Paper - Some factors influencing the early development of the mammalian hypophysis (1935)
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Gilbert MS. Some factors influencing the early development of the mammalian hypophysis. (1935) Anat. Rec. 62(4): 337-
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Some Factors Influencing the Early Development of the Mammalian Hypophysis
Margaret Shea Gilbert
Laboratory of Histology and Embryology, Cornell University, Ithaca, New York
norm mxr menses and one PLATE (SEVEN mavens)
The hypophysis of mammals, from an embryological point of View, consists of two parts——the pars buccalis and the pars neura1is——which arise respectively from the roof of the stomadeum and the ﬂoor of the forebrain. In the period when the developing embryo was conceived to be a mosaic of numerous, predetermined organ anlagen, the hypophysis was thought to arise from two distinct, separately determined anlagen which as a result of their development were joined to form one organ. Recent investigations tend to show that the hypophysis arises as a single structure whose development is peculiarly dependent on its position in the median prechordal region of the head. Thus although many textbooks and special treatises l(e.g., Shumway, ’27; Buoy, ’32; Cushing, ’32; Jordan, ’34) still describe the hypophysis as developing from two distinct evaginations which approach each other, meet and fuse, every known investigation of the early development of the mammalian hypophysis has shown that the roof of the stomodeum and the ﬂoor of the neural tube are closely adherent to each other in the future hypophyseal region before either hypophyseal outpocketing appears, and that this original adherence persists throughout the early development of the hypophysis (Minot, 1897; Parker, ’17; Kingsbury and Adelmann, ’24; Hochstetter, ’24; Schwind, ’28; Brahms, ’32; Gilbert, ’34). Minot suggested that this ‘tdherence of the stomodeal epithelium to the ﬂoor of the forebrain during the early growth of the head is an important mechanical factor in determining the formation of the hypophyseal evagination from the roof of the stomodeum, and this conclusion has been supported by subsequent investigations (pig, Nelson, ’33; rat, Schwind, ’28; cat, Brahms, ’32; Gilbert, ’34). These investigations have shown that in the early embryo the ventral surface ectoderm of the head is closely adherent to the ﬂoor of the neural tube over a median area lying just cephalad of the oral membrane. As the neural tube expands cephalad and then ventrad around the anterior end of the foregut, the ventral suface of the embryonic head is swung ventrad and caudad. This results in the‘ formation of an acute angle between the surface of the head and the oral membrane. This angle is the ‘hypophyseal angle’ (Mihalkovics, 1877) and its rostral wall is formed by that area of ectoderm which is adherent to the ﬂoor of the forebrain. As increasing amounts of mesenchyme grow in between the ectoderm and the neural tube, the surface ectoderm is pushed farther and farther away from the neural tube, except around the hypophyseal angle where the adherence of the stomodeal ectoderm to the ﬂoor of the forebrain persists. This results in the formation of an ectodermal pocket whose apex lies against the brain ﬂoor and whose mouth opens into the stomodeum. This is Rathke’s pouch, the early pars buccalis of the hypophysis. Its formation seems to be the result of the interaction of three developmental processes: 1) the persisting adherence of the surface ectoderm to the neural tube over a limited median area. on the ventral surface of the head; 2) the overgrowth and marked expansion of the neural tube around the apex of the foregut; 3) the general development of mesenchyme between the ventral head ectoderm and the ﬂoor of the forebrain, and the bilateral expansion of this mesoderm accompanying the expansion of the neural tube. The last two factors are general processes characteristic of the development of the entire head, and it was, in part, their role in the development of the hypophysis which led Kingsbury and Adelmann (’24 p. 264) to conclude that: “the hypophysis as a structure is determined by the mode of growth of the whole head.” The ﬁrst factor appears to be peculiarly and directly related to the development of Rathke’s pouch. Any attack on the prob— lem of hypophyseal development must, therefore, be grounded on -an analysis of this region of neuro-ectodermal adherence. The development of the pars neuralis of the hypophysis as an outpocketing from the