Paper - On the structure of the human umbilical vesicle
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Meyer AW. On the structure of the human umbilical vesicle. (1904) J. Anat. 3(2):155-166.
See also later Jordan HE. A further study of the human umbilical vesicle. (1910) Anat. Rec. 67(4): 4(9): 342-353.
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On the Structure of the Human Umbilical Vesicle
From the Anatomical Laboratory of the Johns Hopkins University.
With 5 text figures.
In the history of embryology the discovery and interpretation of the yolk sac must always remain one of the most interesting chapters. The, to us, naive speculations as to its significance, at a time when " anatomists feared to make a thorough examination of ova and preferred rather to preserve them in alcohol," lend a peculiar interest to the study of the early literature on this subject. Many of the embryologies and anatomies of that time give much attention to the yolk sac, and it is not uncommon to find several chapters devoted to the discussion.
The credit for the first description of the human yolk sac seems to lie between Hoboken and Noortwyck. Wrisberg, however, gave the first accurate description of it, in full cognizance of the fact that what he described was a yolk sac comparable to the yolk sac of birds. The latter is referred to by Wrisberg as the "vesicula erythroides " of von Pockel, imconscious of the fact that von Pockel really described the allantois and not the yolk sac, as he believed. It is possible that Noortwyck was the first to recognize the yolk sac of the human embryo. Hoboken did not recognize it, and, according to Mayer, this was left for the great Albinius who first pictured a human embryo with the umbilical vesicle in situ. It was this fact which caused Zinius, in his monograph, to refer to the yolk sac as "de vesicula embryonis Albiniana." Neither this designation, nor that of " vesicula alba " of Hunter, found favor, however, for both were soon displaced by the term " vesicula umbilicalis " first used by Blumenbach.
Up to 1835 the greatest diversity of opinion existed regarding the functions of the yolk sac, and many interesting theories were advanced. Oken, while recognizing the meaning of the organ and demonstrating its occurrence in several of the mammalia, promulgated the idea that the intestine arose in the vesicle itself. Kieser, in 1810, claimed to have proven that the intestine develops in the yolk sac, and that it is then slowly taken into the abdomen. Van Euysch and Ossiander, on the contrary, took it for an hydatid and a pathological formation respectively. Mayer's exhaustive monograph, which appeared in 1835, removed many of these misconceptions; though regarding its functions he says, " ueber den eigentlichen Zweck des Nabelblaschens schweben wir in ganzlicher Ungewissheit." It is interesting to note, however, that most of the early investigators ascribed nutritive and hgematogenous functions to it.
The following study is based mainly upon eighteen normal human umbilical vesicles in the collection of Dr. Mall, to whom I am greatly indebted for the unrestricted use of his extensive collection of human embryos, and for many helpful suggestions. Besides these normal specimens, a number of pathological ones, and some taken from placenta at birth, were examined. They were all stained in alum-carmine" and imbedded in paraffin. An endeavor was made to set the imbedded vesicle so that its long diameter, which usually lay in the same direction as the remnant of the umbilical stalk, was at right angles to the microtome knife. In all cases in which this was not possible, account was taken of the fact in the study of the sections.
As the following table shows the size of the vesicles, not including those taken from placenta at birth, varies from one to six millimeters in embryos from 11 to 110 days old:
Table of Embryos and Attached Umbilical Vesicles
The numbered emhryos in the first column refer to the cabinet of Dr. Mall.
|Length of Embryo in millimetres.||Diam. of Vesicle in millimetres.||Approx. age in days.||Presence of tubules.|
Embryos of the Second Week.
Peters 0.19 0.19 .. None vonSpee 0.37 l.OSbyl.O 13 None No. II 0.80 1.00 by 1.5 13 Several Keibel 1-00 1.00 .. Many V. Spee-Gfe 1..54 1.8 by 1.5 13 Many Embryos of the Third Week.
No.12 3.1 1.5bylbyl 13 Several Janosik 3.0 3.5 15 No mention No. 76 4.5 3.0 19 Many No.80 4.5 4.0 19 Some Embryos of the Fourth Week.
