Early differentiation of motor neuroblasts in the chick embryo as studied by electron microscopy: I. General aspects

Abstract

The early differentiation of the motor neuroblast of the chick embryo has been studied at the fine structural level. Material from embryos from 33 hours to 8 days incubation has been examined, with particular attention to identification of motor neuroblast cells at successive stages of development. Previous to the visible differentiation of neuroblasts the cells of the ventrolateral neural tube are columnar epithelial cells with the following characteristics: many free ribosomes throughout the cytoplasm, scant, scattered granular endoplasmic reticulum, scattered mitochondria, fairly prominent Golgi material medial to the nucleus, junctional zones near the apical ends of the cells, cilia extending into the neurocoel, and large nuclei with prominent nucleoli. The first changes that have been observed in the differentiation of the motor neuroblast (2-day embryo) are the appearance of areas of filaments in the cytoplasm and the formation of an axon. The axon contains filaments, some mitochondria, small smooth-surfaced vesicles, and a few free RNP particles. Subsequently (3-day embryos) the medial end of the neuroblast is withdrawn from the neurocoel, there is a change in organization in the cell body such that the nucleus is medial to the larger area of cytoplasm containing Golgi complex and centrioles, and the granular endoplasmic reticulum begins to increase in amount. The next changes (4-day and older embryos) include the outgrowth of dendrites, which contain many filaments and also most of the organelles found in the perikaryon, and the appearance of filaments in the perikaryon arranged as in mature neurons. The Golgi material is reorganized into a number of separate zones scattered in the perikaryon and dendrites. The granular ER continues to increase in amount, but remains scattered, throughout the period considered. These observations are discussed in relation to other work on development of the nervous system and on differentiation.