Book - Uterine and tubal gestation (1903) 1-10
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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 X. The Blood-Forming Function of the Trophoblast
In another particular, as I believe can be shown, the resemblance between the processes of development in the human being; and those in animals is carried out still further — namely, the formation of blood by the fetal placenta.
In sections through the umbilical vesicle of Tarsius and Tupaja, Hubrecht finds, polynuclear cells which he considers to be mother blood cells. Their nuclei result through fragmentation of a large nucleus. The individual nuclei within the mother cell then become surrounded with a special circle of plasma. When the original mantle of the protoplasm of the large cell is lost, the included cells are freed, forming embryonal nucleated red blood cells. During their circulation in the embryonal fetus the nuclear membrane disappears and the nucleus changes to a drop in which the chromatin diminishes, while the protoplasm begins to resemble the former nucleus. The cells become gradually smaller, and the final forms do not result from the protoplasm but from the nuclei of the cells. The blood cells originate, then, from the original nucleus of a mother blood cell. Hubrecht finds the same steps in the placenta of Tarsius as in the umbilical vesicle. He finds the same to occur in the trophoblast and in the maternal spongia, and even in the vessels which lie between the glands. In the placenta of Tupaja the same steps are noted, but, in addition, the epithelium, as well as the connective tissue of the decidua, is the seat of red blood cells derived from nuclei.
In his ovum v.H. Spee describes the mesoderm layer as follows : He finds two different cell forms occurring side by side and intermingled : (1) Branched cells, extending into fine threads which project in all directions and which may well be considered connective-tissue fibres. Where these cells occur singly — for instance, at certain points in the abdominal pedicle and in the villi— the mesoblast appears exceedingly poor in nuclei. Strongly-stained specimens are necessary to see the numerous fine, wavy, and sharply contoured fibres of the embryonal connective tissue. (2) Between these structures are found larger, strongly prominent spindle cells with oval nuclei, arranged sometimes closely together like bundles, and at other points separated from each other by wide spaces without evident contents. The course of these fibres follows, with noticeable constancy, the curves of the free mesoderm surface — that is, the surface covered with epithelium.
In our opinion many of the second form of cells are possibly ectodermal. In an ovum described by Reichert, on the inner side of the chorion is a substance which he considers to be a mucoid-like deposit and which also extends into the villi. The cavity of the ovum is likewise filled with this substance. Spee considers this to be mesoderm tissue, although Reichert describes the contents of the ovum simply as a mucoid substance without cells: Spee says that the contents of the amnion and of the umbilical vesicle of ova hardened in alcohol never coagulate, but such a condition does take place in the wide mesoderm slit of dead young human ova.
Thus the early embryonal mesoderm tissue of the membrana chorii and of the villi is very poor in cells, and the description of Reichert, characterized by Spee as mesoderm substance, selves to characterize as mesoderm the non-celled material found between the central space and the external layers of cells seen in Fig. 36 and also found in many of the younger villi.
It has been mentioned that Langhans considers the cell layer to •be of mesodermal origin, and that Leopold holds the same view because he finds transitions between the stroma cells and the cell layer. Frankel, who believes that the cell layer and the syncytium are probably identical, finds the question difficult of solution because of the numerous villi whose interior is composed of ectodermal trophoblast cells. We have called attention to this occurrence as a frequent one, and have likewise observed the resemblance between the cell layer of the membrana chorii and the several layers of cells under it which are embedded, however, in mesoderm substance (Fig. 36). The origin of the villi, the fact that they are formed out of solid trophoblast groups, the fact that only later does mesodermal tissue reduce them to the generally single layer of Langhans, the fact that in this reduction to a single layer many trophoblast cells are left embedded in the stroma of the villi and the membrana chorii, and the numerous transitions between these cells and the cells of Langhans, prove that they are ectodermal cells situated in a basis of mesoderm substance in which only later mesoderm nuclei appear.
As the villi grow older, these ectoderm cells are crowded by the growing mesoderm tissue, so that some degenerate, while others take part in the following important changes — that is,
THEY FORM RED BLOOD CELLS.