ﬂoor of the diencephalon is also directly linked with the occurrence of this neuro-ectodermal adherence. The author has shown (Gilbert, ’34) that in cat embryos the pars neuralis cannot develop as an active evagina— tion from the infundibular region of the brain floor (as is usually stated in textbooks), since practically no mitoses occur in this region of the brain during the period of hypophyseal development. It was found that this region of inactivity in the brain ﬂoor includes that area of neural epithelium which is adherent to the epithelium of Rathke’s pouch, and a small area of brain wall immediately surrounding this adherence, or in other words, all of the neural epithelium normally involved in the formation of the pars neuralis. This component of the hypophysis must arise, therefore, under the inﬂuence of growth processes occurring in the adjacent regions of the brain ﬂoor where mitoses are numerous. Analysis of the growth rates and shiftings occurring in surrounding regions of the brain and head suggested that the formation of the pars neuralis as an outpocketing from the diencephalic ﬂoor is the result of two developmental factors, namely, the presence in the brain floor of inactive tissue which is ﬁrmly adherent to the apex of Rathke’s pouch, and the pressure exerted on this region by the adjacent rapidly growing regions. The contours of the diencephalic ﬂoor. are such that the rostral part of the inactive region is subjected to a cephalo-dorsally directed pressure, while the caudal region is subjected to a caudo-ventrally directed pressure. The result of these'combined growth pressures is the rotation of the inactive infundibular region of the brain ﬂoor, and the adjacent wall of Rathke’s pouch, from a dors0—ventral to a cephalo-caudal plane. This rotation of a small segment of the brain ﬂoor produces a small depression or outpocketing in the brain floor which becomes the pars neuralis of the hypophysis (Gilbert, ’34, ﬁgs. 3 and 4). On the basis of these observations it Was concluded that in the cat the pars neuralis of the hypophysis, as Well as the pars buccalis, develops as a result of the reaction of the median region of neuro-ectodermal adherence to the general growth processes occurring Within the prechordal region of the head. Such a conclusion makes the analysis of the origin and nature of this region of neuro—ectodermal adherence doubly important in an attack on the problem of hypophyseal development. ’
The question then arises: can this interpretation of hypophyseal development in the cat be extended to include other mammals? Careful study of embryos of dog, rat, calf, pig, and man has shown a similar scarcity of mitoses in the infundibular region of the brain during hypophyseal development. In these forms too, then, the pars’ neuralis of the hypophysis must develop as a. result of growth processes occurring in surrounding regions of the head.
Descriptive and experimental evidence suggests that the hypophysis does not develop as a result of the presence of predetermined potencies for hypophyseal development in the tissues involved, but rather that it is determined by the normal conﬁguration of materials and growth processes in the prechordal region of the head. The evidence for this conclusion comes _from studies of cyclopean embryos (amphibian), in which the hypophysis is frequently lacking (Adelmann, ’34); from the experimental studies of the embryonic hypophysis, which have shown that hypophyseal development depends on the normal position and relations of its anlage (Smith, ’20; Blount, ’32; Stein, ’33) ; and from descriptive analyses of hypophyseal development (Kingsbury and Adelmann, ’24; Schwind, ’28; Brahms, ’32; Gilbert ’34). It is further supported by the evidence presented in this paper concerning 1) the origin of the neuro—ectodermal adherence and its role in normalihypophyseal development; and 2) the mode of formation of the pars neuralis in various mammals.
Fig.1 Diagrams (X 80) of median sagittal sections of the anterior end of young rat embryos. The median surface of the neural plate is cross—hatched, the surface ectoderm is dotted, and regions of neuro-ectodermal conﬂuence are marked by dashes. The outline of the lateral neural folds is indicated with a broken line. Figures A-E are based on Butel1er’s (’29) reconstructions (his ﬁgs. 1 to 5). A, 1 s0m.; B, 2 som.; C, 4 30111.; D, 5 som.; E, 6 som.; F, 8 son1.; G, 12 som. an, anterior neuropore; fg, for-egut; hyp, hypophyseal region; nf, neural folds; om, oral membrane; pc, pericardial region; to, torus opticus.
