Paper - The histological appearances of the mammalian pituitary body

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Herring PT. The histological appearances of the mammalian pituitary body. (1908) Quar. Jour. Ex. Physiol., 1: 121-159.

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The Histological Appearances of the Mammalian Pituitary Body

By P. T. Herring.

From the Physiology Department, University of Edinburgh.

Received for publication 11th February 1908.

Introduction

The structure and significance of the pituitary body have long been objects of much speculation. Erroneous conceptions of its structure are responsible for some of the many theories which have been advanced with regard to its functions. The pituitary, indeed, derives its name from the old idea that it was a gland which discharges a secretion—pituita—into the nostrils.


Rathke (32) discovered the double origin of the pituitary, and on developmental grounds classed it among glands. Other observers looked upon it as part of the brain. Luschka (23) called it a “nerve-gland” in which the two parts are separated from one another by pia mater. Ecker (8), on the other hand, held the view that both portions of the pituitary combine to form a unit of the nature of a “ blood-vessel gland.”


Burdach (4.-), Luschka (23), and Virchow (46) regarded the posterior lobe as the anterior terminal end of the cerebro-spinal canal, a “filum terminale anterius,” resembling in structure the filum terminale of the spinal cord. Virchow also compared the anterior lobe to the thyroid gland, and described in it vesicles containing colloid material which show a striking resemblance to the follicles of the thyroid. Rogowitsch (34), H. Stieda (43), Schonemann (39), and others have attached great importance to this resemblance, and ascribe similar functions to the two glands. Removal of the thyroid is, according to their observations, followed by a compensatory hypertrophy of certain parts of the glandular lobe of the pituitary.


In 1886 Marie drew attention to a relationship between changes in the pituitary and the disease acromegaly or gigantism. Clinical and pathological experiences have led to the theory which assigns to the pituitary the role of regulating the normal development of the body, more especially of the extremities and bones. The nature of the change that the pituitary undergoes in acromegaly is uncertain, and before any light can be thrown upon its pathology it is necessary that the significance of the various histological elements that constitute the normal pituitary should be understood. Moreover, it appears that acromegaly may occur without any apparent change in the pituitary, and that tumours of the pituitary are not always attended by acromegaly. A feature as constant as acromegaly in affections of the pituitary is the occurrence of polyuria with or without sugar in the urine (Hansemann (16), Sternberg (41)).


Oliver and Schafer (28) in 1895 described the presence of a substance in saline extracts of the pituitary, which, when injected intravenously, produces a rise of blood-pressure. Howell (18) showed that this substance is only present in the posterior lobe. Magnus and Schafer (24) in 1901 noticed that intravenous injection of saline extract of the posterior lobe is followed by a marked increase of urine flow. Schafer and Herring (37) confirmed this observation, and showed the striking parallelism which exists between the suprarenal capsules and the pituitary in development, structure, and functions. In each there are two parts, one of which, a highly vascular epithelium, yields no active extract, while the other, of neuro—ectodermic origin, gives an extract which has a remarkable physiological effect upon the heart and arteries. The view was conjectured that in the epithelial part of each organ the material which is to furnish the active agent of the secretion passes through certain stages of formation, and that its production is merely completed in the neuro—ectodermic part, in which part alone the full activity of the secretion is acquired. That the posterior lobe of the pituitary should furnish an active secretion is difficult to reconcile with the usual views held on its structure. The older anatomists, W. Muller (27), Schwalbe (40), and Toldt (45), looked upon it as a mass of connective tissue cells and fibres which during development have destroyed all trace of the original nerve tissue. Berkley (2), on the other hand, describes in it a complex arrangement of nerve cells and nerve fibres, besides neuroglia and ependyma cells. Kolliker (19) takes up an intermediate position, and believes that there are no true nerve cells, but neuroglia and ependyma, a view similar to the one held by Virchow. Peremeschko (30) first recognised that the posterior lobe has an epithelial investment. Osborne and Swale Vincent (29) state that extracts of the central part of the posterior lobe are more active than extracts of the margin of the lobe, and believe that the epithelial investment would be found to be inactive if it could be properly isolated.


