DEVELOPMENT OF THE SUPRACHIASMATIC NUCLEUS IN RATS DURING ONTOGENY - TYROSINE-HYDROXYLASE IMMUNOPOSITIVE CELL-BODIES AND FIBERS

This study has evaluated differentiation of tyrosine hydroxylase-immun opositive neurons in the suprachiasmatic nucleus as well as the innerv ation of this nucleus by tyrosine hydroxylase-immunopositive axons in rats during ontogenesis. Tyrosine hydroxylase-containing structures we re detected with electronmicroscopic pre-embedding immunocytochemistry at the 22nd fetal day as well as at the second, ninth and 21st postna tal days. Rare uni- and bipolar small tyrosine-hydroxylase-immunoposit ive neurons were observed in the suprachiasmatic nucleus both in fetus es and postnatal rats. These neurons underwent differentiation over th e perinatal period that was mainly manifested in the increase of their size as well as in the development of the Golgi complex, granular end oplasmic reticulum and the onset of the dense core vesicle production. Concomitantly, tyrosine hydroxylase-immunopositive neurons, cell bodi es and dendrites, became innervated by immunonegative axons first maki ng presynapses, and, then, symmetric (Gray-type II) and asymmetric (Gr ay-type I) synapses. In addition to cell bodies and dendrites, tyrosin e hydroxylase-immunopositive axons were regularly observed in ventral, ventrolateral and ventromedial regions of the suprachiasmatic nucleus in fetuses and postnatal rats. Tyrosine hydroxylase-immunopositive ax ons were observed either in simple appositions with the immunonegative neurons or making presynapses in fetuses and symmetric and asymmetric synapses in postnatal animals. The nature of the tyrosine hydroxylase -immunopositive axons and the functional significance in the suprachia smatic nucleus in ontogenesis are discussed.