Paper - The relation of the chorda dorsalis to the entodermal component of the hypophysis (1915)
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Atwell WJ. The relation of the chorda dorsalis to the entodermal component of the hypophysis. (1915) Anat. Rec. 10(1): 19-42.
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The Relation of the Chorda Dorsalis to the Entodermal Component of the Hypophysis
Wayne J. Atwell from the department of anatomy, university of michigan
The mode of termination of the anterior end of the notochord has been variously interpreted by authors. The older writers thought the chorda to present a causal relation to the head flexure of the embryo; to act mechanically in drawing out the infundibulum of the pituitary from the brain wall; or to form, after a similar manner, the entodermal diverticulum known as SeesseVs pouch. Later writers, while not necessarily arguing for a causal relationship, have noted the existence of contacts between notochord and hypophysis and between notochord and fore-gut. Recently the r61e of the entoderm in the formation of the hypophysis has occupied the attention of a number of observers. It is the object of this communication to call attention to an observed relation of the chorda to the entodermal component of the hypophysis and to attempt the explanation of certain connections of chorda and hypophysis.
An exhaustive review of the literature covering the development of the hypophysis will not be here attempted. For a yery complete bibliography on this subject the interested reader may be referred to Stendell^s monograph in Oppel's Lehrbuch and to the recent contributions of Bruni and Woerdeman (1914). Neither will the much-perturbed question of the origin of the notochord receive attention. For a theoretical study of this question a recent article by Triepel may be consulted. Only the literature which seems especially pertinent will be referred to.
The Entodermal Component of the Hypophysis, Rathke, whose name is used in designating the epithelial pouch from which the main body of the hypophysis arises, thought the structure to be derived from the anterior end of the fore-gut and thus to be entodermal. It was not until in 1873 that the now commonly accepted view of the ectodermal and there is no reason to doubt that the oral membrane had its upper attachment fairly between the two as they now exist. On the ventral side of the angle of Seessel's pouch appears a soUd epithelial bud. The notochord follows the configuration of the entoderm as far forward as SeesseFs pocket, where it dips down and comes into close relation with the soUd bud of epithelium just described. An actual contact does not appear to exist, the two structures being separated by a space of 5m.
Numerous other embryos, closely staged, show the notochord in a similar close relation to a solid bud of epitheUum, always in the same relative position. Gradually SeesseVs pouch is reduced imtil it is represented only by a slight indentation of the roof of the bucco-pharynx. The reduction of the size of the epitheUal bud is not always proportionate, so that it is often relatively prominent. A drawing of a 15-day rabbit embryo is presented in figiu-e 3. The hypophysis shows connection with the epithelium by a solid cord of cells. SeessePs pouch is f oimd in the same relative position as in preceding embryos but is now much flattened. A well-marked epithelial bud extends from the ventral side of the pouch near its apex. This bud, arising from the epithehum by a broad base, tapers rapidly so that its apex is drawn out into a fine point. This point is directed toward the cephalic termination of the notochord, which makes a marked ventral bend and comes in proximity to the end of the epithelial bud. Between the two is a line of cells more densely arranged than the surroimding mesenchyme. The end of the notochord shows signs of degeneration: the boundary of its extremity is not well marked, large spaces occur in it, and the cytoplasm of most of the cells stains heavily with Congo red. The appearance presented warrants the interpretation that this is the last region of contact between notochord and entoderm.
By degeneration of the end of the chorda and of the epithelial bud, and by a rapid increase of connective tissues to form the skull base, the notochord becomes farther removed from any connection with the entoderm. A rabbit embryo of 16 days is shown in figiu-e 4. The chondrocranium is well indicated at this stage and the solid stalk of the hypophysis has lost its although he admits his lack of material to demonstrate their ^'imiige Beziehmig zu einander.
