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=The Development of the Mammalian Pituitary and its Morphological Significance=
 
By P. T. Herring
 
From the Physiology Department, University of Edinburgh
 
Received for publication 11th February 1908
 
==Introduction==
 
The development of the pituitary body has been a favourite subject of research by embryologists. Its position in the embryo, forming as it were a meeting-point for the anterior end of the neural canal, buccal invagination, archénteron, and notochord, gives to the pituitary an importance, the significance of which has been the object of much speculation. Some authorities have looked upon its relations to these structures as more or less accidental; others have attached great Weight to them. Kupffer, indeed, regarded the pituitary body as an important key to the phylogeny of the vertebrate head. The morphological significance of the pituitary is also of interest from a
physiological point of view, and some of the theories which have been advanced regarding it will be briefly discussed in this paper.
 
 
Nearly all the work that has been done on the development of the “pituitary body has been concerned with its mode of origin and with the early stages of its growth. The later stages, although probably of greater physiological importance, have been comparatively neglected. The differentiation of the epithelium of the anterior lobe, the relations of epithelium to the nervous tissue of the posterior lobe, and the extraordinary differences
in the vascularity of its several parts are all features which need investigation. Its development in mammals has been followed chiefly in animals in
which the posterior lobe of the pituitary becomes a solid structure at a
comparatively early stage. In the cat, this lobe remains hollow throughout
development, and presents peculiarities of morphological interest which are
not found in the pituitaries of other animals. The structure of the posterior
lobe in the cat is also of a simpler character, and the nature and arrangement of the cells found in it can be interpreted more readily than in the
case of those animals which possess a solid lobe. For these reasons the
pituitary body of the cat forms the basis of the description in this paper.
The embryos of man, ox, and pig, which furnish a different type of pituitary,
have also been eizamined, and some of the more important features presented by them receive attention.
 
==Historical==
 
The pituitary body was at one time thought to be wholly derived from the brain, but Rathke (26) in 1838 described the invagination of mucous membrane which is now known as Rathke’s pouch. Rathke rightly assigned to this pouch the origin of the epithelial portion of the pituitary, but was mistaken in believing it to be derived from the entoderm of the fore-gut. His view was not at once accepted. Reichert (28) failed to find
the invagination, and put forward the theory that the epithelial portion of
the pituitary is a structure of mesodermic origin derived from the anterior
end of the notochord. His (14«) lent additional support to Reichert’s view,
but made no special investigations of the subject himself. Both Rathke
(27) and Reichert (29) subsequently changed their opinions, the latter
believing the anterior lobe to arise from a proliferation of the cells of the
pia mater. Dursy (8) sought to unite the original view of Rathke with
that of Reichert and His, and described the origin of the epithelium of
the pituitary from the fore—gut, and the origin of its vascular stroma from
the tissue of the head of the notochord. W. Miiller (23) demonstrated
that the anterior lobe of the pituitary is derived from Rathke’s pouch,
but fell into the same error as Rathke and Dursy in believing it
to be of entodermic origin. The later researches of Gotte (12) and
Balfour (3) showed that the pouch described by Rathke is derived, not
from the fore-gut, but from the epithelium of the buccal cavity immediately in front of the oral plate. The pouch is now recognised as an
ectodermic structure.
 
 
The posterior lobe of the pituitary was at first believed to represent the anterior extremity of the brain (v. Baer (2)). Gotte (12) showed that in amphibians this is not the case, the infundibulum being a later formation. The researches of Mihalkovics (21), van Wijhe (35), Kupffer (20), and others have demonstrated that the infundibulum cannot be regarded as the representative of the anterior end of the brain axis; it is an outgrowth of the “ Zwischenhirn ” or thalamencephalon.
 
