Identification of early neurons in the brainstem and spinal cord: I. An autoradiographic study in the chick

Abstract

Early stages in chick neurogenesis were investigated with tritiated thymidine (3H-Tdr) autoradioraphy to determine the location and identity of the first neurons produced for the central nervous system. These cells have been shown to arise prior to neural tube closure (Sechrist, '75). Chicks were treated at selected intervals between 20 and 72 hours of incubation with 3H-Tdr in a modified pulse-labeling technique, and terminated on the 18th day of embryonic development (E18), when neuronal types could be determined. Some of the earliest neurons start their final DNA synthesis before 20 hours of incubation (head process, Hamburger-Hamilton stage 5). These are primarily medium-sized cells of the reticular formation in the medulla and at the diencephalic-mesencephalic junction, but also in the intermediate zone of the spinal cord. Motor neurons of the brainstem and spinal cord begin to appear next, after 26-28 hours incubation; the first sensory neurons arise after 32 hours. Other workers (Ramon y Cajal, '60; Tello, '23; Windle and Austin, '36) found that neurons of the reticular formation were the first to differentiate neurofibrils, during the latter part of E2, indicating that fibrillogenesis in these cells begin about 24 hours after the initial cessation of DNA replication.

Time of origin of the neuroblasts in the neural tube of the chick embryo determined by histochemical observation of acetyl - cholinesterase activity.

“…They were medium-sized cells and located in both the ventral and the dorsal halves of the wall of the neural tube. Some authors carried out autoradiographic studies using 3H-thymidine and reported that the first neurons in the central nervous system appeared in the lower half of the rhombencephalon (Sechrist, 1975;Fujita, 1977;McConnell and Sechrist, 1980). As to the region in which the first neurons appeared, our observations were in agreement with their reports.…”

“…Some authors (Fujita, 1964;Martin and Langman, 1965;Langman and Haden, 1970;Hollyday and Hamburger, 1977) regarded the unlabeled cells on the autoradiogram as neuroblasts and others (LaVail and Cowan, 1971;Kanemitsu, 1977;McConnell and Sechrist, 1980) the heavily labeled ones as neuroblasts. The time when 3H-thymidine was administered was considered as the time when these unlabeled or heavily labeled cells differentiated from the neuroepithelial cells to the neuroblasts.…”

Development of vestibular afferent projections into the hindbrain and their central targets

“…Specifically, the study of Rubel et al [86] found that the magnocellular nuclei had their terminal mitosis between E2.5 and 3 whereas the laminar nuclei were generated between E3.5 and 4. Comparable to mammals, the vestibular nuclei are generated somewhat earlier, around E2 [70]. No such data exist for any other vertebrate to date, but would be interesting with respect to formation of vestibular neurons prior to auditory neurons in a given rhombomere.…”

“…The origin of auditory and vestibular nuclei neurons has been studied with thymidine radiography in mice, rats and chicken [1,70,86]. Altman and Bayer [1] described an orderly progression of birth dates for vestibular and cochlear nuclei in rats.…”

Signals from the notochord and floor plate regulate the region-specific expression of two Pax genes in the developing spinal cord

“…In the rat spinal cord, most motor neurons are generated between embryonic days 11 and 13, while the majority of the neurons in the intermediate gray and the dorsal substantia gelatinosa appear on embryonic days 12-15 and 14-16, respectively (Nornes and Das, 1974). In the chick, most neurons in the spinal cord are born after stage 10 with the exception of a few postmitotic cells in the cervical region (McConnell and Sechrist, 1980). As in the rat spinal cord, neurons are born in a ventral-to-dorsal gradient (Langman and Haden, 1970).…”