Early events in the histo- and cytogenesis of the vertebrate CNS.

Development of the vertebrate, viewed on the cellular level, proceeds by sequential steps in which potencies of progenitor cells become progressively and irreversibly restricted. This is known as progression of the major differentiation. Cytogenesis of the CNS may be regarded as one typical example. The period of cytogenesis in the CNS is divided into three consecutive stages. In stage I, the wall of the neural tube is composed solely of matrix cells. In stage II, i.e., the stage of neuronogenesis, some of the daughter matrix cells are determined at the early G1 phase to be differentiated into neuroblasts. The specificity of individual neurons appears to be irreversibly determined at the time of birth of the neuroblasts, as a function of time-and-place of their production. The individual matrix cells that have existed at the very beginning of neurogenesis give birth to a series of progressively different types of neurons in stage II as the major differentiation proceeds. Finally, matrix cells cease to produce neurons. This is the end of stage II. Thereafter, only non-neuronal cells, namely neuroglia and ependymal cells, are produced. This is stage III or the stage of neuroglia production. The sequential nature of the differentiative behavior of matrix cells can be explained by the hypothesis of progressive gene inactivations that accumulate in genomes of matrix cells during development. Different types of neurons are produced from matrix cells at different states of the "major differentiation".(ABSTRACT TRUNCATED AT 250 WORDS)