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Some factors influencing the early development of the mammalian hypophysis.

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SOME FACTORS INFLUENCING T H E EARLY DEVELOPMENT O F T H E MAMMALIAN HYPOPHYSTS
MARGARET SHEA GILBERT
Laboratory of Histology and Embryology, Cornell University, Ithaca, NFW Pork
FOUR TEXT FIQUBES AND ONE PLATE (SEVEN FIQmES)
The hypophysis of mammals, from an embryological point
of view, consists of two parts-the pars buccalis and the pars
neuralis-which arise respectively from the roof of the stomodeum and the floor of the forebrain. I n 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 (e.g., Shumway, '27 ; Bucy, '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 adherence of the
stomodeal epithelium to the floor of the forebrain during the
337
THE ANATOMICAL RECORD,
VOL. 62, NO. 4
338
MARGARET SHEA GILBEBT
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
rostra1 wall is formed by that area of ectoderm which is adherent t o 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 eetodermal 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 t o 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
EARLY DEVELOPhlENT OF MAMMALIAN HYPOPEYSIS
339
first factor appears to be peculiarly and directly related to
the development of Rathke’s pouch. Any attack on the problem 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 evagination 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 rostra1 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 dorso-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
TEE ANATOYICAZ REUORW, VOL. 62, NO. 4
340
MARGARET SHEA GILBERT
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
hy-pophyseal 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. I n 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 coilclusion 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 normal hypophyseal development; and 2) the
mode of formation of the pars neuralis in various mammals.
EARLY DEVELOPMENT OF MAMMALIAX HYPOPHYSIS
341
F i g . 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 mural folds is indicated with a broken line.
Figures A-E are based on Butcher’s (’29) reconstructions (his figs. 1 to 5 ) . A,
I som.; B, 2 som.; C, 4 som.; D, 5 som.; E, 6 som.; F, 8 som.; G, 12 som. an,
anterior neuropore; fg, foregut; hyp, hypophyseal region; nf, neural folds; om,
oral membrane ; pc, pericardial region; to, torus opticus.
342
MARGARET SHEA GILBERT
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
neuropore. The rat embryos and one human series are in
the collection of the Department of Histology and Embryology, Cornell University.l Sixteen of the human series are
in the collection of the Carnegie Institution of Embryology,2
and for the privilege of studying these embryos the author
is indebted to Dr. Q. L. Streeter. One human embryo, from
the collection of the Department of Anatomy, Rochester
University School of Medicine, was kindly loaned to the author
by Dr. Q. W. Corner.
Rat embryos. The early embryonic disc of the rat corresponds closeIy to that of the pig as described by Streeter ('27).
The primitive streak and Hensen's 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. 1B, 6) develops as a fold of the
ectoderm and endoderm caudal to the pericardial mesoderm.
Thin investigation was aided through the constant advice and encouragement
of Dr. B. F. Kingsbury, to whom I exprees my gratitude.
Two som. (CC.1878) ; 4 som. (CC.3709) ; 7 aom. (CC. 4216) ; 8 som. (CC.
391); 9 som. (CC.4251); 13 aom. (CC.318); 13 som. (CC.4783); 14 aom.
(CC.779) ; 14 som. (CC.4529) ; 16 aom. (CC. 470) ; 17 8om. (CC.5872) ; 22 aom.
(CC.4736) j 2 mm. (CC. 250) ; 3.4 mm. (CC. 6079); 4 mm. (CC.5923) ; 4.5 mm.
(CC.6500).
EARLY DEVELOPMENT OF MAMMALIAN HYPOPHYSIS
343
This process places the prechordal plate in the antero-dorsal
wall of the foregut, makes the small stretch of ectoderm 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). No 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. 1B-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.
344
MARGARET SHEA GILBERT
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.1 D ) . 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 a n 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. I n
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 a t 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
EARLY DEVELOPMENT O F MAMMALIAN HYPOPHYSIS
345
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. A s 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. I n 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.
