Neurosecretory endings in the rat neurohypophysis are en passant

A combination of Golgi, Phaseolus vulgaris leucoagglutinin, and transmission electron microscopic (EM) techniques was used to investigate the morphology of neurosecretory axons and their endings in the neurohypophysis of the rat. Light microscopy indicated that the neurosecretory processes are very tortuous, varicose, and branched, often running in close association with blood vessels. EM, as well as reconstruction from serial thin sections, demonstrated that the combination of synaptoid membrane specializations, dense core vesicles, and accumulations of microvesicles, thought to indicate areas of preferential hormone release, can occur anywhere that the axons contact the basal lamina (BL) lining the perivascular space. Usually, but not always, this was accompanied by some degree of axonal dilation. Individual neurosecretory axons frequently entwined around, or ran adjacent to, blood vessels, a short length of axon forming multiple "endings." Thin glial (pituicyte) processes were interposed between these endings. Axonal processes were also seen to end blindly as end-bulbs suspended in the perivascular space where they were often surrounded by pituicyte processes. The morphology observed suggests that reductions in the contact length of individual nerve terminals could be mediated through increased pituicyte coverage along BL. Conversely, a reduction of pituicyte coverage along the BL could lead to more areas of axonal contact with the BL inducing or allowing the formation of more endings, such as has been reported to occur during lactation or prolonged elevation of plasma testosterone levels.