Ultrastructure of the organum vasculosum of the lamina terminalis of the Japanese quail, Coturnix coturnix japonica.

The structure of the organum vasculosum of the lamina terminalis (OVLT) of the Japanese quail has been studied by light and electron microscopy. The OVLT has a palisade arrangement. It forms a part of the terminal plate extending from the optic chiasma to the anterior commissure and is characterized by a special vascular arrangement. The organ consists of ependymal, internal and external zones. The ventricular surface of the organ is covered by non-ciliated ependymal cells characterized by the presence of raspberry-like ventricular protrusions. The ependymal zone is composed of two types of ependymal cells, one being a large, cuboidal cell with columinous cytoplasmic protrusions suggesting secretion into ventricle, and the other a slender tanycyte with long balar plexus of the external surface. The internal zone contains two types of secretory neurons, parvocellular neurons containing a few dense-cored granules 1,000 A in diameter, and mediocellular neurons containing in their perikarya many larger granules 1,300-1,500 A in diameter and well developed granular endoplasmic reticulum and Golgi apparatus. The granular formations are usually observed in the Golgi area of both types of cells, thus indicating of their secretory activities. There are three types of nerve terminals in the neuropil: (1) nerve endings containing spherical, dense-cored granules 800 A in diameter, which display axosomatic synapses with perikarya of the neurons, (2) nerve endings containing dense-cored granules 1,000 A in diameter and clear vesicles 400 A in diameter, and (3) nerve endings containing dense granules 1,300 A in diameter and clear vesicles. Types 2 and 3 do not display axo-xomatic synapses but often show axo-dendritic synaptic contacts with dendrites in the neuropil; thus they are probably axons originating from the parvocellular and mediocellular neurons of this organ. The functional significance of the neurons and axons of the OVLT is discussed and it is suggested that two kinds of neurohormones may be secreted from the OVLT of the Japanese quail.