From the scarcity of stages in the rabbit showing any indication of connection between notochord and hypophysis I am led to believe that a union of these two structures, when it does occur, is accidental rather than of normal occurrence. Figiu-e 1 shows that in early stages the end of the notochord lies close to the dorsal wall of the forming hypophyseal pocket. The presence of the notochord is responsible for an indentation of this dorsal wall showing that considerable pressure occurs between the two. It is not inconceivable, then, that a fusion may occur in a certain number of cases.
On the other hand the constancy of relation between the notochord and an epithelial bud from Seessel's pouch is noteworthy. This bud of epithelium appears constantly a short distance ventral to the top of SeesseVs pouch. It is probably to be considered identical with Selenka's 'Gaumentasche' (solid in higher forms), and St. Remy's 'branche descendante' of the notochord.
Relations of the Chorda in the Chick
My observations on chick embryos, beginning with a stage of 17 somites, are in accord .with the generally accepted view that the notochord loses its attachment to the entoderm first caudally, then gradually forward. A mid-sagittal view of a 19 somite chick is shown in figiu-e 6, and a view of the wax reconstruction made of the hypophysis region of this chick is shown in figure 9, A. The hypophysis anlage is indicated by a region of thick^ ened epitheUum. The connection between notochord and entoderm is lost except at the anterior end of the fore-gut. However, the entoderm shows evidences of this recently-severed connection by a considerable region of thickening anteriorly and by several buds just caudal to the present connection. There is no connection between notochord and hypophysis anlage, nor between the latter and the entoderm of the fore-gut.
An older chick (22 somites, figs. 7 and 9, B) shows the hypophysis as a well marked angle with a broad opening into the oral invagination. The notochord shows an attachment to a drawn out portion of the entoderm at the anterior end of the fore-gut and presents evidences of other connections more caudal. The hypophyseal anlage is not connected with either notochord or entoderm.
Fig. 9 Wax models of hypophysis region of chick embryos. X 75. A, chick of 19 somites; J9, chick of 22 somites; C, chick of 29 somites. The embryos from which these models were made are shown in mid-sagittal section in figures 6, 7 and 8, respectively. A and B ahow cephalic end of notochord attached to bud from anterior end of fore-gut. In C this bud has become fused with superior part of dorsal wall of Rathke's pocket. NCj notochord; 5, Seessel's pouch (anterior end of fore-gut); Rj Rathke's pocket; BW^ brain wall.
An embryo with the oral membrane in process of rupture (29 somites, figs. 8 and 9, C) shows a marked increase in the size of the fore-brain vesicle which has resulted in a compression of the oral invagination and a flattening of the hypophyseal pocket. But the most noteworthy result of this compression has been the bringing together of Rathke's pocket and the epitheUal bud from the entoderm which is apparently further drawn out by its persisting connection with the notochord. The bud, proceeding from the entodermal epithelium at a point sUghtly ventral to the cephalic end of the fore-gut, lies in connection with the ectodermal epithelium of the dorsal wall of Rathke's pocket for a considerable region near its blind end. The connection is a true fusion of entoderm and ectoderm with no demarcation between the two. The notochord makes a bend to maintain its connection with the entodermal bud. It will now be noted that the end of the notochord is nearer to the wall of Rathke's pocket than to the fore-gut (later Seessel's pouch). Here notochord, ectoderm and entoderm are in close relation. However, the notochord does not touch Rathke^s pocket directly but only by means of the epitheUal bud through which it maintains its connection with entoderm. At the anterior end of the fore-gut is another entodermal bud directed toward the notochord. This is to be considered as one of the remains of the earUer, more general union between notochord and entoderm.^
A strikingly similar condition is presented by another embryo of about the same age (latter half of the third day of incubation). The hypophysis region is shown in figure 10. The oral membrane has not broken but is very thin. A bud from the entoderm Ues fused with the superior part of the dorsal wall of Rathke's pocket for some distance. The result of this fusion is to more than double the thickness of the hypophyseal wall in this region. The notochord shows an unquestionable contact with the end of the entodermal bud by a dense line of cells, which is considered to be the anterior end of the notochord in process of disintegration.