 
The proximity of the anterior end of the notochord to the developing pituitary body led to the belief, not only that the notochord enters into the
structure of the pituitary, but that it also exercises a mechanical influence
upon the formation of the infundibulum. Both His and Dursy considered
that a close union between the notochord and the wall of the cerebral
vesicle is the dominating factor in the development of the infundibulum,
but were not agreed as to the exact manner in which this is brought about.
W. Muller believed that the head of the notochord anchors a portion of the
wall of the brain, and that with the growth of the surrounding tissues the
rest of the brain is carried forwards, leaving a diverticulum of its wall, the
infundibulum, attached to the notochord. The attachment is subsequently
dissolved by a proliferation of connective tissue cells. Mihalkovics (21)
and others showed that the head of the notochord does not come into immediate relationship with the brain, and cannot therefore act upon it in
this manner.
 
 
The most complete account of the early development of the pituitary body is that given by Mihalkovics (21), who investigated the subject in
rabbit and chick embryos. Mihalkovics found that the anterior lobe is
developed from Rathke’s pouch, which, in mammals as in amphibians, is
of ectodermic origin. The beginning of the pouch or hypophysial angle
lies in front of the oral plate, Where the epidermis bends round the base of
the brain to the nasal mucosa. In the rabbit, Mihalkovics states that the
end of the notochord is in contact with the epidermis at the back of
Rathke’s pouch. When the oral plate ruptures, its upper stump, containing in its upper part the head of the notochord, bends forward and
narrows the mouth of the epithelial pouch, leading to the formation of a
definite sac—the hypophysial sac. The wall of the sac presses upon the
base of the anterior brain vesicle, giving rise at its upper extremity to a
fold in the wall of the brain which becomes the primitive infundibulum.
Mihalkovics denied that the end of the notochord is ever united to the wall
of the fore-brain; it does not enter into the formation of the infundibulum
at all, but has some influence upon the hypophysial sac, by preventing this
from extending backwards. The primitive infundibular process comprises
the surrounding tissue of the tuber cinereum as well as the origin of
the infundibulum, and the true infundibulum is formed at a later stage
by its own growth from a portion of the primitive infundibular process.
Mihalkovics made a careful investigation of the relations of the notochord,
and found that its head touches the lower part of the posterior wall of the
hypophysial sac in rabbits, but is placed at a higher level in birds; it
exercises no traction upon the sac in either, and, beyond presenting a
barrier to the backward growth of the sac, takes no part in the formation
of the pituitary body.
 
 
The main conclusions of Mihalkovics’ researches have been cofirmed by Kolliker (16), Kraushaar (18), Minot (22), Kupffer (19), Salzer (33),
and others. Kupffer described an additional origin of part of the anterior lobe of the pituitary from the entoderm of the fore-gut. According to
Kupffer, the pituitary body of amphibians is built up from three separate
sources: part of the epithelial lobe is derived from Rathke’s pouch, and
part from the anterior end of the fore-gut, while the infundibulum is of
brain origin. In mammals, e. g. the sheep, the hypophysial pouch appears
behind the “ Riechplakode.” Behind this and ventral to it is a swelling,
the “Haftscheibe,” which is an important larval organ in Lepidosteus.
Then comes the double-layered oral plate (“ Rachenh ant”), and behind
this an outgrowth of entoderm directed dorsally and forwards, known as
Seessel’s pouch. The third portion of the pituitary, the cerebral, appears
later, after the disappearance of the oral plate and median “Riechplakode.”
In the next stage the growth of entoderm increases, but is cut off from
Seessel’s pouch; no cavity is to be found in it, and the end of the notochord remains in contact with it. The infundibulum now  to grow. In
the older embryos, e. g. 11—mm. sheep, the entodermic part degenerates and
appears as a string-like appendage of the notochord; it eventually disappears, and does not enter into the formation of the adult mammalian
pituitary.
 