In the same manner that the external trophoblast cells,, which come in contact with the blood, obtain from it a protoplasmatic envelope, while they themselves form the nuclei, in quite the same manner these cells situated in mesoderm are seen to gradually change their form, become darker, become surrounded by a red-staining granular protoplasm of the same character as the mesodermal tissue. They then represent nucleated red blood cells. They are often seen in stages where the nucleus becomes fragmented or divided into other cells. These larger and smaller nucleated reds are found isolated or in groups, often lying in spaces or slits of the mesoderm of the villi and membrana chorii. These spaces are often surrounded by very long, very dark, and spindle-like cells, evidently forming the endothelium of the future capillaries.
M 1 e have observed the statement that in animals the mesoderm, which enters the trophoblast groups and forms the stroma of the villi, carries with it the branches of the allantoic vessels. Vessels, however, are not present in the villi of the human ovum until the third week, and the villi are then filled with mesoderm or ectoderm cells, and it would be impossible for extensions of the allantoic vessels to enter into the innumerable extensions of the membrana chorii constituting the villi. The process followed, however, is probably the following :
Mesoderm does not enter as a distinct tissue into the trophoblast elements after they have been reduced by the maternal blood to syncytium, but is present between the trophoblast cells from the very earliest period, as may be seen in the description of a tubal ovum. When, therefore, a trophoblast cell group comes in contact with maternal blood, the outer cells are changed to syncytium, underneath which results the cell layer of Langhans, which in the still later periods disappears. The mesoderm substance, when increasing in amount, dilates, the villus and forms the stroma. In the resulting stroma are left the remaining more or less scattered trophoblast cells. From these, nucleated red blood cells are formed, which lie in small slits or spaces, which spaces become lined with endothelium, thus forming capillaries. These capillaries gradually increase and finally become very numerous, having a tendency to lie close under the epithelial covering of the villi. "Whether any of these trophoblast cells take part in the formation of the endothelium, it is impossible to say with certainty. These same processes are observed in the membrana chorii. These capillaries in the villi and in the membrana chorii later unite with the umbilical vessels which make their way through the abdominal pedicle.
Fig. 38. — Membrana chorii of tubal ovum.
The nucleated red blood cells, in their subsequent circulation through the fetus, become changed, and from the third month on ordinary red blood cells are found. In the membrana chorii of our uterine ovum the same stages of capillary development may be observed, but to a less extent than in some of the villi which already possess numerous, and distinct capillaries. In the later stages this process is more marked in the membrana chorii than in the villi, as may be seen from the following description of a tubal gestation with well-preserved fetus.
J The possibility that these nucleated red blood cells came from the fetus was not left out of consideration.
The fetus was lying in the central cavity of the ovum (Fig. 38) attached by an umbilical pedicle which entered the placenta at the site to be mentioned later. The fetus was one centimetre long; the abdominal wall was not yet closed, and the arm and leg formations were just evident as small pinhead knobs. The cavity of the ovum is lined by a membrane composed of flattened cells with distinct nuclei. At various points in the circumference of the membrana chorii, and especially at the point where the abdominal pedicle is attached, are numerous larger or smaller spaces, separated, sometimes, from the maternal blood by only a single or double layer of cells, which at many points are of a distinctly plasmodial character. The membrana chorii has at various points, and especially at the placental site, numerous villi attached to it and numerous small projections of the same structure as villi. These spaces, or sinuses, contain masses of round, red-staining protoplasmatic masses with dark central nuclei, many of which are undergoing division— nucleated red blood cells. It seems possible that many of these sinuses or spaces represent dilatations of the parallel row of cells observed in Fig. 36, wherein were noted, at various points, groups of cells of a trophoblast character. It is probable that those groups too form the nucleated red blood cells observed in these sinuses.
The largest of these sinuses are present near the insertion of the abdominal pedicle, and all of them are separated from the maternal blood of the intervillous space, often by only a single layer. These sinuses may represent, then, areas in the membrana chorii filled with nucleated red blood cells of placental trophoblast origin, which subsequently enter the fetal circulation.
In the pedicle itself a change to the subsequent non-nucleated red blood cells may be observed, for in its substance are fetal vessels and sinuses filled with cells of a different character. They are small, with a thin circumference of protoplasm and a dark nucleus filling out almost the entire cell body. These represent, then, the later stages of the red blood cells.