Humaa 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. Th’e 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 rostra1 mid-line was observed
346
MARUARBT SHEA GILBERT
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). I n 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. 2 b). 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 (CC. 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).
I n 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. I n
the 8-somite Dandy embryo (fig. 2 c) 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
EARLY DEVELOPMENT OF MAMMALIAN HYPOPHYSIS
347
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
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 Bartelmez 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 baaed on study
of sections of the embryos. A, 2 sorn.; B, 7 sorn.; C, 8 som.; D, 10 som.; E, 13
sorn.; F, 16 som.
348
MARGARET SHEA GILBERT
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 thiR 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). I n 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. I n
the 14-somite Athey embryo the brain floor and surface ectoderm are distinct but closely adherent 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.
Other mammals. It seems probable that the manner of
growth of the rostra1 part of the neural plate and the concommittant history of the head mesenchyme as described
ebove for the rat and man, are essentially the same in many
mammals. The early development of the pig, as described by
Streeter ( ’ W ) , corresponds closely to that of the rat. Unfortunately Streeter’s descriptions follow the pig only to the
EARLY DEVELOPMENT O F 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.
Xzlmrnary. 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 CODplete 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 boundary-processes which characterize the lateral
part of the neural plate. From the mode of development of
350
MARGARET SHEA GILBERT
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
iafundibular 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 processes 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.
351
EARLY DEVELOPMENT OF MAMMALIAN 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 of. cat, dog,
rat, calf, pig, and man. In the accompanying table, the
TABLE 1
Data concerning the distribution of mitoses in the diencephalic goor and the rotation of the newo-ectoderml plate in various mammals
MITOSES POUND IN
-__
SIZE
-
ANUliE OF NEUBOCCTODEEXAL PLATE
WITH AX18 OF THE
VENTRAL PLICA
Premammillary
Optic
Postoptic
Infundibular
4
14
2
0
0
0
1
5
0
0
11
29
20
49
2
4
0
0
88
102
80
68
32
25
11
53
65
14
8
5
28
42
38
130
145
135
120
75
62
25
10
153
mm.
Cat
4
6
8
9
Dog
Man
Calf
92
128
14
28
18
4
4
4
16
23
0
0
1
6
7
9
4
19
27
0
0
0
0
2
0
0
10
4
30
3
5
6
7
6
9
11
12
14
Pig
33
3
5
7
9
13
11
12
30
Rat
degreea
6
9
11
14
20
20
..
..
..
..
107
50
1
25
0
0
3
0
9
9
15
19
..
..
25
4
0
7
8
8
1
3
0
9
3
16
53
6
15
21
11
I
.
36
31
28
7
14
68
71
65
9
4
13
8
11
33
15
13
13
8
10
16
6
7
9
1
..
2
28
58
111
55
49
35
8
15
7
9
7
26
15
60
47
30
95
90
70
45
25
10
18
12
24
30
10
68
130
50
160
101
55
2
0
45
15
352
MARGARET SHEA GILBERT
.-
D.11 m m
C.9 m m
F:
CAT.
I
I
CALF.
Figs. 3 and 4 Diagrams ( x 20) of median sagittal sections through the brain
floor and hypophysis of embryos of cat (3a-c), calf (3d-f), rat (4a-e), pig (4d-f ) ,
dog (4g-i), and man (4j-1). I n 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 (a-b 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 buccalis;
in, infundibular region; nh, pars neuralis ; op, optic region; pi, post-infundibular
region; pm, premammillary region; PO, post-optic region.
EARLY DEVELOPMENT OF MAMMALIAN HYPOPHYSIS
A.4mm
RAT.
PIG.
DOG.
Figure 4
MAN.
353
354
MARGARET SHEA GILBERT
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 occursthe 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.
I n 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.
EARLY DEVELOPMENT O F MAMMALIAN HYPOPHYSIS
355
The evidence presented here offers 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.
SUMMARY
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 epithelia are confluent. As contraction of the anterior neuropore is occurring, the cells of this me'dian 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 accompanies the bilateral expansion of
356
MARGARET SHEA GILBERT
the forebrain and optic vesicles, results in the persistence of
a neuro-ectodermal adherence in the median ventral prechordal 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 infundibular region, which is firmly adherent to the developing pars
buccalis of the hypophysis.
