BIRTH AND DIFFERENTIATION OF RETICULAR NEURONS IN THE CHICK HINDBRAIN - ONTOGENY OF THE 1ST NEURONAL POPULATION

To understand better early neuronal birth and differentiation in higher vertebrates, we have examined the time when presumptive neurons in the chick embryo first withdraw from the cell cycle and express neurofilaments. Hindbrain reticular neurons arise prior to the definitive streak stage of gastrulation and represent the first neuronal population to be born. The birth of hindbrain reticular neurons and spinal interneurons occurs in a rostrocaudal sequence that closely parallels regression of Hensen's node. More rostral brain stem neurons are born shortly after those in the hindbrain. Neurofilament expression in reticular neuroblasts first occurs by the 7-somite stage, followed by axon outgrowth by the 15-somite stage. When neural plate morphogenesis is inhibited, neurofilament expression occurs on schedule in neurons that are undergoing or have completed terminal mitosis. Our results suggest that the inductive cues governing the birth and initial differentiation of reticular neurons are imparted by gastrulation and early neurulation.