No. 18 7 26 None No.2 7 7by4.5by4.5 26 Some Embryos of the Fifth Week.
No.187 9 6by4.5by4.5 30 Many No. 163 9 5.2 by 3.7 30 Some No. 113 - 30 Many No.187 10 4 33 Macerated Embryos of the Fifth and Sixth Weeks.
No. 175 13 3.7by3 36 Many No. 167 14.5 5.5 by 5.3 38 Many No. 5 18.5 4.7by4.5 43 Macerated Embryo over Six Weeks.
No. 33 20 5 by 3 by 3 43 Some No.l45Â» 33 4.5bv4by2.5 57 Few No. 176 38 4.8 by 3.6 by 3.5 61 None No. 184 50 5 by 4 by 3.9 70 None No. 171 60 6.5 by 4.3 77 None No. X 110 4.5 by 4 110 None
As all these measurements were made after preservation in alcohol, shrinkage must be borne in mind, although it is of no practical importance since estimations of age were not based upon them.
The vesicles are usually pyriform in shape, somewhat flattened in one diameter, and slightly roughened by protrusion and ridges below which blood islands and blood vessels usually lie. A few specimens are smooth, inflated, translucent sacs without any outward sign of blood islands or blood vessels. Others are collapsed irregularly folded and filled with calcareous-like material. There is never any regularity in the folding of the vesicle, however. Usually the folds were present while the vesicle still lay between amnion and chorion ; while in some cases they were produced during the hardening and imbedding. In several of the inflated vesicles the blood vessels are plainly visible throughout their entire length and can be seen entering the umbilical stalk.
The umbilical stalk is present in the detached vesicles, as a short (5-15 mm.) stump only. It is thread-like, about .75 mm. in diameter, and never appears twisted. In cross sections the cavity of the vesicle can be traced up to the stalk, and after ending blindly a strand of characteristic entodermal cells can be traced for some distance towards the abdominal end; after which the lumen of the stalk reappears at varying intervals. This lumen, which never contains anything but a slight amount of amorphous material, is often completely occluded by the bounding entoderm.
The stalk itself is composed of three layers in the greater part of its extent. On the exterior there is a thin layer of ccelomic epithelium (mesothelium) which continues indefinitely downward over the vesicle itself. In most vesicles it stops at the upper border, but in three specimens it forms a complete outer layer. The entodermal cells which bound the lumen have all the characteristics of those lining the vesicle itself, except for a slight decrease in size. Between these two layers mesoderm is found. Nearer the body of the embryo the latter usually predominates, while it is scarcely represented at all near the upper border of the vesicle.
Besides these three layers the blood vessels form a conspicuous part of the umbilical stalk. They are not constant in number in various parts of the stalk. Sometimes three arteries and two veins are found, while in other cases one vein and two arteries are present. They can generally be distinguished by the character of their walls. The wall of the vein is formed by a single layer of very flat cells, while that of the arteries usually has an additional outer layer, composed of somewhat flattened entodermal cells. This difference in structure, which is evident with the low power of the microscope, is found to disappear soon after the upper border of the vesicle is reached. In the structure of the walls of the blood vessels of the yolk sac itself there is never any difference as far as I am able to ascertain. The position of the vessels in both stalk and vesicle is usually well out towards the periphery, and in some cases only the ccelomic epithelium covers them.
For the microscopic structure of the youngest umbilical vesicles reference to the literature is necessary. Peters, in his monograph, gives the size of both embryo and vesicle as 0.19 mm. Unfortunately, the preservation of the umbilical vesicle of Peter's ovum was not such as to prompt a detailed description of it. We are told, however, that it is composed of entoderm and mesoderm, and in the accompanying plate (Peters, Taf. Ill, Fig. 33) some contents containing globules and cells are represented. In this plate the lower half of the vesicle shows no clear demarcation between mesoderm and entoderm; while in the upper half a fairly clear line of division between the two is indicated. The character of the mesodermal and entodermal cells is not given in the monograph, except that the latter are spoken of as " unscheinbaren Entodermzellen." Blood islands and blood vessels are not represented.
