Book - Uterine and tubal gestation (1903) 1-4
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Bandler SW. Uterine and tubal gestation. (1903) William Wood & Company, New York.
|zygote, morula, and blastocyst stages with implantation occurring in week 2.|
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Part I. The Essentials of Uterine Gestation
Chapter IV. The Early Development of the Human Ovum
The early cell division taking place in an ovum is a more or less even increase, which later gives way to a development along specific lines in certain and well-distinguished areas, so that larger or smaller cell complexes pass through varying phases leading ultimately to the production of embryonal and extraembryonal structures. Preliminary to this stage the cells' of the ovum, which lie loosely together in the earliest period of cell division, become more closely united, so that an epithelium is formed. The epithelial membranes resulting from this early cell division are called germinal layers (Fig. 17). It is in them that the various specific areas of development subsequently appear. On the two germinal layers (ectoderm and entoderm) lining the germinal vesicle or ovum and on the subsequent complexes of these layers, two surfaces are to be distinguished: (1) An inner surface or base facing the cavity of the ovum, and (2) an external or free surface. The various cells of a germinal layer may increase or advance in either direction. If the cells advance from the basal side this constitutes an invagination; if from the free surface, an outgrowth or papulation.
Fig. 17. — Scheme of ovum at stage of simple chorion ectoderm and entoderm. Fig. 17a. — A, amnion. Scheme of ovum with formation of amnion from the ectoderm.
The germinal vesicle increases in size through an increase of fluid in its centre, and is composed of two original boundary layers lying together: an outer, the chorion ectoderm, and an inner, lining the cavity, the entoderm. This combination is called the germinal vesicle and probably represents the stage immediately after embedding. At a certain point is an oval area, the area embryonalis, in which appears an additional very small cavity, the amniotic cavity, in the wall of which the embryo first appears. The exact mode of this production cannot be positively stated, but it follows one or two closely allied forms. It may be formed through an invagination of the ectoderm (Fig. 17a), the folds in the area of invagination lying close together and uniting quickly, thus leaving within the ectoderm a small enclosed cavity. The very smallest amniotic cavities (flying-dog, guinea-pig) are formed by a solid group of cells passing out from the ectoderm and finally becoming separated from it. In the centre of this solid group the subsequent amniotic cavity develops (Fig. 175).
Fig. 17b. — Ovum with ectodermal amnion separated from the chorion ectoderm. A, amnion.
Fig. 17c. — Ovum with ectoderm, entoderm, amnion, germinal plate, but no mesoderm. (Schematic.)
The early amniotic cavity of the human ovum is extremely small. In no animals where an amniotic cavity is formed through the production of reflected and approaching germinal folds does such a small amniotic cavity result. Therefore the amniotic cavity of the human ovum follows either of the two aforementioned processes. By either of these two methods, then, an amniotic cavity passes out from the ectoderm and pushes the entoderm toward the central space of the germinal vesicle. (It is possible that the amnion develops in the embryonal sphere of Fig. 13. We know little about the origin of the entodermal space, and it is described as the lining of the germinal vesicle, although it is possible that it develops in a manner resembling the amniotic cavity.) In the human ovum, in all probability, the early stage consists of small ectodermal amniotic cavity sunk into a depression in the closed yolk vesicle lined with entoderm, these being surrounded by the single layered ectoblast vesicle, the later chorionic ectoderm (Fig. 17c). At this stage no mesoderm is present.
On a portion of the ectodermal lining of the amniotic cavity appears an area, the primitive streak. The cells about it become differentiated into the ectodermal germinal plate (the future embryo). In the primitive streak is a furrow — the primitive furrow — which lies, therefore, in the middle of the later germinal plate. By a growth of cells of the germinal plate on either side and over the primitive furrow (primitive medullary plates) a canal results — the medullary canal (Fig. 30a).
Fig. 18. — Ovum showing the beginning of a mesodermal growth at the caudal end of the germinal plate. (Schematic.)