Origin of the Neuro-Ectodermal Adherence
The origin and early development of the hypophyseal region of the head has been studied primarily in embryos of rat and man. The material utilized consisted of forty-ﬁve series of rat embryos of all stages from the embryonic disc (no somites) through closure of the anterior neuropore (20-23 somites), and eighteen series of human embryos of all stages from the beginning of somite formation through closure of the anterior nenropore. The rat embryos and one human series are in the collection of the Department of Histology and Embryology, Cornell University.‘ Sixteen of the human series are in the collection of the Carnegie Institution of Embryology,” and for the privilege of studying these embryos the author is indebted to Dr. G. L. Streeter. One human embryo, from the collection of the Department of Anatomy, Rochester University Schoolof Medicine, was kindly loaned to the author by Dr. G. W. Corner.
Rat embryos. The early embryonic disc of the rat corresponds closely to that of the pig as described by Streeter (’27). The primitive streak and Hensen’.-2, node are well deﬁned. The notochordal plate is associated with the endoderm and ends anteriorly in a distinct patch of columnar cells which may be identiﬁed as the prechordal plate (pp., ﬁg. 5). Anterior to the prechordal region a slender band of pericardial mesoderm connects the two sheets of lateral mesoderm across the mid—line. The ectoderm is markedly thickened throughout the neural plate. As the neural folds form (1-somite stage, ﬁg. 1 A; see also Adelmann, ’25) mesenchyme cells ﬁll the cavity of the folds. The head fold, which appears coincidently with the second somitic cleft (ﬁgs. 1 B, 6) develops as a fold of the ectoderm and endoderm caudal to the pericardial mesoderm.
- This investigation was aided through the constant advice and encouragement of Dr. B. F. Kingsbury, to whom I express my gratitude.
‘Two som. (GC.1878); 4 som. (CC. 3709); 7 som. (CC. 4216); 8 som. (CC. 391); 9 som. (CCA251); 13 50111. (CC.318); 13 som. (004783); 14 som. (Co. 779); 14 30111. (cc.4529); 16 som. (cc.47o); 17 som. (cc.5s72); 22 som.
(CCX4736); 2 mm. (CC. 250); 3.4 mm. (CC. 6079); 4 mm. (CC. 5923); 4.5 mm. (CC. 6500).
This process places the prechordal plate in the antero-dorsal wall of the foregut, makes the small stretch of eetodermn and endoderm which lies between the prechordal plate and pericardial mesoderm into the oral membrane, and shifts the pericardial mesoderm to a more ventral and caudal position. The neural plate and roof of the archenteron remain in _intimate contact throughout this stage of development (1-4 somites), no mesodem other than pericardial occurring medial to the lateral walls of the foregut (ﬁgs. 6, 7). The ‘median ectoderm of the neural plate remains of uniform thickness as it passes around the apex of the foregut and in the oral plate
it changes gradually to the typically thin body ectoderm which covers the pericardial prominence (ﬁg. 7 ). N o deﬁnite anterior boundary of the neural plate can be identiﬁed even though laterally the neural plate and surface ectoderm are clearly distinguishable (Adelmann, ’25, ﬁg. 9). The neural folds are well developed laterally and anteriorly where they project beyond the median anterior limit of the head fold (ﬁg. +1 B-C). This forward projection of the lateral neural folds determines the appearance of the median ‘terminal notch’ which is such a characteristic feature of the early neural plate in mammalian embryos (rat, Adelmann, '25; man, Bartelmez and Evans, ’26, Corner, ’29; cat, Schulte and Tilney, ’15; pig, Heuser and Streeter, ’29). In the rat, -at least, it results from the failure of the median material of the neural plate to participate in the marked expansion which occurs in the lateral neural folds. A study of the various developmental factors which might be related to this difference in behavior of the median and lateral parts of the neural plate is beyond.’ the scope of this investigation, but attention may be directed to the fact that it is solely that part of the neural plate which is not underlain by mesenchyme that fails to take part in the early formation and growth of the neural folds. It is this lack of mesenchyme under the median anterior or prechordal part of the neural plate, correlated with the manner of formation of the ﬂoor of the forebrain, which determines the development of the hypophyseal region of neuro-ectodermal adherence.
