The histogenesis of the isthmic nuclei in chick embryos (Gallus domesticus). I. A morphological study.

Abstract

The neuromeric mes-rhombencephalic boundary runs between the oculomotor and trochlear nuclei. During morphogenesis the m 2-segment is reduced to a cellfree mantle demarcating a morphological mes-rhombencephalic border. The floor plate and glycogen-containing raphe extend rostralwards to terminate at the level of the m 2-segment ventromedially. The isthmic migration commences within the dorsal and dorsolateral rhombencephalic cell columns caudal to the emergence of the trochlear nerve. The neuroblasts migrate radially out from the synthetic zone of the neural epithelium into the mantle constituting the isthmic migration. The latter migrate longitudinally “en masse” rostralwards into the mantle below the optic lobe. The m 2-segment can, however, be identified as a morphological border between the mesencephalic and isthmic (rhombencephalic) mantles throughout the early embryogenesis. The isthmic migration subdivides into a tectal and a tegmental nuclear group. Both groups contribute to the formation of the isthmic nuclei. The caudal portion of the mesencephalic tectal mantle contributes to a mesencephalic isthmic nucleus: Nucleus isthmi principalis mesencephali (magnocellularis).