An electron microscopical study of the human neurohypophysis

Summary The human posterior pituitary obtained at operation was studied with the electron microscope.The fine structure of the human neurohypophysis was found to be basically similar to that of the other mammalian and cold-blooded vertebrates examined.In addition to the nerve fibres and their swellings two cell types were seen: the pituicytes and the cells in the perivascular spaces — the fibroblasts.The distribution, size and shape of the pituicytes was variable. Long pituicyte processes were seen (sometimes > 20 in length) running between nerve fibres and their swellings, and ending on or in the perivascular space.Several types of large nerve swellings were seen: a) those containing only elementary granules, mitochondria and small vesicles; b) modified swellings characterised by contorted mitochondria and mitochondrial fragments. The latter were often surrounded by single or multiple highly osmiophilic myelin-like membranes; c) multilamellar bodies characterised by concentric osmiophilic myelin-like membranes or lamellae apparently dividing the swellings into separate compartments. The small vesicles, appearing in considerable numbers in such swellings, are suggested to originate from the osmiophilic lamellae.While some true synaptic vesicles may occur in non-secretory nerve fibres, the small vesicles (250–600 Å) seen in most nerve fibre swellings were thought to be derived from elementary granules.Repeated swellings were seen in individual nerve fibres and their functional significance discussed.The electron-dense material within the elementary granules was variable and is suggested to be dependent on the functional state of the neurohypophysis rather than on hormone content. Internal structure was often seen in the elementary granules. The impression is gained that the osmiophilic content of the elementary granules was composed of microvesicles possibly representing molecular aggregations of the hormone-neurophysin complex.