In an embryo of 0.37 mm. described by Graf Spee a marked advance in the structure of the umbilical vesicle exists. In this case the entoderm, which is one-layered, is composed of cubical cells, while the mesoderm is made up of irregular masses of cells with protrusions on the distal half of the vesicle, below which blood islands are found between entoderm and mesoderm. The' latter is thus pushed out while the entoderm in these places is said to be more wavy, its cells of greater variety and stained more intensely.
The distal part of Graf Spee's embryo Gle (an embryo 1.54 mm. long) is said to be full of gaps - " ausserst liickenreich." Some of these gaps have an epithelial lining of flat cells of the nature of embryonic endothelium. Blood Anlagen are found in the wall of the vesicle only. In the proximal third of the latter the entoderm and mesoderm are thin and membranous, while in the distal two-thirds they are of varying thickness. The protrusions on the surface are said to be due to collections of cells between . entoderm and mesoderm. It is of interest to note in this connection that Keibel states that the umbilical vesicle of an embryo 1.0 mm. long, described by him, is in every particular like that of embryo Gle of Graf Spee.
In the later article of Graf Spee, already referred to, he says that in embryos of three or four weeks the entoderm forms true glandular structures with small necks and large distal ends, which in embryos of nine weeks are branched and are found in all parts of the mesoderm and fat oft '????? e origin and fate of these glandular structures, and to throw some light on their possibe function. So far as I have been able to learn, Graf Spee was the first to mention and to describe them, and no one else seems to have suggested any explanation of their presence. These glandular structures which for the sake of brevity I shall call tubules, are present in the walls of nearly all vesicles taken from embryos less than two months old m the collection of Dr. Mall. The vesicles of A^s. n and U of this collection, embryos 0.8 mm. and 2.1 mm. respectively, are Identical in structure. Both are in a state of good preservation, and their structure in cross section as represented in Fig. 1.
FIG. 1. Umbilical vesicle ot ,â€ž embrjo 2.1 mm. long ,Nâ€ž. 12,. x 35, narrow'll,',bules with mesoderm close to the entoderm. These socalled glandular stn^ctnres do not branch and can be traced throng from Here thf. ,â€žâ€žâ€ž1 f * , ' ' â„¢'"'^Â° â„¢ millimeters long.
Here the number of tubules is considerably greater and a direct connec.on be ween many of them and the entoderm exists (Fig. 2). In many a es the tubules end as evaginations of the entoderm "and arc thr^n direct communication with the cavity of the vesicle. Others are "nd" r^ly connected with the entoderm by bands of entoderma Icel ^ht s 111 others he isolated m the mesoderm. As shown in Fig. 3 all transi ions are found from a slight evagination of the entoderm t los d ubules lj.ng detached from the entoderm in the mesoderm. Altl gh they can be traced through a series of fifteen to twenty-five sections Z are never seen to branch. On the other hand the branching described by Graf Spee is well seen in a vesicle taken from an embryo thirteen millimeters long. In such a vesicle (Fig. 4) we find an almost complete canalization of the mesoderm while the entoderm is but little changed. The tubules are much larger and longer and are formed by a layer of flat cells which often approach the cubical type. Contact of tubules is common but definite branching is infrequent. The lumina are wide and contain confused masses of amorphous material similar to that found in the cavity of many of the younger vesicles. They never seem to open directly into the cavity of the vesicle, although often the entoderm only separates their lumina from it. They are of many sizes, shapes and lengths, and lie irregularly distributed in the mesoderm. When not in contact they often have irregular masses of entoderm between them or are separated by mesoderm. Their abundance gives a striking appearance to sections of the vesicle which is well expressed by Graf Spee as " ausserst liickenreich." It is worthy of note that the lumina of the tubules have greatly increased in diameter while the thickness of the bounding endotheliimi has, absolutely as well as relatively, decreased. In many cases the shape of the individual cells also has changed from cubical or pyramidal to a membranous-like layer of greatly flattened cells.
