THE DEVELOPMENT OF THE MAMMALIAN PITUITARY AND ITS MORPHOLOGICAL SIGNIFICANCE
Authors: Herring P. T. 
Published: December 2014

Development of the pituitary body begins very early in embryonic life. In mammals the epithelial portion is derived entirely from the ectodermic wall of the buccal invagination known as Rathke's pouch. Its origin is single and mesial. The epithelium is differentiated at an early stage into two parts, which show differences in arrangement, structure, and vascularity. One of these, which has been termed the pars intermedia, is closely adherent to the wall of the cerebral vesicle from its earliest appearance, and remains attached to it throughout life. It forms a layer of cells of varying thickness over body and neck of the posterior lobe and adjacent parts of the brain, and tends to arrange itself in positions where it can approach as near as possible to the cerebro‐spinal canal. The cells of the pars intermedia are further characterised by the absence of deeply staining granules from their protoplasm, by their tendency to form a colloid substance in the adult organ, and by their comparatively poor supply of blood‐vessels. Its relation to the nervous part of the pituitary and to the adjacent wall of the brain tends to become even more intimate as development proceeds, by the ingrowth of its cells into these structures.

The other portion of buccal epithelium gives rise to the anterior lobe proper. The lower portion of Rathke's pouch, which is not adherent to the brain, forms a solid mass of cells which grow into surrounding blood‐channels and into the cavity of the pouch itself. Its cells become filled with deeply staining granules and form columns without any lumen, separated from one another by blood‐channels of a sinusoidal character. The original cavity of Rathke's pouch persists as a narrow cleft separating the anterior lobe proper from the epithelial investment of the posterior lobe. The cleft remains a closed cavity, which varies in extent in different species and in different individuals of the same species. In the cat embryo there is evidence of some proliferation of cells of the anterior end of the fore‐gut; these soon disappear, and do not enter into the formation of the adult pituitary.

The infundibulum is an invagination of part of the wall of the thalamencephalon which is adherent to the anterior and upper wall of Rathke's pouch. It therefore possesses an epithelial covering derived from the latter. The infundibular process grows backwards, and, in the cat, retains its central cavity. It is lined by ependyma cells which during development become elongated, so that ependyma fibres run obliquely in its neck. The body of the lobe consists of ependyma and neuroglia cells and fibres; no true nerve cells are present in it, and there is very little connective tissue. The posterior lobe of the pituitary is, from the first, a composite structure of epithelium of the pars intermedia and of neuroglia and ependyma, and the relations between the two tissues become more and more intimate. Its vascular supply is derived from a different source from that of the anterior lobe; blood‐vessels grow into it at its posterior‐superior angle and form true capillaries in the lobe.

The intimate nature of the connection between the wall of Rathke's pouch and the cerebral vesicle, and the maintenance of a close relationship between the cells of the pars intermedia and the cerebro‐spinal canal, render it probable that the pituitary body of mammalia is to be regarded as the representative of an old mouth opening into the canal of the central nervous system. Such an arrangement exists in its simplest form in the Ascidian larva. A connection between Rathke's pouch or original mouth‐cavity and the interior of the infundibulum is sometimes seen in the developing cat, and in the adult cat it is not uncommon to find epithelial cells, derived from the buccal cavity, lying inside the posterior lobe in communication with the third ventricle of the brain. The relations between epithelium and nervous tissue are not accidental in the mammalian pituitary. The latter may have arisen, as Willey stated, from a functional neuropore, but is more likely to have been produced in the manner indicated by Kupffer. There is less probability of Dohrn's view being a correct solution of the problem. The question is one of great interest, and is by no means settled. The anterior lobe proper is a gland whose secretion must enter the blood directly, and so pass into the general circulation. The pars intermedia, on the other hand, appears to secrete into the brain tissue, and must be regarded as a brain gland. The nature of these secretions, and the question as to whether that of the pars intermedia is modified by its passage through brain substance, await further investigation.

I have to express my indebtedness to Mr Richard Muir for the care with which he has executed the accompanying illustrations. The expenses of the research have been defrayed by a grant fromn the Earl of Moray fund for the prosecution of research in the University of Edinburgh.