5. Evidence presented in this paper offers additional support to certain conclusions regarding the manner of development of the hypophysis, which were discussed in detail in
another paper.
LITERATURE CITED
AASAR, Y. H. 1931 The history of the prochordal plate in the rabbit. J. Anat.,
vol. 66, pp. 14-45.
ADELMANN,H. B. 1925 The development of the neural folds and cranial ganglia
of the rat. J. Comp. Neur., vol. 39, pp. 19-171.
1934 A study of cyelopia in Amblystoma punctatum with special
reference to the mesoderm. J. Exp. Zoiil., vol. 67, pp. 217-281.
BARTELMEZ,
cf. W. AND H. M. EVANS1926 Development of the human embryo
during the period of somite formation, including embryos with 2-16
pairs of somites. Contributions to Embryology, vol. 17, pp. 1-67.
Carnegic Institution of Washington, pub. no. 362.
BLOUNT,
R. 1932 Transplantation and extirpation of the pituitary rudiment and
the effects upon pigmentation i n the urodele embryo. J. Exp. Zeal.,
V O ~ . 63, pp. 113-141.
BRAHMS,
8. 1932 The development of the hypophysis of the eat (Felis doniestica). Am. J. Anat., vol. 50, pp. 251-281.
BUCY,P. 1932 The hypophysis eerebri. In: The cytology and cellular pathology
of the nervous system. P. B. Hoeber, New York.
BUTCFLER,
E. 0. 1929 The development of the somites in thd white rat (Mus
norvegicus albinus) and the fate of the myotomes, neural tube and
gut in the tail. Am. J. Anat., vol. 44, pp. 381-439.
CORNER, G. W. 1929 A well-preserved human embryo of 1 0 somites. Contributions to Embryology, vol. 20, pp. 81-101. Carnegje Institution of Washington, pub. no. 394.
CUSHINQ,
H. 1932 Pituitary body and hypothalamus. C. C. Thomas, Baltimore.
EAXLY DEVELOPMEXT O F M\IAMMALIBK HYPOI‘HYSIS
357
GILBERT,M. S. 1934 The development of the hypoph>sis: fartors influencing thc
formation of the pars ncuralis in the cat. Am. J. Anat., 1-01. 54,
pp. 287-313.
1929 Early stages i n the dcvclopment of
HEUSER,C. H. AND G. L. STREEYER,
pig embryos, from the period of initial cleavage t o the time of appearance of limli buds. Contribntions t o Embryology, 1701. 20, pp. 1-30.
Cainegie Institution of Washington, pub. 110. 394.
HOCIISTETTER,
I?. 1924 Beitrbge zur Entwicklungagcschiellte des meusehlichen
Gchirns. 11. Teil, 2 Leif erung. Die Fntwcklung des Hirnan2laiigc.s.
S. 49-81. F. Deutirke, Wein nnd Leipxig.
INGALLS,
N. W. 1920 A hunian embryo at the begilllling of segmentation, with
sperial referenre t o the vascular system. Contributions t o Embryology,
vol. 11, pp. 61-90. Carnegie Institution of Washington, pub. no. 274.
J O R DH.
~N
E., 1934 A textbook of histology. Appclton Century Co. New York.
RINGSBURY,
E. F. AND H. B. ADELMAXT~:
1924 The morphological plan of the
head. Quart. J. Mic. Sci., vol. 88, pp. 259-285.
MIIIALIIOVICS,
V. 1877 Entwicklungsgeschiclite dcs Gehirns. Leipzig.
Mmor, C. 8. 1897 Human embryology. The MacMillan Co., New Tork.
NELSON,W. 0. 1933 Studies on the anterior hypopliysis. I. The development of
t h e hypophysis in thc pig (Sus scrofa). Am. J. Anat., vol. 52, pp.
30 7-3 32.
