Paper - A further study of the human umbilical vesicle
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Jordan HE. A further study of the human umbilical vesicle. (1910) Anat. Rec. 67(4): 4(9): 342-353.
|CRL embryo corresponds to Carnegie stage 18 in Week 7. Note that CRL measurements in embryos are affected by histological processing and shrinkage.
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A Further Study of the Human Umbilical Vesicle
Harvey E. Jordan
University of Virginia, Department of Anatomy, Charlottesville, Va.
The material for this study consists of a well-preserved umbilical vesicle of a 13 mm human embryo. The specimen was sent to me in a 5 per cent solution of formalin immediately after hysterectomy, by Dr. J. L. Crenshaw of Charlottesville, Va. It was at once transferred to 95 per cent alcohol, and subsequently embedded in paraﬂin and sectioned at 10 microns. The sections were stained in saffranin. The perfectly normal condition of the embryo itself and of the chorionic villi would seem to preclude all possibility of post-mortem degeneration, pathologic variation or fixation artifacts.
Primarily the object was simply a morphologic study of the entodermal tubules of this stage, supposed to be just past the phase of maximum development. Hence differential stains were not employed. This is to be regretted since numerous blood islands, showing with especial clearness the earliest stage in hematopoiesis, were subsequently discovered. The main criteria for a differentiation of these cells must consequently be morphologic, e.q., presence of pseudopodia (indicating amoeboid motility), sh.ape of nucleus, comparative size of nucleus and cell, granular character of cytoplasm, etc. Staining capacity of the protoplasm, however, yields consistent confirmatory information. Thus a certain type of cell always has a deeply staining homogeneous (haamoglobin-containing) cytoplasm. The points of special interest in this study concern the tubules (“glands”Spee; “crypts”—Selenka) and the blood islands.
An exceptional importance attaches to the human umbilical vesicle of this stage of development and for two reasons: (1) The tubules have just entered upon a functional decline. They appear to be at the height of their development and activity in vesicles of embryos of about 9 mm. (Spee, Meyer, Jordan, Branca.l (2) Schridde proposes to divide haematopoietic phenomena in the human embryo into two sharply defined periods. The earliest period ends at about the 10 to 12.5 mm. stage of growth. Meanwhile the blood cells (all of a single type, “primary erythroblasts” with haemoglobin) have origin in blood channels. “Blut—réiume,” of the umbilical vesicle and connecting-stalk. Moreover the original cells arise only from “vessel-wall cells” and proliferation is strictly intravascular. In embryos of 13 mm. the liver has assumed the haematopoietic function. Three different types of cells are said to arise simultaneously from the outer layers (i.e., extravascular) of the hepatic capillaries: Myeloblasts, sec~ ondary erythroblasts, and giant cells. The secondary erythroblasts are told from the primary by their smaller size. Both from the standpoint of material and results, Schridde’s position is unique. On the basis of very extensive observations on material from various higher mammals, prepared with almost faultless technique, Maximow rejects Schridde’s hypothesis. Furthermore, as regards the source of the blood cells during the stages of hepatic hematopoiesis, these two investigators disagree. Maximow derives the blood mother—cells (“haamatogonia,” “lymphocytes,” “primary wandering cells” of Saxer) from latent mesenchyme. Schridde absolutely denies the presence of such in the embryonic liver. It seems clear that more evidence is demanded. The 13 mm. human embryo is of the exact stage required. A careful study of its umbilical vesicle and the liver may be expected to give indication of the more probable conditions respecting primary hematopoiesis.
In anticipation of ensuing results it may be said that the evidence is all in favor of a continuous and identical haematogenetic process. And, in the sense that only one source of origin and only one line of cells can be recognized, the members of which appear identical in umbilical vesicle, liver, and heart, the evidence supports the monophyletic theory of blood cell formation. Moreover, except for occasional endothelioblasts which become transformed into blood cells, the proliferating cells in the hepatic capillaries would seem to have been carried there by the blood currents. The verity of such a conclusion Schridde denies, claiming that the secondary are not daughter but sister cells of the primary erythroblasts.
The umbilical vesicle (fig. 1) here under consideration measured 6 mm. x 4 mm. It is almost spheric in shape. Externally it is faintly corrugated. Sections reveal at-hicker wall distally and on one—half of its surface (fig. 2). These thicker regions contain the tubules and blood islands (fig. 3). The tubules are of two types; open and blind. The former are mostly cylindric in shape and generally open into the cavity of the vesicle by a constricted neck. The lining cells are similar to those lining the main cavity and continuous with them at the neck. The lumen is filled with an amorphous coagulum apparently identical with that of the cavity. Beyond the neck the tubules bend almost at right angles and generally pass distally, though occasionally proximally. The tubules sometimes branch once. Occasionally two limbs proceed from the neck, one proximally and one distally. The tubules are disposed parallel to the long axis of the vesicle. They vary in length from 100 to 200 microns.