The pituitary body is found in all vertebrates, and, although differing widely in structure and in the arrangement of its component parts, possesses many features common to all. In fishes, the posterior lobe has a complex vascular structure of a glandular nature, which was called the “saccus vasculosus” by Gottsche (12). L. Stieda (44) proved that the saccns vasculosus communicates with the brain cavity, and Rabl-Riickhard (31) named it an infundibular gland. Their researches have been confirmed by Kupffer (21). The function of the saccus vasculosus is unknown, but its secretion, if it is a secretory gland, apparently mixes with the fluid contents of the ventricles of the brain. According to Kupffer, the posterior lobe of the mammalian pituitary in its early development retains for a time a glandular structure. In the adult mammal the epithelial investment of the posterior lobe is regarded by Ktilliker as the representative of an infundibular gland. B. Haller (14«) states that in mammals—as a type of which he takes the mouse——and in all other classes of vertebrates the anterior lobe of the pituitary and epithelial investment of the posterior lobe form a gland, the tubules of which open by a small median and ventral mouth into the space between the pia and dura mater. Haller believes that the pituitary in all vertebrates secretes directly into the subdural space. Edinger (9) denies that this is true of the human pituitary, Salzer (36) could find no opening in the pituitary of the rat and mouse, and Sterzi (42) found none in the pituitary of Petromyzon.

There are other views on the structure and functions of the pituitary body. Boeke (3) and Gemelli (11) describe appearances in the posterior lobe of fishes which they regard as indicative of sense organs. Cyon (6) looks upon it as an organ which regulates the amount of blood passing to the brain. Guerrini (13) and others believe that the pituitary produces a secretion which has a vague antitoxic action.

Our knowledge of the structure of the pituitary body is, therefore, far from exact, and is inadequate to account for the physiological effects which follow intravenous injection of extracts, especially of the posterior lobe. Even the important question as to whether the glandular portion secretes directly into the subdural space is still unsettled. The work, the results of which are given in this paper, was begun with the intention of investigating the physiological histology of the posterior lobe, but the two portions of the pituitary were found to be so closely associated that no part would be complete without careful consideration of the other. The development and comparative anatomy of the pituitary body have been examined, but are only touched upon in this paper where reference to them throws light upon the particular point considered.

Material and Methods Employed

The cat furnishes some of the best material for the study of the pituitary body, for in this animal the posterior lobe retains throughout life its original cavity in free communication with the third ventricle of the brain. The structure of the posterior lobe in the cat is thus rendered simpler because the arrangement of the cells which line the cavity persists in the adult in much the same manner as obtains in the developing organ. The parts which are derived from the buccal mucous membrane form an almost complete investment for the nervous portion, and the original lumen of the epithelial pouch also persists throughout life in the form of a well marked cleft. The so-called colloid cysts are also prominent features in the pituitary of the cat.


The pituitary of the monkey more closely resembles that of man, and is a type in which greater fusion of the original elements from which it is developed has taken place. The posterior lobe is solid throughout. Its investment by the epithelial portion is not so complete as it is in the cat, and only a small cleft remains as the representative of the original buccal pouch.


The pituitary of the dog offers in some respects a type which‘ is intermediate between that of the cat and that of the monkey. The posterior lobe is solid, but the cavity of the third ventricle of the brain is continued downwards and backwards towards the neck of the posterior lobe. The epithelial investment is very complete, and the cleft in it well developed as in the cat. The colloid cysts are more numerous than in the pituitary of the monkey, and their arrangement and structure present features which distinguish them from those of the cat’s pituitary. The morphology of the pituitary bodies of the cat, dog, and monkey will be described briefly, and the structure of the various parts more minutely detailed in the cat.


For the investigation of the finer structure of the pituitary body Flemming’s fixative gives the best results; a 10 per cent. solution of formol and saturated corrosive sublimate have also been employed. Sections have been cut serially in a vertical antero-posterior plane; these show the relations of the Various parts of the pituitary to one another better than do sections cut in other directions. Most of the material has been cut in paraflin, but the freezing microtome has also been used, and the Golgi preparations cut by hand.


The structure of the anterior lobe is shown to the best advantage by staining with eosin and methylene blue, or by the employment of some of the many methods devised for the staining of blood films. Many preparations were made by Ca_jal’s silver reduction method, which is especially valuable for showing the fibrils of the neuroglia, and the ependyma cells of the posterior lobe. Cox’s modification of Golgi’s method was also adopted for the investigation of the nervous elements. Fresh tissues have been teased out and examined in salt solution and in osmic acid, and chromic acid fixed preparations have been cut by the freezing microtome. The blood-vessels were also injected from the common carotids with carmine gelatine, and the vascular supply of the pituitary body studied in thick sections.