Fig. 18a. — Ovum with completed growth of mesoderm. (Schematic.)
There begins, with the appearance of the amniotic cavity and the differentiation of a portion of its lining into the ectodermal germinal plate, the development of mesoderm at the caudal end of the latter (Fig. 18). This growth of mesoderm begins close to the inner surface of the chorion ectoderm and forms (1) a thick mesodermal mass, the caudal knot of the primitive streak, extending about the caudal side of the amnion up to the chorion (Fig. 18) ; (2) thin mesodermal layers which grow between the chorion ectoderm and the ectodermal amniotic cavity, and the entodermal yolk vesicle.
EARLY DEVELOPMENT OP THE HUMAN OVUM.
In the schematic drawings the entodermal yolk vesicle has been drawn large to make the relations clear. Whatever its size may have been at the beginning, and whether it originates as a very small vesicle in an embryonal nodule or not, it is certain that on growth of the mesoderm the amniotic and yolk cavities are found in the embryonal area of the ovum and take up a relatively small portion of it.
Beginning mesodermal slit
Fig. 19. — A part of the periphery of the ovum of Peters, showing actual conditions pictured schematically in Fig. 18a. Yolk sac is very small compared with Fig. 18a,, and naturally the amount of mesoderm below the germinal plate and the yolk sac is much larger than in Fig. 18a. Fig. 19 is the E.F. of Fig. 20.
The ectoderm of the ovum develops hugely, forming the chorionic ectodermal cover or trophoolast.
This stage is well represented in the embryonal formation of Peters (Fig. 19). The ovum is surrounded by chorion ectoblast formation. Cells are more plentiful in the mesoderm near the chorion, but the mesoderm is thick only in the region of the embryonal formation. The internal part of the oval cavity is poor in cells, between which is a weakly staining, fibred, granular
44 EARLY DEVELOPMENT OP THE HUMAN OVUM.
mass. The embryonal formation consists of two small epithelial cavities, the ectodermal amniotic and the entoderm yolk vesicle surrounded by mesoderm and embedded in a thickening of the mesoblast near the chorion. The amniotic cavity is entirely closed. Its wall is differentiated into the very thin amniotic membrane and into the germinal plate, composed of high cylindrical cells. Between these and the entoderm cells of the yolk, the future umbilical vesicle, is a layer of mesoderm cells, which are separated from the ectoblast area of the amnion by a membrana prima which always develops at the border between ectoderm and mesoderm.
EARLY DEVELOPMENT OP THE HUMAN OVUM.
DIVISION INTO EMBRYONAL AND EXTRA-EMBRYONAL AREAS.
Through the appearance of a slit in the entire circumference of the mesoderm beginning at the caudal end of the germinal plate, but not dividing the dense mass of mesoderm at the caudal end, the amnion with its germinal plate and the future umbilical vesicle are separated from the chorion ectoderm up to the dense mass of mesoderm, which then constitutes the point of union between the embryonal formation and the chorion ectoderm. This
Extra embryonal area
Fig. 19a. — Ovum, Fig. 18a, after the development of the mesodermal periembryonal slit. (Schematic.) The adherent band of mesoderm connects the embryonal and extra-embryonal divisions.
point of union contains the adherent band (future abdominal pedicle) and the caudal knot of the primitive streak (Fig. 19a).
The amnion and embryo develop from a small area in the germinal covering, and with the umbilical vesicle are separated from the extra-embryonal area of the ovum, with which they are connected through the adherent band of mesoderm alone. The embryo develops from a portion of the ectodermal lining of the amnion.
The extra-embryonal area of the ovum forms the troplioblast, villi, the chorionic membrane, and the placenta.
OHAPTEE V. THE TBOPHOBLAST IN THE OVA OP ANIMALS.
THE EARLIEST DEVELOPMENT OF THE ECTODERMAL EXTRA-EMBRYONAL AREA OF ANIMAL OVA.