Three other embryos, with the oral membrane in various stages of rupture, show distinctly this fusion of entoderm and
Such appearances are not uncommon, some embryos presenting two or even more such structures, which may persist later than the rupture of the oral membrane.
ectoderm with the remams of the anterior end of the notochord in relation to the place of fusion. These five chicks cover quite Veil the latter part of the third day of incubation, yet in none of them could be found the fine communication between the cavities of Rathke's and Seessel's pouches as described by St. Remy.
Fig. 10 Reconstructed outline drawing, in mid-plane sagittal section, of hypophysis region of chick embryo in latter half of third day of incubation. X 55. Brain floor shaded. NC notochord; F(j, fore-gut (later Seessel's pouch) ; Rj Rathke's pocket; OM, oral membrane. Shows same relations as figure 8.
Fig. 11 Reconstructed outline drawing, in mid-plane sagittal section, of hypophysis region, chick embryo, 96 hours of incubation. X 55. Brain wall shaded. iVC, notochord; 5, remains of SeessePs pouch; R, Rathke's pocket; X, transition from ectoderm to entoderm. Fusion between Seessel's and Rathke's pockets now lost. A line of cells connects cephalic end of notochord to a bud from dorsal wall of Rathke's pocket.
Fig. 12 Reconstructed outline drawing, in mid-plane sagittal section, of hjrpophysis region, chick embryo, 120 hours of incubation. X 55. Brain wall shaded. NC^ notochord; 5, remains of Seessel's pouch; 72, Rathke's pocket; X, transition from ectoderm to entoderm. Large blood vessel shown between Seessel's and Rathke's pockets. Notochord shows traces of an attachment to a bud from dorsal wall of Rathke's pocket.
In older stages the fused entodermal bud loses its connection with the fore-gut but remains attached to the ectoderm of Rathke's pocket and causes a thickening of a small portion of its dorsal wall. With this thickening the notochord shows a connection which gradually becomes less definite as its cells are drawn out and separated (St. Remy maintains that they are changed into connective tissue). The hypophysis grows most rapidly at its blind end and thus the point of entodermal fusion (with the degenerated end of the notochord in relation) comes to lie relatively nearer to the mouth of the hypophyseal pocket. This has been noted by Woerdeman, who finds (pig) that in young embryos a fusion of notochord and the dorsal wall of Rathke's pocket is found near the apex of the pocket, while in an older stage the insertion of the notochord is into the middle of the dorsal wall.
A chick of 96 hours incubation (fig. 11) shows Seessel's and Rathke's pockets in commimication in the mid-plane. On the dorsal wall are found two epithelial thickenings. From the more cephalic of the two a line of cells leads to the end of the notochord. This would seem to indicate that this thickening is the location of the former ento-ectodermal fusion. The thickening is bud-like and may be what Bruni calls the 'diverticolo medio.' It is worthy of note that while the outermost part of this thickening is entodermal (remains of the bud that fused with the hypophyseal wall), yet none of this entoderm need be supposed to Ue next to the lumen of the hypophyseal sac. At this stage it is rather diflScult, from mid-sagittal sections, to determine the boundary between Rathke's and SeesseFs pouches. An examination of more lateral sections indicates that this is at the point marked X (fig. 11). The epithelial thickening caudal to this point is the tip of Seessel's pocket. Bruni calls this the
' gemma della tasca di Seessel." A chick of 120 hours incubation (fig. 12) shows much the same relations but SeessePs pouch has again assiraied a more definite demarcation from Rathke's pocket. This is due to growth of mesoderm dorsal to Rathke's pocket and especially to the development of a prominent blood vessel growing between the two. This vessel has been spoken of by several authors as a connecting branch between the int^pial carotids, common in bird embryos. The notochord still shows indications of a connection with a bud (which sometimes shows a lumen) from the inferior part of the dorsal wall of Rathke's pocket. This must be Brum's 'diverticolo medio.' Small blood vessels occur in this location. If the interpretation of the point X as the transition between ectoderm and entoderm is correct, then the hypophysis proper contains no entodermal component except the relatively few calls fused with the dorsal wall at about the time of the rupture of the oral membrane. It is hoped that the fate of SeesseFs pouch may serve as the subject of a future study.
