The hypothalamo-hypophysial system in Acipenseridae. IX. Formation of monoaminergic neurosecretory cells in the preoptic nucleus region during early ontogeny.

"Falck-positive" monoaminergic neurosecretory cells (M-NSC) are found in stage I-II sturgeon larvae in the dorsal zone of the preoptic nucleus (PN) by means of fluorescence and electron microscopy. Single poorly differentiated M-NSC are located mostly among the ependymal cells. At this stage the M-NSC have very short dendrites protruding into the third ventricle. In the process of differentiation the M-NSC move subependymally and their dendrites become longer. Axons of the M-NSC can be clearly traced only in stage IV larvae. Perikarya and dendrites of M-NSC contain little rough endoplasmic reticulum and numerous neurosecretory granules (NSG) 60-150 nm in diameter. Ventricular dendrites of the "Gomori-positive" peptidergic neurosecretory cells (P-NSC) containing NSG 110-200 nm in diameter are observed within the PN in stage III larvae. Thus the M-NSC differentiate earlier than the P-NSC. The intraventricular terminals have kinocilia and show typical macroapocrine secretion. This fact supports the view of a receptory as well as a secretory function of the M-NSC and P-NSC. It is proposed that in early ontogeny monoamine as well as peptide neurohormones are secreted into the cerebrospinal fluid. This transventricular pathway is considered phylogenetically ancient.