The resemblance of this process to the one noted by Hubrecht in Tupaja and Tarsius carries out still further the resemblance of the various processes in the human placenta to those in the placentas of animals. That this theory and explanation, whereby red blood cells are formed of trophoblast ectodermal cells, is a rational one and proven by the examination of our specimens seems to me evident. In addition, it serves to explain the presence of trophoblast cells in the interior of the villi — a fact which has led many, who have considered them mesodermal because situated in the mesodermal stroma, to likewise call the cells of Langhans mesodermal. The formation of nucleated red blood cells from trophoblast cells, on the addition of a protoplasm of mesodermal origin, would serve to make their resemblance to syncytium very great; for the latter is composed of trophoblast nuclei with a protoplasm largely composed of maternal blood plasma. If, as many sections have suggested, the capillaries were lined by endothelium which is also formed of these trophoblast nuclei, the resemblance of this process to the formation of syncytium would be further increased; for the syncytium simply plays the part of endothelium, separating the other fetal cells at all times from the circulating blood.
Uterine (as well as tubal) ova furnish us with the following positive conclusions:
- The human ovum possesses an ectodermal growth of cells, the trophoblast, consisting of closely-grouped cells.
- When vascularized by maternal blood, a second external layer, consisting of plasmodial mononuclear and polynuclear elements, results.
- Elements of the maternal blood circulating in the spaces and lacunas of the trophoblast contribute to the protoplasm of the syncytium. Among other elements, the secretion of the uterine or tubal epithelium may likewise contribute to the formation of the syncytial protoplasm. At any rate, much of the protoplasm (but none of the nuclei) is of maternal origin.
- On the villi and the membrana chorii the plasmodial cells form the outer syncytial layer, ivhile the closely-grouped cells beneath it furnish the single layer of Langhans.
- The stroma of the chorionic villi is formed of mesodermal tissue in which are later found capillaries communicating with the umbilical vessels and containing fetal blood. Clear in almost every detail, then, a trophoblast formation, consisting of an inner layer of separated cells and an outer or plasmodial layer such as is found in the placental development of animals, is present in human placentation.
Note. — It is to be noted that the embedding of the guinea-pig's ovum reveals symplasmatic changes in the decidua which make Spee's theory of the maternal origin of the syncytium very attractive. Certain it is that the decidual cells undergo "syncytial changes." Were we to go but one step further and add to the maternal blood symplasmatic decidual structures as an external factor in the production of syncytium, then the theory of the exclusively fetal trophoblastic origin of this tissue would fall to the ground. The distinction between fetal and maternal cells is often decidedly difficult, and in that fact lies the crux of the problem. If the view we have chosen be correct, it must still be granted that the decidua undergoes primary or secondary changes which make its resemblance to real syncytium striking. Though often and in many specimens led to favor Spee's view by microscopic appearances, yet our choice of views (by no means surely excluding, in whole or in part, the views of Spee) seems well founded.
"Structures not wisely called 'syncytial' may be formed from various forms of tissue, as uterine epithelium, uterine connective tissue, fetal ectoblast, etc. Therefore, especially when we consider the variations in placental formation in various species of animals, it is not correct to speak of 'the syncytium' as if all syncytial formations were alike. Therefore, in the guinea-pig, I did not use this word, but chose the term 'symplasma' to designate those structures (developed from connective tissue) comparable to syncytium" (v. Spee). Van Beneden calls this tissue, developed from ectoblast in animals, Plasmodium. Plasmodium is, then, in the human ovum "trophoblast." We believe "syncytium" to be the "Plasmodium" of animals. Decidual and other uterine cells undergo the so-called "syncytial changes." It would be advisable to call these "symplasmatic" cells. The question then is: Is syncytium of symplasmatic or plasmodial origin? "We hold the latter view.
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Reference: Bandler SW. Uterine and tubal gestation. (1903) William Wood & Company, New York.
Cite this page: Hill, M.A. (2020, August 10) Embryology Book - Uterine and tubal gestation (1903) 1-10. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Uterine_and_tubal_gestation_(1903)_1-10
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