Comparison of Relations in Rabbit and Chick
In comparing conditions found in the rabbit with those in the chick it is seen that in both the anterior end of the notochord tends to maintain a connection with the fore-gut, as St. Remy states. This connection is aided by a bud drawn out from the epithelimn. In the rabbit the bud remains for some time, gradually decreasing as the epitheUum of Seessel's pouch flattens out. In the chick the rapid growth of the fore-brain and the sharpness of the cervical flexure bring this bud into contact with the growing hypophyseal wall and the two become fused. Soon afterwards the bud loses its original connection with the entoderm but remains fused with the wall of Rathke's pocket. Thus a small mass of entodermal cells becomes miited with hypophysis anlage. The notochord shows indications of a connection with this mass of entodermal cells which now form a bud on the dorsal wall of Rathke's pocket. By this means the attachment of the notochord id transferred from Seessel's pouch to Ratlike's pocket.
In the rabbit, on the other hand, the notochord does not usually come into connection with Rathke's pocket. A fusion may occur in a small percentage of cases but must be considered accidental. When such a union occurs a bud of entodennal epithelium may be directed toward the point of union.
These observations may be offered to explain certain connections between notochord and hypophysis as seen by KoeUiker (in the rabbit) and Woerdeman (in the pig). The latter has in two cases, seen a more definite fusion than can be demonstrated in the two rabbit embryos which show evidences of such connection used in this study. These observations on the rabbit and particularly on the chick, may also serve to explain certain entodermal components of the hypophysis as noted by Kupffer (amphibia and mammals), Valenti (amphibia and birds), St. Remy (chick), and Nusbaimi (dog). The findings here recorded, it seems to me, do not substantiate the view, taken by most of these writers, that the entodermal contribution to the hypophysis is of fimdamental importance, especially phylogenetically; nor do they aid in the belief that the hypophysis represents a paleostome.
While the fate of SeessePs pouch and the participation of its epithelium in the composition of the hypophysis may be doubtful in the chick, it cannot be stated that this structure takes part in the formation of the hypophysis of the rabbit. As may be seen from the figures (1-5) Seessel's pouch isat all times sharply defined from Rathke's. This is especially well brought out in numerous wax reconstructions made from embryos closely staged before, during and after the rupture of the oral membrane.
For the rabbit it has not been possible to demonstrate a structure similar to the 'diverticolo medio' which Bruni describes for the rat.
For the rabbit:
1. In the rabbit the anterior end of the notochord tends to maintain its connection with the entoderm represented by a bud from SeesseUs pouch.
2. The entoderm cannot be said to contribute to the formation of the hypophysis of the rabbit.
3. Usually the notochord does not come into connection with the wall of the hypophysis in the rabbit.
For the chick:
4. In early stages of chick embryos the anterior end of the notochord is attached to a solid bud of epithelium which extends from a point slightly ventral to the cephaUc end of the fore-gut.
5. By the growth of the forebrain and the sharpness of the cervical flexure, this entodermal bud comes into contact with the growing hypophyseal sac and fuses with it. The fusion occurs at about the time the oral membrane ruptures.
6. The bud is soUd; a lumen such as is described by St. Remy could not be demonstrated.
7. The fused bud soon loses its connection with the entoderm. It remains fused with the dorsal wall of Rathke's pocket, however, and contributes a small mass of cells to the hypophysis anlage.
8. The notochord still shows indications of an attachment to the fused bud. Thus the attachment of the notochord (which is gradually becoming less and less a definite attachment) in transferred from Seessel's pouch to Rathke's pocket.
9. The relation of the notochord to this small entodermal increment of the hypophysis, and the fact that no lumen is to be found, tend to disprove the view that the entodermal contribution is anything more than an accidental union of parts.
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