Fig. 2. Umbilical vesicle of an embryo 7 mm. long (No. 2). X 35.
Fig. 3. Tubules from the vesicles of an embryo 7 mm. long (No. 2). X 35 ; (a) Simple evagination of entoderm - first stage; (5) Same, second stage; (c) Isolated tubule.
In older vesicles these tubules occur but rarely. This is usually the case in vesicles of the ninth and tenth weeks, although one vesicle taken
Fig. 4. Umbilical vesicle from an embryo 13 mm. long (No. 175). X 2.5.
from a normal embryo of the fifth week has already reached the stage of those three or four weeks older. Generally these older vesicles have a very different structure than those of four or five weeks and contain masses of calcareous matter.
It seems then that these tubules make their appearance during the second week, reach their greatest development by the fourth or fifth week and then gradually disappear by the eighth or ninth week. These stages are well represented in embryos Nos. 11 and 12; 113 and 175; and 145, 176 and 184 respectively. This conclusion is at variance with the observation of Graf Spee on embryo Gle, but as the widest variations as to the presence, structure and size of these tubules exist the contradiction does not seem surprising. As a rule the only constant characteristic was their direction. This was almost invariably in the direction of the long diameter of the vesicle, for only occasionally was a tubule cut at other than a slight angle to its long diameter. Even when such was the case it could generally be accounted for by the fact that the plane of the microtome knife was not at right angles to the long diameter of the vesicle.
In spite of the large amount of material at my disposal, I am unable to reach any satisfactory conclusion as to the meaning of these tubules. Their presence is not at all a constant one. Vesicles of the same age and size often present wide divergencies of structure which are hard to reconcile. I feel justified, however, in suggesting an explanation of the manner of formation, which an examination of the material at my disposal will, I think, corroborate. Two methods of formation can be distinguished: (1) evagination of the entoderm and (2) development from irregular extensions of entoderm into the mesoderm. That the first step in the formations of many tubules is a slight evagination of the entoderm, as Graf Spee has stated, is very evident. I have found all transitions between such a stage and perfect tubules lying isolated in the mesoderm. This isolation can be readily brought about by a gradual deepening of the original evaginations accompanied by constriction and consequent fusion. This process seems to be further indicated by the occurrence of tubules which communicate with the cavity of the vesicle by their ends only, while others are closed at both ends and lie isolated in the mesoderm close to the entoderm. It seems highly probable to me that an active proliferation of the mesoderm might play a part in this separation of the tubules and their further removal to the periphery of the mesoderm.
Even if correct, however, this explanation cannot account for those tubules in whose lumina masses of unmistakable mesoderm are found. This is the case in No. 33, an embryo twenty millimeters long. In this specimen there are striking evidences of the formation of tubules by proliferation from irregular extensions of entodermal cells. Such inclusions of mesoderm might evidently result from tubule formation by invagination of the entoderm, but it is hard to find any satisfactory evidences of such a process of inclusion. That another method than that of evagination of the entoderm must have been followed, however, in the case of No. 32, is clearly indicated not only by the masses of mesoderm contained in the tubules, but especially by the fact that strands of mesoderm are found in various stages of inclusion by the entoderm. That an active proliferation of the entoderm into the mesoderm does occur, is further indicated by those specimens in which almost the entire wall of the vesicle is composed of entoderm (Fig. 5), for in young specimens the entoderm is composed of a single well-defined layer of cells (Fig. 1).
