Neurons in the dorsal tegmentum of the midbrain of the teleostsPoecilia sphenopsandP. latipinnawere examined by use of electron microscopy. A nucleus of neurosecretory neurons was identified in the subependymal region just dorsal to the medial longitudinal fascicle. This nucleus has been called the dorsal tegmental magnocellular nucleus (DTMN). The most distinguishing cytological feature of these cells is the presence of large granular vesicles, 100–180 nm in diameter. These vesicles resemble neurosecretory granules characteristically found in preoptic and lateral tuberal magnocellular neurosecretory cells. Presynaptic terminals on these cells contain small clear vesicles, and some among them contain small dense-core vesicle
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