As the lateral neural folds continue to expand anteriorly they are shortly joined across the midline anterior to the foregut by a transverse ridge of ectoderm, the torus opticus (ﬁg. 1D). This ridge, which deﬁnitely marks the anterior boundary of the neural plate, is not a fold of ectoderm comparable to the lateral neural folds in having an internal thickened neural layer and an external thin ectodermal covering, but it is a solid outgrowth from the prechordal neural plate in which neither neural plate nor surface ectoderm are distinguishable (ﬁgs. 8, 9). As this outgrowth from the prechordal neural plate increases in length, the cells in the caudal part of this ridge gradually become arranged into two epithelia, neural plate and surface ectoderm (ﬁgs. 1 E to G, 9 to 11). As a rule these two layers of cells do not immediately become distinct epithelia since the limiting membrane between them is slow in developing, irregular in appearance, and incomplete_in extent, with cells frequently lying across the apparent boundary between the two epithelia. This segregation of the cells into two epithelia is most regular and rapid in the middle of the growing ridge, and is delayed at the cephalic and caudal margins. The cephalic margin (ventral lip of the anterior neuropore) continues as a solid undifferentiated mass of cells until after the closure of the anterior neuropore. In the caudal part of this region the differentiation into two epithelia proceeds slowly and irregularly, the basement membrane between the cell layers appearing to be incomplete for some time after the separation of surface ectoderm from neural tube has been completed in more anterior regions. This area, where separation of the surface ectoderm and neural epithelium is delayed, becomes the hypophyseal region of neuro-ectodermal adherence. Caudally it ends abruptly a.t the point Where this solid outgrowth from the prechordal neural plate started, i.e., immediately in front of the prechordal plate. Laterally it ends at the medial edge of the mass of mesenchyme which projects into the lateral neural fold. Early in development a blood Vessel, the future internal carotid artery, develops in the medial edge of this lateral head mesoderm, running anteriorly from the ﬁrst aortic arch along either side of the prechordal plate and torus opticus. Thus the internal carotid artery characteristically lies on either side of the hypophyseal region of neuro-ectodermal adherence throughout early development.
During this period ( 5 to 13-somite stage) no mesenchyme is present in the head medial to the lateral walls of the foregut, but abundant mesenchyme completely underlies the lateral parts of the neural plate. As the anterior neuropore closes, this lateral mesenchyme grows cephalad, ventrad and mediad around the optic vesicles, gradually separating the surface ectoderm from the neural tube over the anterior surface of the head. At the same time, the prechordal plate is raised out of the roof" of the foregut as a keel-shaped mass of mesenchyme lying between the reestablished roof of the foregut and the ﬂoor of the neural tube. Normally neither this prechordal mesoderm nor the lateral head mesenchyme grows into that median ventral region of the head which was formed by the outgrowth of a solid bud of cells from the prechordal neural plate. As a result, after closure of the anterior neuropore, there remains on the ventral surface of the head a small area in which the surface ectoderm and neural tube are in close contact. In the caudal part of this region of contact the ectoderm and neural tube are always closely adherent to each other, and in many cases the cells are not arranged into two completely distinct epithelia. This is the prospective hypophyseal region of neuro-ectodermal adherence.