In discussing the histology of gestation it is important to note that in this field, as well as in that of embryology, many of the most important and valuable points have been learned through the study of like processes in animals. The results obtained in investigations in the latter have been applied more or less closely to the various developmental changes in the human being. Though ofttimes erroneously applied, they have been the source through which many vexed questions have been settled. More recent investigations, however, prove that it is not so much the erroneous applications of the results gained in the study of animals as it is errors made in these observations which have so long delayed a satisfactory conclusion concerning the various processes involved in human placentation. It is necessary, for that reason, to consider first the more recent studies concerning animal placentation, as reviewed by Strahl.
Marchand examined the placentas of rabbits gravid from eight to sixteen days. In the earliest specimens he finds on the ovum a growth of ectoderm elements consisting of two layers: (1) a deeper layer of separated cells and (2) a superficial plasmodial. Since the latter is still covered in part by the zona, it is necessarily of fetal origin. In addition he observes transitions from the cell layer of the ectoderm into the plasmodial. The deeper cells become larger and are arranged in groups. Their cell boundaries disappear, and, through a union of these cells, a plasmodial layer is formed, covering the deeper ectoderm, but, as a rule, distinctly outlined from it. This ectodermal Plasmodium unites with the syncytially-changed uterine epithelium, forming the first connection between the ovum and the uterine wall. In the placenta of nine to ten days Marchand finds blood (1) in the maternal vessels possessing an endothelial lining and (2) in ectodermal spaces in the cell layer. Into these spaces, which possess no endothelium, the blood from the maternal vessels enters.
THE TROPHOBLAST IN THE OVA OP ANIMALS. 47
In the placenta of eleven days few of these unlined ectodermal spaces containing blood remain. Most of them are now lined with a continuous layer of elements rich in protoplasm, which Marchand considers originate from the maternal vessel endothelium ( ?), which has undergone a "syncytial change."
Maximow, in his investigations on rabbit placentae, denies, in contrast to Marchand, the existence of a fetal ectodermal Plasmodium as early as the eighth day. He believes that, when the ovum attaches itself to the uterine wall, the ectoderm consists of only separated cells, which come in contact with the epithelium of the uterus and cause it to degenerate. When the ectoderm, after disappearance of the uterine epithelium, reaches the maternal vessels, the glycogen-containing cells of the uterine connective tissue change to large polynuclear structures. In these cells he finds elements resembling red blood cells in form and color. Only later, when the ectoderm enters into the decidua, does it divide into two layers: (1) one with cell boundaries, the cytoblast, and (2) one without cell boundaries, the plasmodiblast. The latter is found only where the villi come in contact with the glycogen-containing cells, at the ivalls of the maternal vessels. It is not found where the villi grow into the connective tissue between the vessels. From the tenth day on, spaces filled with maternal blood are found in the fetal Plasmodium and there develops an ecto-placenta, into ivhose ectodermal villous formations the mesodermal elements bringing the allantoic vessels enter.
Opitz finds in the rabbit a double ectodermal layer during the process of attachment of the ovum in the uterine wall: (1) an internal layer formed of separated cells, and (2) an external plasmodial layer. The latter causes the uterine syncytium to disappear and aids the attachment of the ovum to the uterine wall freed of epithelium. In the Plasmodium spaces occur into which maternal blood empties from the eroded maternal vessels. From the ovum, ectodermal cell groups enter into this ectodermal Plasmodium, are vascularized, and later become also plasmodial. Those syncytial cells which Marchand considers as resulting from maternal endothelium, Maximow, however, views as ectodermal Plasmodium when found in the fetal portion of the vessels, but considers them in the maternal portion to be endothelial. Opitz considers these cells to be entirely of ectodermal origin. It is to be noted that Maximow and Opitz mention the destruction of the uterine epithelium. Later fetal mesoderm with the allantoic
48 THE TROPHOBLAST IN THE OVA OP ANIMALS.
vessels enters into the placenta and on its external surface the remnants of the ectoderm are found as flat covering cells. The epithelium of the uterus plays no part in the development of the placenta, but forms a syncytium which is absorbed by the ectoderm cells.
