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 day of incubation, the earliest neurofibrillar differentiation occurred in neuroblasts located at the diencephalic-mesencephalic junction and in the rhombencephalon; some of these neuroblasts were believed to become reticular neurons. Since calretinin, a cytosolic calcium-binding protein of the "E-F hand" family, occurs in reticular neurons, the present study investigated immunohistochemically whether the early differentiating reticular neurons are also the first neurons to express this marker during chick embryo development. The first calretinin-immunoreactive neuroblasts appeared at stage 11 (40-45 h of incubation according to the series of Hamburger and Hamilton), and were located in the basal plate of the diencephalic-mesencephalic junction and of rhombomeres adjacent to the otic placode and in the alar plate and intermediate zone of the cervical spinal cord. In bromodeoxyuridine-injected embryos, these earliest calretinin-immunoreactive neurons were shown to express the calcium-binding protein 11-16 h after their last mitosis. By stage 11 up to the 14th day of incubation (stage 40), the calretinin-immunostained neurons increased in number and ultimately formed a chemically defined subset of neurons belonging to the tegmental reticular formation and raphe region of the brainstem. In the meantime, early calretinin-immunostained nerve processes were shown to form two conspicuous longitudinal bundles which run in the ventral and lateral margins of the brainstem and spinal cord.