 
Kupffer came to the conclusion that the intimate relationship between infundibulum, mouth, and intestine is not an accidental one, but denotes an ancestral communication between the brain tube and the anterior part of
the intestinal canal. A structure resembling in many respects the early
stages of development of the vertebrate pituitary is found in Ascidians, and
is known as the subneural gland. J ulin (15) in 1881 pointed out that
this gland is probably homologous with the hypophysis of higher vertebrates,
and since then it has been frequently spoken of as the Ascidian hypophysis.
Kupffer believes that the direct ancestors of vertebrates showed the same
relations as are seen to—day in the tailed Ascidian larva. In a scheme of the
ancestral vertebrate he describes the mouth (“ Palaeostoma ”) opening dorsally
in front of the brain. The brain tube is in communication with the anterior
part of the intestine by a canal running through the base of the anterior
brain vesicle. This canal has developed upon it a subcerebral gland.
Ventral to the palaeostoma is the “ Haftorgan ” on the anterior pole of the
body. In the course of development the new vertebrate mouth (Neostoma)
is formed, in agreement with Dohrn’s hypothesis, from a pair of gill-clefts
below the “Haftorgan.” The part of the intestine between the old and
the new mouth, or preoral intestine, is reduced, but persists to a certain
extent in some vertebrates. The palaeostoma is lined by epidermis, and its
representative in vertebrates is Rathke’s pouch; it also forms the outer part
of the nasal duct (“ Nasenrachengang ”) of Myxine, and the entire nasal
duct of Petromyzon. The remains of the canalis neurentericus anterior,
with its appertaining glands, are to be seen in the infundibular process and
saccus vasculosus. In mammals, the only representative of the preoral
intestine is the transitory appearance of the solid mass of cells formed from
Seessel’s pouch, but in amphibians it persists as part of the anterior lobe of
the pituitary.
 
 
Kupffer’s views on the morphological significance of the pituitary body
have not met with general acceptance. Willey (36) states that the present
relation of the hypophysis to the infundibulum in the craniates, however
intimate it may be in some cases, is, nevertheless, incidental and secondary.
Willey believes that the hypophysis arose in connection with a functional
neuropore. B. Haller (13) criticises Kupf f er’s results and differs from him
in many important particulars. He believes the nasal duct of Cyclostomata
to be a secondary structure and not related to the origin of the hypophysis.
He also states that the anterior lobe of the pituitary of mammals and
other vertebrates is a tubular gland which pours a secretion into the subdural space. The latter statement has not been confirmed by subsequent observers. Gaskell (9) quotes Haller’s results in support of the theory that the glandular hypophysis was originally the coxal gland of Arthropoda.
 
 
Kupffer’s description of the threefold origin of the pituitary body has
received support from observations by J. Nusbaum (24) and SaintRemy (31). Nusbaum found that in dog embryos of 9 mm. Seessel’s
pouch is well developed, and its anterior extremity abuts against the
posterior wall of Rathke’s pouch. In 80 per cent. of older embryos examined
it gives rise to a column of cells which unites with the epithelium of
Rathke’s pouch, and thus enters into the formation of the anterior lobe.
In the remaining embryos no such appearance is seen, and the anterior
lobe is entirely ectodermic in origin. Traces of a lumen were noticed by
Nusbaum in the column of cells growing from the fore-gut, but not a
definite communication between the interior of the buccal invagination and
the fore-gut. The connection is not preserved for long, and the entodermic
cells disappear, with the exception of a few which join the posterior
wall of Rathke’s pouch. What further part these cells play—if any—in
the formation of the anterior lobe of the pituitary Nusbaum did not determine.
 
 
Saint—Remy (31) described a budding of Seessel’s pouch in the embryo
chick towards the seventieth hour of incubation. The bud acquires a fine
lumen, and, reaching Rathke’s pouch, affords a direct communication between
the interior of the latter and the fore-gut. The connection lasts a little,
then disappears, the cells of Seessel’s pouch never actually uniting with
those of Rathke’s pouch. Saint-Remy agrees with Kupffer that the
entodermic origin is rudimentary in birds and mammals, and does not enter
into the formation of the adult pituitary body. It is, however, of morphological importance, and betokens the existence in lower forms of vertebrates
of a communication between the intestine and the buccal invagination.
 