Blind tubules result from the former type by occlusion of the neck followed by constriction and eventual separation from the lining epithelium. They appear cystic, are lined with more ﬂattened epithelium and contain the same amorphous coagulum. None of these contain mesenchyme as recorded by Meyer. Moreover, the line of demarcation between entoderm and mesenchyme is here always distinct.
The entodermal cells lining the cavity of the vesicle vary from the cubic to the polyhedral type. The former is the prevailing type where the wall is thinnest. These cells have a deeply—staining almost homogeneous cytoplasm, and a centrally located nucleus. Distally and on one surface the lining epithelium is of the stratified polyhedral type (fig. 3). These cells contain a large centrally placed nucleus with one or several nucleoli and a reticulum with occasional net—knots. The cytoplasm is greatly vacuolated and contains irregularly shaped flakes (cell detritus?) of deeper staining substances. Sometimes the nucleus appears suspended by several strands in the otherwise almost empty cell.
In the tubules similar types of cells occur: the ﬂattened cells in cystic tubules or those with wide lumina, and the polyhedral cells (always in a single layer) in those with narrow lumina. In a general way the height of the epithelium varies inversely as the size of the lumen.
Besides the two types of tubules above described there are occasional solid cords of entodermal cells. These may be the results either of an original solid invagination of entoderm or of proliferation in a tubular structure.
The mesenchymal layer of the wall corresponds most closely to embryonic connective tissue. The predominating type of cell, however, is spindle—shaped and the whole structure is more compact and fibrillar. The amount of connective tissue in diﬂerent regions varies inversely as the number of tubules. It contains everywhere blood vessels and capillaries. Distally the mesenchymal layer of the vesicle contains the blood islands.
The mesothelial covering (coelomic epithelium) is considerably more ﬂattened than in the earlier vesicle (of a 9.2 mm. embryo) previously described. Neither in this nor in the younger vesicle could cilia be demonstrated on these cells as described by Branca.
The main points of difference between the two vesicles from the 9.2 mm embryo and from the 13 mm embryo respectively are as follows: (1) The older vesicle is slightly larger. (2) It contains fewer tubules (more of the blind variety and solid cords). (3) It contains blood—islands. (4) Its mesothelial cells are more ﬂattened. (5) Its entodermal cells are more irregular and show signs of degeneration, e.g., extreme vacuolization of protoplasm, absence of the “mucinous masses,” and, in the cystic tubules and the more ﬂattened cells of the vesicle, a disappearance of cell borders coupled with a decrease in size and staining capacity of the nuclei.
The above facts indicate that the human umbilical vesicle grows for a short time after the first month and while the entoderm is undergoing the early phases of degeneration. The continued growth coincident with a degeneration of some of its elements (entodermal) indicates that the vesicle has a double function_ viz., hamatogenous, and some function of the entodermal cells. That the umbilical vesicle subserves a heematogenous function in some degree has never been disputed. But this function is limited entirely to the mesenchyme. In the two specimens studied no evidence appears of an origin of giant cells (supposed blood mother—cells—Saxer) from the entoderm as held by Spec. Nor is the transition from entoderm to mesenchyme indistinct as described by Meyer. Moreover, the haematogenous function is commonly regarded as more or less incidental to a more primary function of the umbilical vesicle, especially as concerns its entoderinal elements. The blood vessels are viewed as the purveyors of some sort of pabulum elaborated by the entodermal cells. The several theories regarding the significance of the umbilical vesicle are: (1) that it has a hepatic function (Spec, Paladino, Saxer); (2) that it has an absorptive function like the intestine (Branca); (3) that it is a rudimentary or vestigial organ, “mor— phologically significant, but functionally nil” (Selenka); (4) that it has primarily ahmmatopoietic function (Hubrecht, Bonnet). A hepatic significance is urged on the basis ( 1) of a structural resemblance between liver and umbilical vesicle (Spec and Saxer); (2) of the presence of glycogen (Paladino); (3) of the presence in both of giant-cells—(Spee). This hypothesis is invalidated by the following facts: (1) The resemblance between liver and the wall of the umbilical vesicle is only general, not detailed (Branca). (2) Giant cells are found at this stage, also in the mesonephros and the heart. (3) Most embryonic tissues contain glycogen (Gage). A nutritive significance is urged by Branca on these grounds: (1) Supposed presence of small amount of yolk in the human "yolk sac.” (2) Morphologic similarity between the lining cells of the sac and those of the small intestine. The details include (a) terminal bars, (b) ciliated borders, (c) position of nucleus, and (d) cell contents, which he likens to ergastoplasm (prozymogen) and zymogen. (3) Common origin of the intestine and yolk sac from primary entoderm. To these might be added Bonnet’s contention that yolk must needs be present to supply haemoglobin for the first erythroblasts.