PARKER,
K. >I. 1917 The developruelit of the hypophysis etJrehri, preoral gat,
and related strurtures in the marsupials. J . Anat., vol. 51, pp. 181-949.
PAYNE,
F. 1929 General description of a 7-somitc embryo. Contributions t o
Embryology, vnl. 1G, pp. 115-1 24. Cbrnegie Institution of Washington,
pub. no. 361.
SCHULTE,
H. W. A m E’. TILXEY 1915 De~elopment of the neuraxis in the
domestic eat t o the stage of twcnty-one soniitcs. Annals of S. Y .
Aead. Scj., 8 0 1 . 24, pp. 319-346.
SCHWIND,
J. F. 1928 The development of the 1iypoph)sis eerebri of t h e albino
rat. Am. J. Anat., vol. 41,pp. 295-319.
SITUMWAY,
W. 1927 Vertebrate embryology. John Wiley and Sons, New York.
SMITH,P. E. 1920 The pigmentary, growth, arid endocrine disturbances indaced
in the anuren tadpole by the early ablation of tlic pars hucralis of t h e
hypophysis. Ani. Anat. &fern., no. 11, p. 1-151.
STEIK,K. F. 1933 The loration and differciitiation of the presumptive eetoderru
of the forebrain and hypophysis as shown by chorio-allantoic grafts.
Physiol. Zool., voJ. 8 , pp. 205-235.
STREETER,
G. L. 1927 Development of the inesoblast and notochord in pig embryos. Contrihtions to Embrpology, vol. 19, pp. 73-92.
Carnegie
Institution of Washiilgtou, pub. no. 380.
VEIT, 0. 1919 Kopfganglicnlcisten bei einen rncnschlichen Embryo von 8
Somitenpaarea. Aiiat. Hefte, Bd. 56.
VEIT, 0. AND P. i%CH 1922 TTntersuchung eiiies in situ fixierten operativ geivonnen menschlichen Eies der veirten Toehe. Zeit. f . Anat. u. Ent., Bd. 63.
TlIE ANATOMlCAI~ RECORD, VOL.
62, KO.
4
PLATE 1
EXPLANATION OF FIGURES
5 Rat, 1 somite. x 160. A median sagittal section of the anterior p a r t of the
embryonic disc before the appcarancc of the head fold. Anterior end t o the right.
Compare with l a .
6 Rat, 2 sornites. X 160. A niedian sagittal sedan through the heaa fold
and foregut. Rostral end to the right. Conipare with 11).
7 Rat, 4 Rornites. X 160. A sagittal section which is median for the rostral
end of the neural plate and foregut. Rostra1 end to the left. Compare with 1c.
8 Rat, 5 somites. X 160. A sagittal section which is median f o r the rostral
end oP the neural plate and foregut. The rostral ciid of the embryo (left) here
vonsists of a mass of tissue, four nuclei 111 length, in which the floor of the neural
ertodrrm of the oral membrane are confluent. Compare with 1d.
9 Hat, 7. somites. x 160. A sagittal section which is median for the rostral
end of t h e neural plate. The mass of tissue in which neural plate and surface
cctoderni are coilfluent is seven t o eight iiuclei in length. Bostral end t o 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 rostra11 Inass of tissue arc sorted into
neural plate and surface ectoderm ahead of the prcelinrdal plate. Rostral elid t o
right. Compare with 1e.
11 Bat, 12 somites. x 160. A nearly niediaii sagittal section tlirougli the anterior end of an embryo with a small neuropore. The floor of the forebrain is
closely adherent t o the surfare ectoderm f r o m the level of the prechordal plate
t o the ventral lip of the anterior neuropore. Rostral end t o the left. Compare
with 1g. an, anterior neuropore; fg, foregut; hyp, hjpophyseal region; np,
neural plate; om, oral memhraile; pc, pericardixl cavity ; pp, prerhordal plate;
t o ; torus opticus.
358
EAXLY DEVELOPMENT O F MANII.\LALlAN HPPOPHYSIS
YARC.1RET SWEA GIT,RERT
359
PLATE 1
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