The chorion may be defined as the trophoblast along with its somatopleuric lining of extra-embryonic mesoderm. It is through this simple type of chorion that nourishment passes by diffusion in the early stages of implantation, but, with increasing differentiation and the formation of an early embryonic circulation, the chorion undergoes structural modifications leading eventually to the formation of the placenta. The strands of syncytiotrophoblast which form the boundaries of the lacunae form the primary chorionic villi. These soon show a core of cytotrophoblast and commence to branch. Strands of extra-embryonic mesoderm appear within the cytotrophoblast of the villi and these shortly become continuous with the layer of extra-embryonic mesoderm forming the deeper part of the chorion. It is disputed whether the mesodermal core of the villus arises in situ by delamination from the cytotrophoblast or whether it actively invades the solid villus from the chorionic aspect.
About the end of the third week of development (twenty to twenty-one days) blood vessels differentiate in situ in the mesodermal cores â€¢ of the villi (Hertig x 935) an d become connected with other vessels which are laid down in the chorionic mesoderm. These in turn become linked up with the primitive intraembryonic circulation by vessels (allantoic, see p. 35 ) which run to the embryo in the body stalk. These villi become more complex by branching. Some of them pass right through the peripheral layer of syncytiotrophoblast which bounds the now confluent lacunae, and are attached to the decidua. These are termed anchoring villi, but the majority of the villi hang free in a blood-filled space, the intervillous space, in which maternal blood slowly circulates. There is no communication between this maternal blood and the embryonic blood cells in the vessels of the villi. The barrier of tissue between the two circulations is the placental membrane (see later).
Fig. 8. Longitudinal Section of early Embryo and Membranes.
1, Extra-embryonic coelom ; 2, amniotic cavity ; 3, yolk sac ; 4, body stalk ; 5, allantoic duct ; 6, embryonic disc ;
7, chorionic villi.
The villi are at first scattered over the surface of the entire chorion, and it is then known as the chorion frondosum. During the fourth month the villi disappear from that part of the chorion related to the decidua capsularis, this portion being termed the chorion laeve. The persisting part of the chorion frondosum becomes transformed into the mature discoidal placenta.
In the human subject the placenta is a disc-like, flattened cake with the chorion attached to its margins. The foetal surface is loosely covered by the amnio tic sac and gives attachment to the umbilical cord. Between the amnion and the foetal surface of the placenta are found large vessels, the major branches of the umbilical (allantoic) arteries and vein. From the uterine aspect it can be seen that the placenta is subdivided into a number (15 or more) of areas by septa. These areas are called cotyledons and each corresponds to a bunch of chorionic villi. At birth the placenta is about 20 centimetres in diameter, some 3 centimetres in thickness and weighs about 500 grams. The attachment of the umbilical cord is usually eccentric. Sometimes the attachment of the cord is to the outer margin of the placenta forming the so-called battledore placenta ; in other cases the cord sub-divides before reaching the placenta giving rise to the placenta furcata. The vessels too, may spread out in the investing membranes instead of at the placental area, and form what is known as the velamentous placenta. Accessory lobules of the placenta separated from the main mass are not uncommon (placenta succenturiata) . They probably arise from abnormal persistence of a group of villi of the chorion laeve.
Functions of the Placenta
The placenta functions in a triple capacity ; it isolates the foetal from the maternal organism so that the blood stream of each remains distinct ; it permits the exchange of nutritive substances from the maternal to the foetal circulation, and of waste products from the foetal to the maternal ; hormones probably also pass from the mother to the child ; and the placenta itself elaborates hormones (oestrogens) which pass into the maternal bloodstream, and of which large quantities are found in the urine of pregnant women.
The cellular layers which separate the maternal blood in the intervillous spaces of the placenta and the foetal blood in the chorionic capillaries obviously determine the rate of transmission of substances from the one circulation to the other. These cellular layers are collectively known as the placental membrane. During the latter part of gestation there is a progressive thinning of this placental membrane due to disappearance of the cytotrophoblastic layer of the villi, which are then clothed only by thinned-out syncytium. Without any great increase in size the placenta becomes functionally more efficient. Gellhorn, Flexner and Heilman (1943) have been able to study the transfer of radio-active sodium across the human placenta at various stages and find that the permeability to this substance is increased about seven-fold in the full-term placenta as compared with that of the tenth week of gestation.