In embryos of the seventh to tenth week the entoderm and sometimes the tubules can be found in various degrees of degeneration. This is true of Nos. 145, 176 and IS-t, embryos of 33, 38 and 50 mm. long, respectively. As the mesoderm is generally increased in thickness in these specimens it seems as though the degeneration of the entoderm is accompanied by a proliferation of the mesoderm. The latter at this time takes on the characteristics of a streaked fibrous connective tissue, and becomes compacted. The degeneration of the entoderm is apparent not only in the inner layer which lines the cavity of the vesicle, but is seen especially well in the groups of entodermal cells which lie scattered throughout the mesoderm. In many cases the entodermal cells are represented by granular detritns without any remnants of nuclei, while in
Fig. 5. Umbilical vesicle from an embryo 7 mm. long (No. 18). X 25.
other cases the cell outlines are faintly seen, and the nuclei are well preserved. Large amounts of cellular detritus can be found in the cavity of such a vesicle, and it does not seem unlikely that the cell remnants found among the calcareous contents of vesicles taken from placentae at birth have this origin. This cellular detritus is especially well seen in Nos. 187 and 176, the cavities of which vesicles are almost completely filled with granular debris containing many large cells having the characteristics of entodermal cells. In older vesicles, those from embryos of sixty and one hundred millimeters, for example, we find, on the contrary, a condition almost identical with that found in full-term vesicles, except that the walls of the latter are more compacted and look still more as though composed of mature fibrous connective tissue.
The signs of degeneration in these older vesicles are not limited to the entoderm, however, for many of the blood vessels show marked degeneration of their walls and of the nucleated red blood cells contained within. The vessels are often pigmented and without a proper lining. The pigment reminds one strongly of blood pigment and looks very much indeed like hsematoidin. The entire absence of vessels in the old vesicles and their extreme vascularity in the early stages alone seem sufficient to indicate a gradual degeneration.
The walls of these vesicles, as already stated, vary greatly in thickness and in the character of the cells composing them (Figs. 2, -i, 5). Usually the greatest thickness is found at the distal end. Both entoderm and mesoderm are present in all vesicles except that of No. 187, below eight weeks of age. In these specimens the ccelomic epithelium in addition extends over the entire surface of the vesicle. This envelope is invariably composed of a single layer of very much flattened cells with elongated nuclei.
The mesoderm also presents great variations in thickness, though not in the character of its cells. These cells, though cuboidal or cylindrical in a few instances, not infrequently look like embryonic connective-tissue cells in the young vesicles, while in those of ten weeks and older it has the characteristics of fibrous connective tissue, as already noted. In these specimens it is denser, and stained more deeply near the cavity of the vesicle. The tubules and blood vessels invariably lie in the mesoderm, but are frequently surrounded by extensions or by groups of entodermal cells. In younger vesicles the blood vessels and blood islands usually cause an elevation of the mesoderm above the points where they lie.
The entoderm is composed of a single layer of cuboidal, pyramidal, and exceptionally in a small area, of cylindrical cells in vesicles of two to four weeks, but is absent in those over seven weeks of age. In a few specimens no distinct demarcation between entoderm and mesoderm can be found, though usually they are clearly defined in all the younger vesicles (Fig. 1).
A series of six imibilical vesicles taken from placentae at birth were found almost identical in structure with tlxe vesicles of JSTos. 184, 171 and X. The walls of these vesicles are composed of a dense, wavy layer of fibrous connective tissue of varying thickness, which blends more or less with amnion and chorion. The cavity contains an irregular mass of calcareous matter among which cell remnants are plainly visible. Even those vesicles which are inflated sacs contain a small amount of calcareous matter, while those which are compressed and irregularly folded contain a firm mass of calcareous substance, which completely fills the cavity of the vesicle. Eemnants of the early blood vessels or of tubules are never found nor can any recognizable remnants of the entoderm be detected. Unless, as previously suggested, the cells lying among the calcareous matter have this origin. The striking similarity between the structure of these vesicles and those from embryos of the third month plainly shows that the condition of the vesicle as found at birth is reached early in the life of the embryo.
The occasional large size of the umbilical vesicle in full-term placentae which contained a normal foetus, is very remarkal)le. I have seen vesicles that measure fifteen by ten millimeters. Such occurrences are hard to reconcile with the supposition that the umbilical vesicle reaches it? greatest development in the fourth week. oSTor is it easy to see how mechanical forces can produce these large inflated vesicles. The only suggestion that occurs to me without further study of full-term vesicles, is that hypertrophy takes place at the time when the transformation of the wall of the original vesicle into fibrous connective tissue occurs.
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