 
Dohrn (7) looked upon the pituitary as the remains of a preoral gillcleft. Salvi (32) has brought forward evidence in support of this View,
and states that in reptiles part of the pituitary is developed from the walls
of the premandibular cavities, which he believes to be the representatives
of gill-clefts. Valenti (34) describes the origin of the anterior lobe in
amphibians from an invagination of the fore—gut arising some distance
behind Seessel’s pouch. The invagination, he considers, has not the significance attributed by Kupf fer to Seessel’s pouch, but is rather to be regarded
as the representative of a gill-cleft. Valenti therefore supports Dohrn’s
theory. Dohrn’s View was based chiefly upon the assumption of a bilateral
origin of the anterior lobe of the pituitary. Dohrn himself described a
bilateral origin in Hippocampus, and Gaupp (10) found something similar
in reptiles. Gaupp, however, described a median origin in addition to
lateral ones, and believes all to be formed from the buccal cavity.
 
 
Yet another interpretation of the significance of the pituitary has been put forward by Beard (4), who believes the anterior lobe to be homologous with the permanent mouth of Annelids.
 
==Author's Observations==
 
My own results are confirmatory of those of Mihalkovics and Kupffer. In a 4—mm. cat embryo, the youngest I have had the opportunity of examining, the appearance is that indicated in fig. 1. The oral plate (f) between buccal invagination and fore-gut has just ruptured. Immediately in front of the oral plate is the hypophysial angle described by Mihalkovics. The anterior limb of the angle is composed of buccal epithelium, which in this situation is closely adherent to the wall of the anterior cerebral vesicle. The posterior limb of the angle, also composed of buccal epithelium, leaves the wall of the brain and bends downward to form the anterior layer of the upper stump of the oral plate. Atthisstage there is no invagination of the wall of the cerebral vesicle to form the infundibulum, but its site is indicated by a definite depression. The anterior end of the notochord does not touch the posterior limb of the hypophysial angle, but is separated from it by a large blood-channel (0). Behind the oral plate is a small dorsal invagination of the wall of the fore-gut, which is the only indication of anything resembling Seessel’s pouch. Its wall is not thickened, and there is no evidence of any entodermic origin for the pituitary in this specimen.
 
 
'''Fig. 1.'''—Mesial sagittal section through head of 4-mm kitten. (Diagram.) a, hypophyaial angle formed by buccal mucous membrane; b, depression in wall of cerebral vesicle where the intundibulum is formed; c, blood-channel; d, anterior end of lore-gut or Seessers pouch; e, head of nctochord ; 1', upper stump of ruptured oral place.
 
 
FIG. 2.—Mesial sagittal section through part of head of a. 6-mm. kitten. a, buccal lnvaglnatlon or Bsthl-re’: pouch; b, beglnnlng of lnvsginatlon of well of cerebral vesicle to form the lnlundlbulnr process; 4:, blood-channel; d, clump of cells derlved from anterior and of tore-gut; 9, head of notochord.
 
 
 
In a. 6-mm. cat embryo (fig. 2), the remains of the oral plate have disappeared. The hypophysial angle has become a definite sac (a), Rathke’s
pouch. This change appears to have been brought about by a bending
forwards of the upper stump of the oral plate and a proliferation of the cells in its wall. The pouch is Widening out behind the neck, and the latter
is found to be constricted when the sections next in series to it are examined.
The anterior wall of Rathke’s pouch is closely applied to the Wall of the
cerebral vesicle, and at the dorsal extremity of the pouch an invagination
of the wall of the cerebral vesicle is forming the primitive infundibulum.
 
 
The head of the notochord bears no immediate relation to Rathke’s
pouch, and is separated from it by a clump of cells which is continuous with
the epithelium of the fore-gut, and appears to be formed by a proliferation
of the cells of the latter. A large blood-vessel (c) is also seen in this
specimen, lying between Rathke’s pouch and the head of the notochord.
 