Human embryos. A similar developmental sequence leads to the formation of the hypophyseal region in human embryos. The head fold appears approximately coincident with the onset of somite formation, and as in the rat, it consists of a folding forward in the median plane of the ectoderm and the subjacent roof of the archenteron. The ectoderm of the head fold cannot be divided into neural plate and oral plate ectoderm. This was determined by a study of the 2—somite Ingalls’ embryo (ﬁg. 2 a). The failure of the lateral neural folds to be connected across the rostral mid-line was observed by Ingalls ( ’20), since he remarks that: “The median groove (neural groove) is continued over the anterior surface of the head, spreading out and terminating in the bucco-pharyngeal membrane just above the attachment of the somatopleure” (p. 66). In this young human embryo, as in the rat, the neural plate cannot be bounded at its rostral end even though its lateral boundaries are distinct, as is shown in a section through the neural plate of the Ingalls’ embryo by Bartelmez and Evans (’26, ﬁg. 30). As development proceeds the lateral neural folds are joined across the median plane rostral to the foregut by a torus opticus, which consists at first of a solid mass of cells in which neither ectoderm nor neural plate can be deﬁnitely distinguished (ﬁg. 2b). Bartelmez and Evans (’26, p. 29) describe this condition in a. 4-somite embryo (University of Chicago collection, H 279) as follows: “The ﬁrst point to attract attention is the growth of the nervous system beyond the end of the pharynx. At the growing tip the neural groove is shallow as compared with the rest of the forebrain, and it is impossible to ﬁx the boundary between neural and somatic epithelium.” That an actual fusion of ectoderm and neural plate exists in the terminal part of the neural plate floor was determined by a study of the photographs of each section of this embryo, which have been deposited in the Carnegie Collection (00. 3709). This conﬂuence of neural plate and surface ectoderm in the torus opticus was also observed in the 7 -somite Payne embryo (ﬁg. 2 b). In this embryo, the presence of mesenchyme between the neural plate and surface ectoderm on either side of this median area of fusion causes the appearance of a slight median groove in the surface ectoderm, which Payne (’25) properly designates as the anlage of the oral hypophysis. In the 8-somite Dandy embryo (ﬁg. 2 cf) the cells within the caudal part of the torus opticus have been segregated into two epithelia. The onset of differentiation of this originally solid plate of. cells into neural and somatic. epithelia varies in different embryos, if developmental stage is determined on the basis of number of somites formed. Thus although this differentiation has begun in the Dandy embryo, the anterior neuro-somatic boundary has not yet appeared in the 8-somite Veit embryo’ and the 10-somite Corner embryo. Veit and Esch (’22, p. 353) described the cranial end of the neural plate in their 8-somite embryo as follows: “Die Hirnanlage—biegt dann aber, erst langsam, dann rasch, nach ventral urn in Kriimmung iiber das Kranialende des Vorderdarmes und endet mit sein Spitze in der Rachenmembran, gegen welche sie nicht abzugrenzen ist.” Veit (’19) in his earlier study of the embryo ﬁgured a transverse section through the caudal part of the torus opticus (his fig. 6), which shows this continuity of neural and somatic ectoderm. The relation of this median neuro-ectodermal conﬂuence to the lateral neural folds is well shown in Corner ’s model of the 10-somite embryo (ﬁg. 2 d). In the 13-somite Wallin embryo (fig. 2 e) the ﬂoor of the neural tube from the anterior neuropore to the level of the prechordal plate is closely adherent to the surface ectoderm, and in the median plane the two epithelia are conﬂuent. In the 14-somite Athey embryo the brain ﬂoor and surface ectoderm are distinct but closelyadherent epithelia, except in the future hypophyseal region where the two lamina are still conﬂuent. This embryo offers distinct support for the contention that there is a real adherence between surface ectoderm and forebrain ﬂoor, in that the shrinkage attendant upon ﬁxation of the embryo produced a. transverse split within the brain ﬂoor rather than causing the separation of the two epithelia. The reverse condition, however, is found in the 16-somite Mitchell embryo (ﬁg. 2 f), where the obviously distinct ectoderm and brain floor are separated by a slight shrinkage space. Other embryos than the ones described were also studied (see footnote 2), and in every case the condition of the developing floor of the forebrain and its relation to the surface ectoderm supported the hypothesis advanced above, namely, that the development of this region of the neural plate‘ and tube occurs in the same fashion in man and rat.
Fig.2 Diagrams (X 80) of median sagittal sections of the anterior end of young human embryos. Markings and abbreviations as in ﬁgure 1. Diagrams A, C, E, and F are taken from Bax-telmez and Evans (’26), B from Payne (’25), and D from Corner (’29). The boundaries between neural plate, region of neuroectodermal conﬂuence, and ectoderm are the author’s interpretation based on study of sections of the embryos. A, 2 som.; B, 7 som.; C, 8 sum; D, 10 som.; E, 13 801.11.; F, 16 som.