A word must be said about the removal of the pituitary body for purposes of examination. In order to investigate the question raised by B. Haller as to the presence of an opening on the median ventral aspect connecting the epithelial cleft with the subdural space by means of a lymph space, it is almost essential to remove the sella turcica and part of the brain from below, to decalcify the bone and cut sections of the pituitary in situ. This can be more readily done in the young animal. For most purposes it is suflicient in the adult animal to dissect the bone piecemeal from the dura mater, which forms an envelope to the pituitary, thickened at certain points, especially behind. Great care must be taken not to rupture the thin layer of epithelium which in the cat is continued backwards from the anterior lobe, to be reflected at the place where the blood-vessels enter the posterior lobe to form a closely fitting investment over the ventral aspect of the latter. Removal of the pituitary from the cranial cavity by raising the brain and dissecting from above is almost invariably followed by rupture of the neck of the posterior lobe. The dura mater should always be preserved intact without being pulled upon, and the best way to do this is to dissect off the bone from below, disturbing the base of the brain as little as possible. A portion of the brain can then be cut out with the pituitary attached, and the piece trimmed after hardening.



Fig. 1. Mesial sagittal section through pituitary body and sella turcica of new-born kitten. (Semi-diagrammatic.) a, optic chiasma; b, tongue-like process of pars intermedis; c, third ventricle; d, anterior lobe proper; e, epithelial cleft ; f, central cavity of posterior lobe ; g, nervous substance of posterior lobe ; h, posterior reflection of epithelium.

Morphology

The relations of the anterior and posterior lobes of the pituitary to one another, and to their immediate surroundings, can be most readily appreciated by reference to the comparatively simple pituitary of the new-born kitten. Fig. 1 is a diagram of a mesial sagittal section through the pituitary and sella turcica of a new-born kitten. The infundibulum cerebri is a continuation of the brain backwards and slightly downwards, and consists of a comparatively thin wall of brain substance enclosing a cavity which is a continuation of the third ventricle. The infundibulum has a funnel-shaped origin from the base of the brain, narrowing as it passes backwards to a tubular neck, then expanding to form a hollow club-shaped body which makes up the larger portion of the posterior lobe. The central cavity also enlarges behind the neck of the infundibulum.


The anterior lobe, composed of epithelial cells, lies below, and its thickest portion is in front of the infundibulum. It extends for some distance anteriorly, forming a tongue-shaped projection which reaches to the under surface of the tuber cinereum. The anterior lobe also spreads further laterally, and enfolds the sides of the infundibulum, the neck of which is encircled completely, so that, as in the figure, a portion of the anterior lobe appears above it. In some kittens the wrapping of the epithelium round the posterior lobe is more complete, and the only part of the lobe which is never covered by epithelium is a small part behind where the blood-vessels make their entrance. A narrow and somewhat S-shaped space lies inside the epithelium close to and following in its outline the under surface of the nervous portion of the posterior lobe, but separated from it by several layers of epithelium. The space or cleft is, as Kolliker (20) pointed out, the remnant of the cavity of the pouch of buccal epithelium from which the anterior lobe is derived. The layer of epithelium which lies between the cleft and the part of the posterior lobe developed from the brain is comparatively thin, and very closely applied to the nervous substance, thus forming an investment to it which is more or less complete according to the degree in which the anterior lobe has grown round the posterior. The cleft extends laterally, and in some cases almost surrounds the body of the posterior lobe. The posterior lobe as separated from the anterior by the cleft is therefore a composite body derived from the brain and from buccal epithelium, and it is to this structure of elements derived from two sources that the name of posterior lobe is usually applied, although strictly speaking the epithelial investment belongs developmentally to the anterior lobe. The cleft is sometimes more complicated, and branches of it may run into the substance of the anterior lobe. Serial sections show no opening below such as has been described by B. Haller (14), nor does there appear to be an opening at any point; the cleft is a closed cavity in the kitten, but very great care has to be taken in the removal and preparation of the pituitary to prevent rupture of the thin layer of epithelium which is continued backwards from the anterior lobe.


The greater portion of the anterior lobe is a solid structure made up of columns of cells and wide blood-channels. Granules are present in many of the cells of this part, but are not so marked a feature of the anterior lobe in the new-bom kitten as they are in the adult cat. Colloid cysts are not found in the pituitary of the new-bom kitten.