 
At this stage it is difficult to determine where ectoderm ends and
entoderm begins; the upper stump of the oral plate has disappeared as such,
and its representative is uncertain. There is no indication of a pouch in
the fore-gut, but the clump of cells appears to be derived from the wall of
the latter. Minot (22) makes the fold of epithelium at the posterior
margin of Rathke’s pouch homologous with the upper lip of Petromyzon.
If this is the case, the fore-gut must begin behind this fold. The close
relation between the head of the notochord and the cell clump makes it
likely that the latter is derived from the fore-gut, for, in the 4«-mm. embryo,
the head of the notochord is some distance behind the oral plate, and the
epithelium opposite it is that of the fore-gut. The clump of cells is the
only structure which resembles the proliferation of entoderm described by
Kupffer. It is not found in any of the older embryos that I have examined,
but the amount of suitable material at disposal for this purpose has been
limited. Rathke’s pouch is the only part that enters into the formation of
the anterior lobe of the pituitary; it is single and median in origin, and
there is no indication in the embryos of the cat and the pig of any other
“Anlage” for the anterior lobe. I have not found any communication
between the epithelium of Rathke’s pouch and that derived from the foregut, as described by Nusbaum in the dog, but cannot say that this does
not occur. In the specimens I have examined there is nothing to indicate
in the slightest degree that Rathke’s pouch is reinforced by epithelium
from the fore-gut. The epithelial proliferation of the latter disappears as
stated by Kupffer, and takes no part in the formation of the pituitary.
 
 
One of the most important characteristics of the developing pituitary is
the close union maintained between buccal and cerebral portions from the
earliest stage. Minot (22) emphasised its importance in mechanically
keeping the two parts together, and thus explaining their intimate relations.
Salzer (33) also noted it, and states that he could find no connective tissue
between the infundibular process and hypophysial sac. With these observations I thoroughly agree. The buccal epithelium in the anterior part of
the hypophysial angle is intimately connected with the epithelium of the
cerebral vesicle, without the interposition of connective tissue. In the
further growth of the embryo this close union is preserved, but in other
parts connective tissue develops and separates the buccal epithelium from the wall of the cerebral vesicle. N o doubt this process is contributory to
the formation of Rathke’s pouch and infundibulum, but it is probably of
morphological significance as well, and betokens the existence in an ancestral
vertebrate of a communication between buccal cavity and neural canal.
 
A later stage of development is shown in fig. 3, which is taken from a pig
embryo of 12 mm. The buccal mucous membrane is now widely separated from the wall of the cerebral vesicle, except at that part Where the
anterior wall of Rathke’s pouch closely adheres to it. The infundibulum
is only beginning to form, and the cells lining the cerebral vesicle at this
point have proliferated and elongated, and look more like ependyma
cells. There is no indication of any proliferation of cells of the fore—gut.
Rathke’s pouch is median in situation, its neck is constricted, and serial
sections show that there is no lateral origin of the pituitary. The notochord persists, but has no immediate relation to Rathke’s pouch. It takes no part mechanically or otherwise in the formation of the pituitary. Its
situation, nevertheless, is not without significance; its arrest behind the
anterior end of the brain tube allows the latter to communicate with the buccal
epithelium and possibly with the fore—gut in some animals. Had the notochord grown further forward, a median origin for the anterior lobe of the pituitary would have been impossible. The median origin of the pituitary,
or rather the ancestral condition which this implies, may indeed explain
why the head of the notochord has been arrested in this situation.
 
 
 
FIG. 3.—Mesia.l sagittal section through part of head of a 12-mm. pig embryo. a, Rathke’s pouch ; b, beginning of infundibular process; 2, blood-channel ; d, remnant of Seessel's pouch (7); e, notochord.
 
 
 
Fig. 4.—-Mesisl sagittal section through part of head of an 18-mm. kitten. a, hypophysial sac now closed below; b, lntundibular process; g, neck of sac connected with nasal mucous membrane; h, cartilage of sphenoid bone.
 
 
 
In an 18-mm. cat embryo considerable changes have taken place (fig. 4).
Rathke’s pouch has become a closed sac, but its wall is still connected by
a stalk of epithelium with what is now becoming nasal mucous membrane.
 
The narrowing of the neck of the sac, its closure and ultimate disappear-
 
 
 
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