In Tarsius, Hubrecht finds that the uterine wall, through a change of the connective tissue, forms a trophospongia, betiveen ivhich the glands degenerate. On the growth of the embryonal ectoderm or trophoblast, a mixture of trophoblast and trophospongia occurs, with a decided increase in the volume of this early placental formation. Then the development of the mesodermal villous elements takes place, and the mesoderm villi surround themselves with a trophoblast covering. Between the trophoblast cells lacunae form, into which maternal blood passes. Finally, the trophospongia becomes a thin layer which represents the boundary toward the deeper maternal tissue.
In Tupaja, when the ovum rests upon the uterine wall its ectoderm causes the uterine epithelium to disappear. The trophoblast grows decidedly, with the formation of giant cells, and occasionally forms a combined layer with the uterine epithelium in which maternal and embryonal nuclei lie in a common Plasmodium. The maternal portions degenerate, but the ectodermal tissue is divided into ( 1 ) a deeper layer, the cytotrophoblast, and (2) a superficial syncytial layer, the plasmoditrophoblast, which constitutes only a temporary differentiation. In the uterine wall, in the meantime, the connective-tissue trophospongia is formed; the border between ectoderm and maternal connective tissue disappears, for these tissues infiltrate each other. In this resulting union the trophospongia is overcome by the trophoblast and the maternal blood passes from the maternal capillaries into the embryonal trophoblast spaces. The mesodermal fetal cells, then enter with the allantoic vessels and receive on their external surface a covering of trophoblast.
Although various investigations have shown that the variations in the development of the placenta in different animals are quite unexpected, and that among the individual mammalia, even when closely related, decided differences are found, yet the trend is more toward the view that, as a rule, the syncytium originates from fetal ectoderm. According to Strahl and Selenka, in quite a group of placenta? the uterine epithelium plays no unimportant role. On the other hand, Frankel, as a result of extensive investigations, comes to the conclusion that the
THE TROPHOBLAST IN THE OVA OF ANIMALS. 49
higher the organization of the placenta and the more firm the connection between maternal and fetal tissues, so much the less is the maternal epithelium preserved.
He finds that in the rodents and in the insectivorae, whose placentae, of all the animals he examined, stand nearest to the human placenta, the uterine epithelium ceases at the border of the placenta. Only in the pig does it remain. In the cow and sheep the epithelium in the cotyledo shows a tendency to degeneration. In the cat, rabbit, squirrel, guinea-pig, rat, mouse, and mole the uterine epithelium takes no part in the formation of the placenta. A so-called syncytial formation, however, he finds to occur in the most varying tissues. The higher in the animal plane the animal stands, the more does the chorionic epithelium groiv into the maternal connective tissue robbed of its epithelium.
Opitz finds that in the guinea-pig the foundation for the placenta is the fetal ectodermal plasmodium vascularized by maternal vessels. The placentae of the rabbit, the guinea-pig, and the human being agree in that earlier or later the ectodermal surface of the ovum comes in contact with the connective tissue of the uterine mucosa. Finally, between maternal blood and the fetal vessels found in the mesoderm of the villi only one layer of the ectodermal syncytium is left. The apparently great differences are to be explained by the fact that in the rabbit and guinea-pig the projections of ectoderm and mesoderm (the villi) come into union with each other, while in the human being they are always separated.
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Reference: Bandler SW. Uterine and tubal gestation. (1903) William Wood & Company, New York.
Cite this page: Hill, M.A. (2020, July 14) Embryology Book - Uterine and tubal gestation (1903) 1-4. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Uterine_and_tubal_gestation_(1903)_1-4
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