Branca regards the entodermal cells as agents for the preparation and transference in usable form of yolk contained in the vesicle and needed as food by the embryo. This hypothesis appears untenable for the following reasons: (1) The presence of yolk is not established. (2) Common origin need not imply identity of function, e.g., cells of villi (absorptive) and chief cells (secretory) of fundus glands. The umbilical cells may functionally resemble more closely the chief cells (3) Absence of A ergastoplasmic granules in cells of absorption. (4) On the basis of mere staining reaction these masses of granules may with equally good reason be regarded as “mucinous masses.” (5) The presence of ciliated borders on some of the cells vitiates a strict homology. (6) The flakes and granules in the distal portions of the cells are too large and irregular to be regarded as zymogen granules. (7) The structural peculiarities, even as interpreted by Branca, are more like those of secretory cells. (8) The haemoglobin may have the same source as the lipoid and glycogen content of the cell.
Branca moreover, urges in support of a nutritive function that the umbilical Vesicle cannot be considered as a merely rudimen tary structure since a decrease in size involves an atrophy of the constituent elements as in the case of the epiphysis and the notochord. But an organ in becoming rudimentary need not necessa-rily decrease via an atrophy, but only a decrease in number, of its elements, e.g., vermiform appendix. Again, an organ may become rudimentary in part and still retain an important collateral function, e.g., hypophysis.
The human umbilical vesicle would seem to have lost its function of yolk absorption and elaboration, but to have retained the very important coincident function of haematopoiesis. When the liver takes up the Work of blood cell formation, the umbilical vesicle decreases in size byreason of an atrophy of its elements and eventually disappears. It is not simply a vestigial structure, but appears to have a necessary function in supplying the progenitors of the fcetal blood cells. Accordingly it must be regarded as the earliest haematopoietic organ.
Furthermore, the complicated histologic structure due to the presence of the entodermal tubules must be interpreted in the light of the phylogenetic history of the mammals. In sauropsidan ancestors with meroblastic yolk-laden eggs, the entodermal cells functionated in the elaboration and absorption of the yolk. This process involved the initial secretion of a liquifying ﬂuid. Thus prepared in soluble form yolk was transferred to the blood vessels. The entoderm seems to have retained its secretory function but
Fig. 1. Photograph of opened chorionic vesicle of 13 mm. human embryo, showing umbilical vesicle and amnion intact, X §. Made by Mr. Frank P. Smart, University of Virginia.
there is no yolk to liquefy and absorb. The amorphous coagulum in the tubules and cavity is perhaps the representative of this yolk—dissolving secretion. That a ﬂuid is actually secreted by the tubules is the more probable when one considers the varying character of the lining cells. This is best interpreted in terms of pressure exerted by a liquid content of the tubules. The apparently gratuitous extension of the entoderm seems due to hereditary factors consequent upon a sauropsidan ancestry.
On the basis of very many facts of comparative anatomy, Hubrecht argues cogently for the primitive character of the Primates in many respects (yolk sac, allantois, etc.), and against a sauropsidan ancestry of the mammals. The structure of the umbilical vesicle, as regards more particularly its tubules and blood islands accords better with the assumption of a sauropsidan descent. The evidence suggests more forcibly a secondary modification, along the lines of greater hematopoietic significance, of a formerly predominantly nutritive organ.
Hubrecht also emphasizes the hematopoietic significance of the yolk sac chieﬂy on the grounds, (1) that the liver during early stages cannot be said to be capable of supplying a sufficient number of blood cells for assisting in metabolic processes, and (2) that due to the presence of a decidua capsularis no nutritive material can enter the extra embryonic coelom to be transported by the blood vessels to the embryo. In the absence of yolk the sole purpose of the vessels is to produce (in the first instance) and transport blood cells to the embryo. But these facts are urged in favor of its primitive character. The peculiar characters of the entoderm, however, seem more intelligible as secondary modifications of primitive sauropsidan conditions.
