Electron Microscopy Study of Anterior Neuropituitary of the Sterlet Acipenser ruthenus

Structural organization of sterlet anterior neuropituitary (ANP), a homologue of the median tubercle of Tetrapoda, was studied by method of electron microscopy. The ANP was conventionally divided in three zones in dorsoventral direction. Neurosecretory fibers (NsF) were concentrated mainly in the central and in the external zones. Terminals of NsF are directly adjacent to the basal membrane (BM) of the brain vascular sheath to form a morphological substrate for release of neurohormones into the pericapillary space. The presence of invaginations of the brain connective tissue sheath, which are in a close contact with NsF, indicates an increase of the area of neurosecretory contacts. Neurosecretory granules (NsG) of various shape and size are revealed in NsF and their terminals. Comparison of light-microscopy immunohistochemical data with localization of NsF terminals containing NsG of different diameters allows reconstructing distribution of neurosecretory elements of various ergicity in the ANP. Patterns of co-localization of various biologically active substances were revealed in the same varicose dilatations or terminals. Detection of typical synaptic contacts in the sterlet ANP indicates possibility of the cholinergic pituitary regulation, alongside with the peptide- and monoaminergic regulation.