Structure and ultrastructure of the neurohypophysis of the viviparous teleost, Cymatogaster aggregata Gibbons

Abstract

The neurohypophysis (NH) of the viviparous seaperch Cymatogaster aggregata Gibbons is composed of an anterior region made up of predominantly B-type neurosecretory fibers and small numbers of A-type fibers and a posterior region of predominantly A-type and relatively few B-type fibers. The anterior region interdigitates with the pars distalis and the posterior region with the pars intermedia. Glial cells and blood capillaries are common to both regions of the NH. In the posterior region glial cells and capillaries are largely situated along the perimeter of a zone of degranulated A-type (and some B-type) fibers; the zone of fibers is immediately adjacent to the 'boundary membrane' of the pars intermedia.The 'boundary membrane' is formed of two membranes separated by an amorphous matrix. In the posterior NH, the membrane completely separates neurosecretory fibers and adenohypophysial cells, whereas in the anterior NH, the separation is not complete and some neurosecretory fibers directly contact cells of the pars distalis. The 'boundary membrane' is closely connected with blood capillaries and may be involved in the transportation of adenohypophysial and (or) neurohypophysial hormone(s) to the capillary system.Terminals of A-type fibers with "synaptoid" structures are found adjacent to blood vessels, glial cells, and the 'boundary membrane' in the pars intermedia. Terminals of B-type fibers are found adjacent to the 'boundary membrane' in all regions of the pituitary and also close to glial cells: B-type fiber terminals adjacent to acidophils of the proximal pars distalis and blood vessels are less common.