The relation of the pituitary body to the sella turcica is shown in fig. 1.


The gland lies on the body of the sphenoid bone, and is separated from it by the dura mater, which is thickened in front and behind, and contains blood—vessels, and what appears to be a lymph space. This space, which was described by B. Haller, is not a marked feature in the kitten, but is more pronounced in the foetus of the ox, where, at an earlier stage in development, it penetrates for some distance into the body of the sphenoid bone. Ossification of the bone at this point is delayed by the gradual disappearance of the epithelial stalk which connects the anterior lobe of the pituitary with the buccal epithelium. No trace of this connection is found in the new-born kitten, and there is no evidence of any opening of the glandular tubules or cleft of the pituitary into the lymph space either in the foetal ox or kitten.


Fig. 2. Mesial ssgittsl section through the pituitary body of an adult cat. (Semi-diagrammatic.) a, optic chiasma: b, tongue-like rocess oi pars intermedia; c, third ventricle ; d, anterior lobe proper; e, epithelial cleft ; f, cavity 0 sterior lobe ; f’, cavity of neck of posterior lobe; g, nervous substance of posterior lobe; i, epithelial nvestment of posterior lobe. The dark shading indicates the distribution of the characteristic cells of the anterior lobe; the lighter shading shows the position ot epithelium belonging to the pars intermedis.


The pituitary body of the adult cat is very similar in structure to that of the kitten, but presents several important modifications. In mesial sagittal section the posterior lobe appears larger than the anterior; the latter is, however, the larger, and extends further laterally, embracing the posterior lobe. The central cavity of the posterior lobe persists, and a tapering process of it runs upwards and backwards towards the place of entry of the blood-vessels into the infundibulum (fig. 2). This process is always present, and frequently runs up to the epithelial investment. The neck of the infundibulum is narrow and its lumen small.


The anterior lobe and epithelial investment of the nervous portion of the posterior lobe are separated by the cleft, which, as a rule, persists in its entirety. Occasionally the cleft is closed up to a large extent, especially in its posterior part, and the two layers of epithelium are more or less fused, but a space always remains between the main part of the anterior lobe and the epithelial investment of the neck of the infundibulum. The main mass of the anterior lobe in front of the cleft contains cells holding granules which stain deeply with eosin. These granular cells are not present in the epithelial investment of the posterior lobe, nor are they found in the tonguelike process of the anterior lobe which runs forwards towards the optic chiasma. The epithelial investment of the posterior lobe is well marked and thickened, especially round the neck of the infundibulum, where there are many layers of cells. The cells are frequently arranged in groups round a central lumen which contains a colloid material. These colloid vesicles are for the most part small and scattered at intervals; they are especially well developed round the neck of the infundibulum and in the tongue-like process, where they show a resemblance to the vesicles of the thyroid gland. They are rarely found among the eosinophil cells of the anterior lobe, and appear to be placed never far distant from the nervous portion of the pituitary.


Fig. 3. Mesial sagittal section through pituitary body of an adult cat. (Photograph.) Compare with fig 2.


Considerable variations occur in different cats in the relative size and arrangement of the parts described. This is especially the case With the eosinophil cells of the anterior lobe, which are sometimes continued far backwards over the posterior lobe, but separated always from it by the



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Histological Appearances of the Mammalian Pituitary Body 151


like the so—called colloid material, and may be of that nature ; but is unlike in many respects the colloid of the thyroid gland. In the posterior lobe of the dog’s pituitary somewhat similar appearances present themselves, but in this case the epithelial islets frequently consist of a number of cells which group themselves round a central cavity containing colloid material. Isolated cysts, the walls of which are composed of a single layer of cells, are not uncommon, and were noted by Lothringer. In the dog, too, the larger cysts of the epithelial investment are not always complete, and the contained material may abut against the neuroglial tissue. The colloid substance, when completely or partly enclosed by epithelial cells, varies considerably in its staining properties in different parts of the same pituitary. As a rule it takes on little depth of colour with stains, and is unlike the colloid of the thyroid in this respect; it has a hyaline rather than a colloid appearance. Occasionally, however, the substance is denser and takes on a deeper stain, and in the dog sometimes looks as though it were a swollen cell with disintegrating nucleus. The hyaline material of the nervous portion of the posterior lobe also varies in appearance, and in its staining properties; most of it might be of the same nature as the so-called colloid of the epithelial investment and of the intermediate part generally, but in a diluted form. Some of this material must be the product of the epithelial cells, for it occurs in places where no other kind‘ of cell is present. The universal occurrence of this material in the nervous substance, often at considerable distances from epithelial cells, is dificult to explain, unless we can suppose it to be carried from them by lymphatic vessels. The substance often does lie in distinct spaces lined by what appears to be endothelial cells. The general tendency of the direction of the material seems to be towards the neck of the infundibulum in the cat, and it increases in amount towards this situation. Large masses are sometimes seen lying among the ependyma cells, and similar material may be present in large amount in the central cavity, communications between the two being evident in places.