Other mammals. It seems probable that the manner of growth of the rostral part of the neural plate and the concommittant history of the head mesenchyme as described above for the rat and man, are essentially the same in many mammals. The early development of the pig, as described by Streeter (’27), corresponds closely to that of the rat. Unfortunately Streeter’s descriptions follow the pig only to the EARLY DEVELOPMENT or MAMMALIAN HYPOPHYSIS 349
stage where the torus opticus is beginning to develop (7 somites). The careful study of the early development of this region in the rabbit by Aasar (’31) indicates that the same developmental conditions occur in this mammal". Aasar’s diagrams (his ﬁgs. 10 to 13) and photographs (his ﬁgs. 16, 21.) of median sagittal sections through the prechordal region of rabbit embryos with 6 to 10 somites correspond closely with the similar ﬁgures of the rat which are given in this paper. Young cat and dog embryos (Cornell collection) in which the neuropore has just closed show the same median ventral area of neuro-ectodermal adherence or even conﬂuence which is seen in rat and human embryos of a comparable age. Summary. The development of this prechordal region of neuro-ectodermal adherence seems to be an integral part of the early development of the head region during closure of the anterior neuropore in many mammalian embryos. It apparently results from: 1) ' the failure of the median prechordal neural plate material to take part in the typical formation of neural folds; 2) the subsequent outgrowth from the prechordal neural plate of the torus opticus, in which neural plate and surface ectoderm are indistinguishable; 3) the delayed segregation of the cells within the future hypophyseal region into neural plate and surface ectoderm; 4) the complete absence of mesenchyme from this region of the head. The importance of the last named factor is indicated by the failure of the hypophysis to develop in experimentally produced cyclopean embryos (Amblystoma) where an abnormal massing of mesoderm in the median prechordal region of the head frequently occurs (Adelmann, ’34). When the normal bilateral expansion of the mesoderm occurs, the development of this median region of neuro-ectodermal adherence clearly depends on the peculiar character of the median prechordal region of the neural plate. This part of the medullary anlagen fails to take part in the marked expansion, the formation of true neural folds, and the development of a sharp neurosomatic bounda.ry—-processes which characterize the lateral part of the neural plate. From the mode of development of the prechordal floor of the neural tube and the adherent surface ectoderm it is clear that these two epithelia arise as a single structure from a common source, namely the neural ectoderm which ﬁrst covers the apex of the foregut, and are only secondarily separated into two epithelia with distinct prospective fates. Explanation of this peculiar behavior of the prechordal neural plate region therefore becomes one of the problems which must be included in any analysis of the early development of the head region in mammalian embryos.
Early Development of the Pars Neuralis
The importance of this neuro-ectodermal adherence in the development of both the pars -buccalis and pars neuralis of the hypophysis has been discussed in a previous paper (Gilbert, ’34), which described early hypophyseal development in the cat. The conclusions drawn from that investigation have been summarized in the introduction to this paper. As pointed out there, the evidence on which the conclusions concerning the pars neuralis were based consisted primarily of two types of observations: 1) the scarcity of mitoses in the infundibular region of the brain ﬂoor, and the abundance of mitoses in surrounding regions of the brain in embryos of all stages during hypophyseal development; 2) the formation of the hypophyseal diverticulum from the brain ﬂoor through the rotation of the neuro-ectodermal plate (region of neuro-ectodermal adherence), as a result of the manner in which the growth pressures from the surrounding regions were exerted on the inactive infundibular region of the brain ﬂoor.
The previous paper presents a detailed analysis of the way in which these two processe affect hypophyseal development in the cat. This paper presents evidence to show that the same developmental conditions exist in the embryos of other mammals. Since the analysis and interpretation of the data with reference to hypophyseal development are the’ same for these mammals as for the cat, the reader is referred to the previous paper for a discussion of the signiﬁcance of these data in relation to the development of each component of the hypophysis.
A scarcity of mitoses in the infundibular and adjacent regions of the brain ﬂoor during the period of hypophyseal development was observed in numerous embryos ofr cat, dog,
rat, calf, pig, and man.