The amniotic cavity is commencing its formation in the eight day human ovum (p. 15). In the twelve-day ovum it is seen to lie between the ectodermal cells of the embryonic disc and a layer of flattened epithelial cells overlying this and continuous with its margin. The outer surface becomes covered with extra-embryonic splanchnopleuric mesoderm. Extension of the extra-embryonic coelom separates the amnion from the inner surface of the chorion except at the caudal end of the embryonic disc where a mass, the body stalk, remains (see Fig. 8). When the embryonic head, tail and lateral folds arise, the line of attachment of the amnion is carried ventrally, so that it appears to become constricted and the embryo appears to rise up in the amniotic cavity. The amnion is growing actively and its cavity is increasing in size. Finally, its walls come in contact with the inner aspect of the chorion and the extraembryonic coelom is thus obliterated. With this growth the amnion comes to cover the outer aspect of the body stalk and its contained structures (yolk sac, allantois and allantoic vessels) which is now known as the umbilical cord.
The Amniotic Fluid
The amniotic cavity contains fluid from the time of its first formation. This fluid is derived from the walls of the cavity. In the later stages of pregnancy foetal urine is added to the amniotic fluid. The body stalk elongates considerably in the formation of the umbilical cord so that the embryo floats in the amniotic fluid suspended by it, and is thus mechanically protected against sudden shocks, blows or pressure. The fluid assists in the maintenance of a constant environmental temperature for the foetus and allows of its free movements in utero. During parturition, the fluid helps to dilate the cervix uteri, acting mechanically within the amniotic sac as a fluid wedge.
At full term about 1*5 litres of amniotic fluid are present, but the volume relative to the volume of the foetus has diminished somewhat in the latter onethird of pregnancy. Flexner and Gellhorn (1942) using radioactive sodium and heavy water have shown that, in the guinea-pig, at least, there is constant exchange of these substances between the maternal circulation and the amniotic fluid. The exchange of water is such that a volume equal to that of the amniotic fluid, is exchanged every hour.
While the normal volume of the amniotic fluid is about i*5 litres, an increase (hydramnios) or a decrease (oligamnios) may be found. In the latter event difficulty in parturition may arise.
The Yolk Sac
The formation of the primary yolk sac and its later diminution in size with the development of the extra-embryonic coelom has already been mentioned (p. 19). In the later part of the third week of development areas of angiogenesis are found in the splanchnopleuric mesoderm (extra-embryonic) which clothes the yolk sac, and soon a network of vessels covers its surface. These become organized as paired vitelline arteries and veins which link up with the early intra-embryonic circulation. The yolk sac is concerned during early development with transfer of nutritive material from the trophoblast to the embryo, but this function is only transient until the chorionic villi become vascularized. The further fate of the vitelline vessels is considered on page 114.
When the embryonic disc becomes folded into a cylindrical embryo the large yolk sac naturally becomes constricted at the site of the future umbilicus and a portion of it becomes incorporated in the body of the embryo as the entodermal lining of the primitive gut. This part is connected with the remainder of the yolk sac (definitive yolk sac) by an elongated duct buried in the mesoderm of the body stalk (vitelline duct ; see Fig. 8). These two structures later degenerate and disappear ; the yolk sac may usually be found, however, as a little vesicle situated towards the placental end of the umbilical cord until the fifth or sixth month of pregnancy.
During the third week of development, a diverticulum grows into the mesoderm of the body stalk from the caudal wall of the yolk sac.
This is known as the allanto-enteric diverticulum or allantois. This grows towards, but does not reach, the chorion, since it is a rudimentary structure in the human. Paired arteries and veins of great importance arise alongside it in the mesoderm of the body stalk. These form a connection between the vessels of the chorion and those of the embryo and are termed the umbilical arteries and veins.
The Umbilical Cord. â€” The umbilical cord arises by the elongation of the body stalk. Fig. 8 shows how it is formed. It is formed mainly of mesoderm covered externally by the amnion. In section this mesoderm is seen to be loosely arranged in a jellylike mass (Whartonâ€™s jelly) and has embedded in it the umbilical vessels and the vitelline and allantoic ducts. One of the umbilical veins (the right one) and the vitelline duct soon disappear. The allantoic duct persists as a microscopic structure in the proximal part of the cord until full term. At this time the cord is a long twisted rope-like structure about 55 centimetres in length. It shows well-marked spirals running from left to right ; these spirals are probably caused by the vessels contained within it, growing more rapidly than the matrix of the cord. The length of the cord varies and may be as little as 10 or 15 centimetres, but sometimes it is excessively long and may measure as much as 105 centimetres.
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Cite this page: Hill, M.A. (2020, September 27) Embryology Book - Aids to Embryology (1948) 4. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Aids_to_Embryology_(1948)_4
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