In fig. 15 a typical portion of the neck of the infundibulum of a cat's pituitary is seen. The central cavity is lined by ependyma cells, outside which are cells with large nuclei and little protoplasm. Occupying the central cavity is a mass of hyaline material, aud masses of a similar substance lie beneath the ependyma cells and between the ependyma fibres. In other places in the posterior lobe the material is distinctly present in lymph channels accompanying the blood-vessels. Evidence strongly points to the probability that the material is on its Way to the central cavity, and so into the ventricles of the brain. In this sense the posterior lobe of "the pituitary is a gland which pours its secretion into the third ventricle of the brain. It is possible that the ependyma and neuroglia cells have also a secretory function, but improbable that they secrete the material described. They may, however, have some influence upon it. The most likely supposition is that the ependyma and neuroglia cells form a scaffolding for the posterior lobe of the pituitary, upon which is built up a covering of epithelial cells.1The relations of the two structures become very intimate; their blood supply is derived from arteries which enter the nervous substance posteriorly; the veins begin immediately below the epithelium, and return through the nervous substance. Interchange of material between blood-vessels and cells must be through the medium of lymph, seeing that most of the epithelial investment is extra-vascular. Cells from the investing layer grow into the nervous framework, giving rise to epithelial columns and cell islets. Secretion goes on either by an emptying of material from the cells into the lymph, or possibly by a breaking down and destruction of the whole cell. The latter indeed is the more probable fate of isolated epithelial cells, and seems to occur at times in the epithelial investment itself. The material known as colloid substance, which has been supposed by many to be identical with the colloid of the thyroid gland, occurs in comparatively large amounts in the pituitary of the eat, an animal which cannot long survive thyroid extirpation. Further chemical and experimental research is necessary to prove


Fig. 15. Section of part of neck of posterior lobe of the pituitary body of an adult cat. From specimen fixed in Flemming’s solution and stained with eosin and methylene blue. Shows colloid material lying among ependyma fibres and in the central cavity of the neck. a, ependyma cells lining central cavity; b, colloid material in central cavity; c, ependyma fibres; d, colloid material: 4:, granular body lying among ependyma fibres.


  • 1 In the posterior lobe of a Belgian hare, a ganglion with large ganglion cells and medullated fibres was found. The ganglion occupie a large portion of one side of the posterior part of the lobe, and the fibres appeared as though entering it obliquely from the side and not through the neck of the lobe. Red bone marrow was also present, and the ganglion may have be part of one of the Gasserian ganglia, which, in the rabbit, are very close to the pituitary by a large part of the nervous portion of the lobe was destroyed by it. The intermediate portion was large in amount, and its cells thickly massed around the neck. The animal was a healthy adult.


whether the colloid substance of the pituitary is identical in composition, or in its physiological action, with the colloid of the thyroid gland. The physiological action is apparently quite different, but this may be due to the presence in the lobe of other substances. Any secretion formed in the pars intermedia of the pituitary must pass into the adjacent substance of nervous origin either by blood—vessels or lymphatics. There is a possible exception to this rule in the tongue-like process of the cat’s pituitary, but even here blood—vessels from it pass into the adjacent anterior lamina connecting the neck of the infundibulum with the tuber cinereum, and the tubules are surrounded by connective tissue containing lymphatics, the course of which is unknown. They may possibly accompany the blood-vessels. The epithelial cells do not resemble in staining properties the cells of the medulla of the suprarenal capsule; they have no affinity for chromic acid. Nor do they resemble them in their relations to blood-vessels, the absence of which is so characteristic of most of the pars intermedia.