It remains to describe the blood islands. They arise in the mesenchyme exactly as described for a number of birds and mammals: Portions of the syncytial mesenchyme become transformed into irregular cords of cells, the peripheral cells of which form the endothelial wall, the central cells blood corpuscles (fig. 4). The only detailed study of blood islands in the human umbilical vesicle previously made, as far as I can learn, is that of Schridde. But he describes “Blutraume” as the original structures. Only subsequently do the first blood cells arise from cells of the vessel-wall. The evidence from a study of the blood islands in my specimen is unequivocally opposed to this procedure. Moreover, the details of early haematogenesis are exactly similar to those described by Maximow for rabbit, guinea pig, cat, rat and dog. Among the central cells of a blood island are seen three successive stages yielding three distinct types of cells: (a) lymphocyte, (b) megaloblast, and (c) normoblast. All these cells can be seen in mitosis, the first two types more abundantly. Occasionally a cell of the vessel-wall is seen to round up and become free as a lymphocyte, as described by Schridde for all the cells.
The first lymphocytes have a 1ight~staining granular cytoplasm and a kidney-shaped nucleus with nucleoli and karyosomes. The
Fig. 2. Photomicrograph of a transverse section of the umbilical vesicle near the mid-region, showing the character of the wall and the content of the vesicle X 30. Reduced é in reproduction. Made by Dr. Leopold Jaches, Cornell University Medioal College, New York City.
later stages and generations of these have large, round and more strongly basophile nuclei, also with nucleoli and karyosomes and a narrow shell of basophile cytoplasm. These cells are actively amoeboid as seen by their pseudopodia. They are the progenitors of both other cells like themselves (lymphocytes) and red blood cells.
Fig. 3. Photomicrograph of region (A) of fig. 2, more highly magnified, showing a branching tubule, and a small blood island (in the angle between the blood vessels in the upper part of the illustration) X 240. Made by Dr. Leopold Jaches. STUDY OF THE HUMAN UMBILICAL VESICLE 351
Fig. 4. Photomicrograph of large blood island. The upper row of cells shows several types of lymphocytes. The majority of the remaining cells are normeblasts. The large lighter staining cell to the left of the center of the island is a megaloblast, X 350. Made by Mr. Frank P. Smart.
The megaloblasts contain smaller, lighter-staining, spheric nuclei, and have a great amount of light-staining granular cytoplasm. The earlier generations of these are the largest cells seen (2 to 3 times the size of post-foetal erythrocytes).
The normoblasts are smaller both as respects the nucleus and the cells. The nucleus is more chromatic than that of the megaloblast. It also contains nucleoli and a reticulum with net-knots. The cytoplasm is homogeneous, probably due to the presence of haemoglobin, and stains more deeply than that of the megaloblasts. These cells are mostly polyhedral in shape due to the crowding produced in consequence of rapid proliferation of megaloblasts. Cavities begin to appear between the cells, and the latter subsequently ﬂoat free in the lumen.
The erythroblasts have a smaller, homogeneous, pale-staining nucleus; and a paler homogeneous cytoplasm and frequently an oval shape, recalling the red blood cells of amphibia.
When attention is now turned to the liver of this specimen, one sees here cells in every respect similar to those described in the umbilical vesicle. Lymphocytes, megaloblasts, normoblasts and erythroblasts appear, identical in form and size. The relative number only varies. The lymphocytes and megaloblasts are rarer. The erythroblasts are more abundant. The normeblasts greatly preponderate. The latter are actively proliferating. Only rarely is a blood cell seen arising from the endothelial wall of the hepatic capillaries. Since no extravascular masses of proliferating blood cells, as described by Schridde for a 13 mm. embryo, appear in this specimen, and only very rarely an extravascular lymphocyte, the presumption is strong that the blood cells of the liver and heart have been carried there by the current from the umbilical vesicle. Here they find favorable harbors for continued proliferation. There appears no evidence to furnish ground for dividing haamatopoietic phenomena in the first weeks of human development into the two stages described by Schridde.
Concerning the method of enucleation of erythroblasts in the formation of erythrocytes whether by intracellular absorption (Schridde) or by extrusion (Howell and Maximow) nothing can be decided here. Obviously also no observations can be made regarding the origin of leucocytes and small lymphocytes since these do not yet appear. The evidence, however, as far as it goes agrees with Maximow’s findings for the early stages in the guinea pig, etc., and to this extent accords with the monophyletic theory of blood cell formation.
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