[Ontogenesis of pro-opio-melano-cortin neurons in the rat]

Immunocytochemical study of the pro-opio-melano-cortin neurones in rat fetuses and newborns indicates that: 1) they appear precociously in the mediobasal hypothalamus, as early as day 13 of fetal development, that is to say 3 days before pro-opio-melano-cortin-containing pituitary cells; 2) between day 13 and day 17 they display synchronous changes in their intracellular patterns of immunoreactivity, arguing in favour of their cyclic secreting activity, which precedes the establishing of their projections; 3) beginning with day 18 until birth, the somatal immunoreactivity disappears transitorily; during the same period and the first postnatal days, their immunoreactive fibres extend in various brain regions.