In the accompanying table, the
Data concerning the distribution of mitoses in the diencephalic ﬂoor and the rotation of the neu/ro-Bctodermal plate in various mammals
MITOSE S FOUND IN
ANGLE OF NEUROECTODERMAL PLATE VVITH AXIS OF THE
Optic 535$ Pu‘:l’I’$’;" M‘f?_’;ﬂ' vnnman PLICA
mm. degrees Cat 4 4 2 2 1 11 102 6 14 4 0 5 29 80 8 20 0 0 0 33 68 9 49 0 O 0 88 32 Dog 3 25 11 8 16 28 135 5 53 14 5 23 42 120 7 65 14 4 0 38 75 9 92 28 4 O 130 62 13 128 18 4 1 145 25 Man 6 4 0 0 0 10 153 7 19 0 2 10 30 . . . 9 27 0 0 4 20 107 11 . . . . . . . . . . 50
12 7 0 3 9 15 30 25 0 0 9 19
Hat 3 25 4 8 9 16 111 5 36 0 1 3 53 55 6 31 7 3 2 28 49 7 28 8 O 2 58 35 Calf 6 7 6 11 8' 26 95 9 14 15 13 15 15 90 11 68 21 13 7 60 70 12 71 33 8 9 47 -15 14 65 15 10 7 30 25 Pig 6 9 9 16 10 so 160 9 4 3 6 18 10 101 11 13 5 7 12 68 55 14 8 4 9 24 130 45 20 11 1 1 O 50 15
Figs. 3and 4 Diagrams (X 20) of median sagittal sections through the brain ﬂoor and hypophysia of embryos of cat (33,-c), calf (3d-f), rat (43-c), pig (4d—f), dog (4g-i), and man (4j-1). In 3a., a section of the entire head, the part shown in the other diagrams is enclosed in broken lines. In each diagram the angle which the neuro-ectodermal plate makes with the axis of the ventral plica (ash in 3a) is stated. The rotation of the neuro-eetodermal plate during formation of the pars neuralis of the hypophysis can be observed in each species. bh, pars buocalis; in, infundibular region; nh, pars neuralis; op, optic region; pi, post-infundibular region; pm, premammillary region; po, post-optic region.
number of mitoses in the various regions of the brain ﬂoor in representative embryos of various sizes from each of the species is recorded. For the purpose of determining the distribution of mitoses in various regions, the diencephalic floor has been arbitrarily divided into ﬁve regions (ﬁg. 3 A): 1) optic; 2) post—optic; 3) infundibular region——that part of the brain ﬂoor which is adherent to the wall of Rathke’s pouch; 4) post—infundibular, which eventually forms the dorsal wall and neck of the pars neuralis; 5) premammillary. Mitoses are relatively numerous in the optic and premammillary regions at all stages, and scarce in the infundibular region, and, in many instances, in the adjacent post-optic and postinfundibular regions. It is these last three regions which comprise the inactive region of the diencephalic ﬂoor, and which are subjected to the growth pressures of the adjacent optic and premammillary regions.
The rotation of the neuro-ectodermal plate, which results from the mode of action of these growth processes on the inactive region, can be best analyzed when the position of the neuro-ectodermal plate in each embryo is determined from a standard line of reference. The line chosen must be one which lies near to the diencephalon, but is not shifted by movements occurring within the brain. The line which best satisﬁes these conditions is the axis around which the cranial ﬂexure occurs—— the axis of the ventral plica (line a—b, ﬁg. 3 A). If the plane occupied by the neuro-ectodermal plate in each successive stage of development is projected onto this line of reference, changes in the position of this region may be readily measured. In the table and in ﬁgures 3 and 4 the position of the neuroectodermal plate in each embryo is expressed as the angle which its plane of position makes with the axis of the ventral plica. These ﬁgures and diagrams show how the rotation of the neuro-ectodermal plate from a dorso-ventral to a cephalocaudal plane results in the formation of a small pocket in the ﬂoor of the diencephalon, which becomes the pars neuralis of the hypophysis.