A question of importance arose in the early investigations of the structure of the pituitary. Peremeschko (30) described the cleft of the epithelial part as being continuous with the central cavity of the neck of the infundibulum, and so with the third ventricle of the brain. If this were the case it would furnish a proof of Kupffer’s view (21) that the epithelial portion represents a “ palaeostoma ” or old mouth of an ancestral form of vertebrate, and there would be in the mammalian embryo a communication between the neural canal and buccal cavity. Subsequent observers have denied the accuracy of Peremeschko’s observations. In the pituitaries of the pig and man, in which Peremeschko described the continuation, there is obviously no such thing, for the body of the infundibulum is solid in both cases. Nor is there any indication of it in the embryo of either pig or man, so far as I have been able to see. It is far more likely to occur in the pituitary of the cat, in which the central cavity of the infundibulum is well developed and prolonged far backwards. In the adult cat the epithelium, as already stated, frequently invades the posterior end of the cavity, so that epithelial cells may even form part of its lining. This peculiarity affords some support for Kupfi'er’s view. In the adult cat I have never been able to find a direct communication between the cleft and the cavity of the infundibulum, although many specimens have been examined with this object. In one embryo kitten, however, I have found such a communication. Cleft and cavity in this specimen are undoubtedly in direct continuity at the postero-superior angle of the posterior lobe. In other kittens, at a comparatively late stage of embryonic life, a direct continuity is occasionally seen between tubular epithelium at the end of the cleft, and the ependyma cells lining the -central cavity of the infundibulum. In the kitten this coming together of the two portions occurs some time after the epithelial duct between buccal mucous membrane and epithelial portion of the pituitary has disappeared, so that there is never a direct continuity between the neural canal and the exterior. The observations, nevertheless, give support to Kupffer’s views on the morphological significance of the pituitary.

Vascular Supply of the Pituitary Body

The arrangement of the blood-vessels in the pituitary body has already been described along with the structure of its several parts, but a general survey of the vascular distribution in the cat's pituitary may be given. The most noticeable feature of the injected organ is the difference in vascu- larity of the two lobes. The anterior lobe is filled with wide channels, making it one of the most vascular structures of the body; the posterior lobe, on the other hand, resembles in the number and arrangement of its vessels the adjacent white matter of the brain. A large blood sinus is found on either side of the pituitary body below. The anterior lobe is supplied by arteries, apparently from the internal carotid, which enter it at the sides of the infundibulum above, and break up immediately into large, thin-walled vessels. The posterior lobe is supplied by a median artery which enters it at its postero—superior extremity; branches run forwards near the central cavity, and break up into capillaries. The veins of the posterior lobe are situated immediately beneath the epithelial investment, and converge towards the place of entrance of the artery; leaving the lobe in this situation, they turn outwards to join the large lateral sinus on either side. Some of the veins from the anterior lobe appear to take a similar course, passing through the epithelial investment of the neck of the infundibulum to run in the nervous portion; others leave the anterior lobe at its neck and pass outwards into the lateral sinus. The presence of so many large arteries and veins in the immediate vicinity of the pituitary body would render the operation of removing it an extremely diflicult one, even were the organ so situated as to be convenient of access. Minot (26) states that the blood-vessels of the anterior lobe of the pituitary will probably be found to have a sinusoidal development. This appears to be partly true at any rate of the anterior lobe of the pig’s pituitary.


Fig. 16. Mesial sagittal section of pituitary body of adult cat ; blood-vessels injected with carmine gelatine. (Photograph.) a, optic chiaama; b, tongue-like process of pars intermedia; c. third ventricle; d, anterior lobe; e, pars intermedia lying above neck of posterior lobe; f, posterior lobe; 9, central artery entering posterior lobe at its poatero-superior angle; h, large vein, lying between nervous substance and epithelial investment of posterior lobe.

Conclusions

We may conclude from the histological appearances of the mammalian pituitary body that it is an organ of physiological importance It may be divided into two parts, which show structural differences probably indicative of distinct functions.


The anterior lobe, consisting of large granular cells and numerous blood- vessels, is a gland producing an internal secretion which is poured directly into the blood. It is a blood-vascular gland, the function of which is undetermined, but which may exercise an influence on growth. The care- ful examination of the pituitary body in cases of acromegaly may throw some light upon this question; at present any statement as to its probable functions must be purely speculative.


