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Nos observations demontrent l'existence de deux types de cellules glandulaires dans la pars intermedia de l'hypophyse du rat: des cellules propres, a M.S.H., dont les grains de secretion sont detruits par la fixation osmiee et relativement bien conserves par la fixation au glutaraldehyde; des cellules a petits grains denses, osmio-resistants, qui s'apparentent morphologiquement aux elements corticotropes de l'antehypophyse. L'existence bien connue d'A.C.T.H. ou, plus precisement, d'un facteur ≪A.C.T.H.-like≫ dans le lobe neuro-intermediaire nous fait considerer comme tres vraisemblable la nature corticotrope de ce deuxieme type cellulaire de la pars intermedia.

La fixation osmiee et l'etude ultrastructurale permettent de preciser que les grosses vacuoles intraneuronales des cellules du N.S.O. de la Souris, qui apparaissent apres une surcharge osmotique prolongee, ont un contenu lipidique.

Dans le tiers posterieur du recessus infundibulaire du Rat existe une zone periventriculaire colorable par le P.A.S. dont l'etude ultrastructurale revele les caracteres glandulaires L'intervention possible de cette ≪glande infundibulaire periventriculaire≫ dans la regulation de certaines fonctions antehypophysaires est discutee.

Apres section de tige les fibres neurosecretoires disparaissent et les pituicytes reprennent un aspect et une organisation ultrastructurale de type embryonnaire. L'absence de reaction inflammatoire mesenchymateuse et de proliferation pituicytaire cicatricielle de type gliose expliquent les possibilites de regeneration des fibres neurosecretoires.

The morphological effects of colchicine on the entire neurosecretory (NS) tract and on various hypothalamic nuclei have been studied. The perturbation in axonal flow, indicated by the accumulation of NS material, coincide with fragmentation of the cytoplasmic membranes, i. e. the Golgi apparatus and the endoplasmic reticulum, whereas the neurotubules remain relatively well preserved. Autophagic destruction of NS material is observed along the entire length of the NS fibres. The rapid and systematic changes in the axoplasmic reticulum, known to store calcium, lead us to envisage a role for this system — similar to that of the sarcoplasmic reticulum — in controlling the transport of NS vesicles. The junctional zone between the stalk and the neural lobe seems to play a particular role in the transport of NS material to the posthypophysial terminals of the NS axons. Colchicine provokes an increase in dense-cored vesicles in most of the neurons of the other hypothalamic nuclei studied: arcuate, suprachiasmatic, periventricular and ventromedial. Membranous alterations are also observed in these sites. Colchicine administered to animals which were hypothyroid, castrated or adrenalectomized, reveals stimulated neurons, identified by their excessive content of dense-cored vesicles. These neurons display no specific localization, for they occur in all hypothalamic nuclei, irrespective of the stimulation. The frequency of stimulation of neurons of the periventricular nucleus is striking.

The pars tuberalis of the rat, mouse, garden door mouse, European hamster, cat, cattle, chicken and newt is composed of two main cell types: specific secretory cells and follicular cells. The specific cells are characterized by comparable morphologic features in the investigated species, despite differences in the diameter of the secretory granulated vesicles; the ultrastructural morphology of these cells is different from that of any of the known cell types of the adenohypophysis. The follicular cells are devoid of secretory granules, they do not only line the numerous follicular cavities of the pars tuberalis but may also be found in the periphery of the cell cords (border cells). In addition, gonadotrophic cells are found; they predominate in the distal portion of the pars tuberalis and are definitely activated by castration or hypophysectomy. Experimental interventions on most of the major endocrine systems did not cause any noticeable ultrastructural changes in the specific cells. It appears certain that the pars tuberalis secretes a specific hormone whose function remains to be determined.