Organization and development of brain stem auditory nuclei of the chicken: Ontogeny of N. magnocellularis and N. laminaris

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Abstract
Nucleus magnocellularis (NM) and nucleus laminaris (NL) are, respectively, second‐ and third‐order auditory nuclei in the chicken brain stem. In this report the morphogenesis of these nuclei is examined. The times of origin of the cells of these nuclei were studied with 3H‐thymidine autoradiography. The number of cells in each nucleus and their rostro‐caudal distribution within each nucleus was determined in Nissl‐stained sections at 9, 11, 13, 15, 17, 19, and 21 days of incubation. For the above ages the volumes of NM and NL were also calculated planimetrically and the rostro‐caudal distribution of volume within each nucleus was studied. The results indicate that the times of origin of NM and NL neurons do not overlap — the cells of NM are formed at around 60 hours of incubation, while those of NL are formed at around 84 hours. At nine days, both nuclei have a roughly equal complement of cells; however by 21 days (hatching) the number of NM neurons has decreased by 18%, while the number of NL neurons has decreased by 84%. In both nuclei most of the cell death occurs in the period between 11 and 13 days, during which time NL assumes a laminar appearance. A rostro‐caudal gradient of cell death was found for both nuclei. The times of volume increases in NM and NL commence at the end of the period of cell death, beginning at 13 days for NM and at 17 days for NL, whose cell death is more protracted. For both nuclei a rostro‐caudal gradient of volumetric expansion was also found.