The posterior lobe is made up of two structures. Of these, the part developed from the brain and consisting of neuroglia and ependyma cells and fibres acts as a framework. It is more or less surrounded and invaded by epithelium, which probably furnishes its active part. There is histologi- cal evidence of a secretion produced by the epithelial cells, which apparently passes into lymph-vessels, and is destined to enter the ventricles of the brain. The posterior lobe of the mammalian pituitary is a brain gland, not by virtue of tissue of brain origin, but by the growth into it of epithelial cells of ectodermic origin. Extracts have the property of producing marked effects on cardiac and plain muscle fibres comparable in some respects to the action of the medulla of the suprarenal capsule. They have also a selective action upon the kidney, causing dilatation of the renal blood-vessels and diuresis. Disturbances of the posterior lobe of the pituitary are - probably responsible for the occurrence of the diabetic conditions which have been so frequently recorded at some time or other in the history of cases of acromegaly and of affections and lesions associated with the base of the skull.

Summary

Three types of mammalian pituitary body are recognised. In one, e.g., the cat, the posterior lobe is hollow and its cavity is in free communication with the third ventricle of the brain, while the epithelium of the anterior lobe affords an almost complete investment for the posterior lobe; in the second type, e.g., the dog, the body of the posterior lobe is solid, but the neck is hollow, and communicates with the third ventricle: the posterior lobe is here again almost completely surrounded with epithelium; in the third type, e.g., man, monkey, ox, pig, and rabbit, the body and neck of the posterior lobe are solid, although traces of a cavity are occasionally found in the neck; in this type the epithelium does not invest the posterior lobe so completely, but is aggregated around the neck and spreads over and into the adjacent surface of the brain.


The epithelial portion of the pituitary body is differentiated into two distinct parts: an anterior lobe proper, consisting of solid columns of cells, between which run wide and thin-walled blood-channels ; and an intermediate portion, which lies between the anterior lobe and the nervous tissue of the pituitary, forming a closely-fitting investment of the latter.


The anterior lobe contains cells which are clear or hold in their protoplasm varying amounts of deeply-staining granules. They are probably different functional stages of one and the same kind of cell, and the granules give rise to a secretion which is absorbed by the blood-vessels.


The intermediate portion consists of finely granular cells arranged in layers of varying thickness closely applied to the body and neck of the posterior lobe and to the under surface of adjacent parts of the brain. The part of it which is separated from the anterior lobe by the cleft is almost devoid of blood—vessels. In the cat the portion lying in front of the anterior lobe has a tubular appearance and is very vascular. Colloid material occurs between the cells of the pars intermedia, and in most situations appears to pass into the adjacent nervous substance, to be absorbed by blood-vessels or lymphatics.


The nervous portion of the pituitary body is made up of neuroglia cells and fibres. Ependyma cells line the central cavity in the cat and send long fibres forwards and upwards towards the brain, most of which terminate in the outer part of the neck. There are no true nerve cells and the nerves supplying the pituitary probably reach it through sympathetic fibres accompanying the blood—vessels (Berkley). The nervous portion is invaded to a large extent by the epithelial cells of the pars intermedia. Columns of epithelial cells grow into it, especially in the region of the neck, and islets of these cells are frequently found throughout the posterior lobe; in the pituitary of the cat epithelial cells may even grow into its central cavity.

A substance histologically resembling the colloid of the thyroid gland, but probably of a different nature, occurs in large quantities in the nervous portion of the posterior lobe. It appears to be a product of the epithelial cells, and, in the cat at any rate, to be carried by lymphatics into the central cavity, and so into the third ventricle of the brain. In this respect the posterior lobe of the pituitary is an infundibular gland. Whether this substance is modified by its passage through the nervous substance or not is unsettled. Its distribution corresponds with the site of the tissue, the extracts of which have active physiological results when injected into the blood.

The anterior lobe of the pituitary is extremely vascular and its circulation sinusoidal. The posterior lobe is supplied for the most part by a central artery which enters it at its postero-superior angle and runs forward giving of? branches; the veins begin immediately below the epithelial investment and run backwards in this situation, to emerge near the entry of the artery. The veins of both lobes enter large blood sinuses lying close to the sides of the pituitary body.

Histological evidence is against the statement of Bela Haller that the

anterior lobe is a tubular gland which pours its secretion directly into the subdural space.

I am indebted to Mr Richard Muir for the care with which he has executed the drawings and photographs accompanying this paper.

The expenses of the research have been defrayed by a grant from the Moray Fund for the prosecution of research in the University of Edinburgh.

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