Early ontogeny of serotonin-immunoreactivity in the sheep brain. An immunohistochemical study.

Using immunohistochemistry with specific antiserotonin anti-sera, the ontogeny of serotonergic neurons was studied in the foetal sheep brain. Serotonergic-immunoreactive perikarya first appeared rostrally on day 25 of pregnancy, in the medio-ventral part of the mesencephalic flexure, and caudally, on day 28, in the medio-ventral part of the cervical flexure. The development of this system is very rapid, because on day 40 of gestation, all serotonergic nuclei present in the adult were visible. Compared with other species such as rodents or primates, serotonin appears early in the sheep nervous system, and the development of the serotonergic system is even more rapid. Serotonergic immunoreactivity was seen in some cell bodies in the growing adenohypophysis between days 40 and 50. This phenomenon has not been observed in other species. Because serotonin appears very early and is present in growing areas of the nervous system, it could play a trophic role in the development and maturation of the sheep central nervous system, as has been described previously in other species.