First appearance and distribution of calretinin-immunoreactive neurons in the early development of the chick central nervous system

Silver impregnation studies in chick embryos have shown that, by the 2nd da y of incubation, the earliest neurofibrillar differentiation occurred in ne uroblasts located at the diencephalic-mesencephalic junction and in the rho mbencephalon; some of these neuroblasts were believed to become reticular n eurons. Since calretinin, a cytosolic calcium-binding protein of the "E-F h and" family, occurs in reticular neurons, the present study investigated im munohistochemically whether the early differentiating reticular neurons are also the first neurons to express this marker during chick embryo developm ent. The first calretinin-immunoreactive neuroblasts appeared at stage 11 ( 40-45 h of incubation according to the series of Hamburger and Hamilton), a nd were located in the basal plate of the diencephalic-mesencephalic juncti on and of rhombomeres adjacent to the otic placode and in the alar plate an d intermediate zone of the cervical spinal cord. In bromodeoxyuridine-injec ted embryos, these earliest calretinin-immunoreactive neurons were shown to express the calcium-binding protein 11-16 h after their last mitosis. By s tage 11 up to the 14th day of incubation (stage 40), the calretinin-immunos tained neurons increased in number and ultimately formed a chemically defin ed subset of neurons belonging to the tegmental reticular formation and rap he region of the brainstem. In the meantime, early calretinin-immunostained nerve processes were shown to form two conspicuous longitudinal bundles wh ich run in the ventral and lateral margins of the brainstem and spinal cord .