The evidence presented here oifers additional support for the conclusion reached in the previous investigation——that the neural lobe of the hypophysis is formed by the action of two rapidly growing regions of the brain on an inactive region which lies between them. On the basis of this evidence, and of similar evidence regarding the development of the pars buccalis of the hypophysis (Kingsbury and Adelmann, ’24; Schwind, ’28; Brahms, ’32) it may be suggested that the early development of the entire hypophysis depends on the normal conﬁguration of materials and growth processes in the prechordal region of the embryonic head, rather than on a localized determination of hypophyseal development.
1. Evidence presented in other investigations of the early development of the mammalian hypophysis has been summarized to show that the initial development of the pars buccalis and pars neuralis is primarily dependent on the early adherence of the stomodeal epithelium to the ﬂoor of the forebrain in the hypophyseal region of the embryonic head.
2. The origin and mode of development of this adherence between stomodeal and neural epithelia has been studied in early embryos of rat and man. It has been shown to arise during closure of the anterior neuropore as a result of the peculiar manner of development of the median prechordal part of the neural plate. Whereas the lateral parts of the developing neural tube are formed by neural folds in which neural and somatic epithelia are separate and sharply bounded epithelia, the median prechordal part of the neural tube is formed by the outgrowth from the ectoderm of the head fold
of a solid mass of cells in which the neural and somatic epi thelia are conﬂuent. As contraction of the anterior neuropore is occurring, the cells of this median prechordal region are gradually segregated into internal neural and external somatic epithelia. The separation of the two epithelia occurs most slowly in the future hypophyseal region. This occurrence coupled with the withdrawal of mesoderm from the median plane which accompanie the bilateral expansion of the forebrain and optic vesicles, results in the persistence of a neuro-ectodermal adherence in the median ventral pre-chordal region of the head.
3. Evidence is presented to show that in embryos of cat, dog, rat, calf, pig and man, the pars neuralis of the hypophysis cannot develop as an active outgrowth from the diencephalic ﬂoor, since growth as measured by mitotic proliferation is exceedingly small in the infundibular region of the brain ﬂoor during hypophyseal development.
4. The formation of the pars neuralis has been shown to occur as a result of the reaction of growth processes occurring in surrounding regions of the brain on the inactive infundibu V lar region, which is ﬁrmly adherent to the developing pars buccalis of the hypophysis.
5. Evidence presented in this paper offers additional support tolcertain conclusions regarding the manner of development of the hypophysis, which were discussed in detail in another paper.
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5 Rat, 1 somite. X 160. A median sagittal section of the anterior part of the embryonic disc before the appearance of the head fold. Anterior end to the right. Compare with 1a.
6 Rat, 2 somiltes. X 160. A median sagittal section through the head fold and foregut. Rostral end to the right. Compare with lb.
7 Rat, 4 somites. X 160. A sagittal section which is median for the rostral end of the neural plate and foregut. Rostral end. to the left. Compare with 1 C.
8 Rat, 5 somites. X 160. A sagittal section which is median for the rostral end of tlie neural plate and foregut. The rostral end of the embryo (left) here consists of a mass of tissue, four nuclei in length, in which the ﬂoor of the neural plate and surface ectoderm of tl1e oral membrane are conﬂuent. Compare with 1 (1.
9 Rat, 5 somites. X 160. A sagittal section which is median for the rostral end of the neural plate. The mass of tissue in which neural plate and; surface cctoderm are conﬂuent is seven to eight nuclei in length. Rostral end to the right.
10 Rat, 6 somites. X 160. A sagittal section which is median for the rostral end of the neural plate. The cells of the rostrall mass of tissue are sorted into neural plate and surface ectoderm ahead of the prcehordal plate. Rostral end to right. Compare with 1 e.
11 Bat, 12 somites. X 160. A nearly median sagittal section through the an— terior end of an embryo with a small neuropore. The floor of the forebrain is closely adherent to the surface ectoderm from the level of the prechordal plate to the Ventral lip of the anterior neuropore. Rostral end to the left. Compare with lg. an, anterior neuropore; fg; foregut; hyp, hypophyseal region; np, neural plate; om, oral membrane; pc, pericardial cavity; pp, yrechordal plate; to; torus options.
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