Process of differentiation of cerebellar Purkinje neurons in the chick embryo.

The microscopic and ultrastructural differentiation of Purkinje neurons has been studied in 40 chicken embryo cerebella, from the 10th incubation day to hatching, and the transverse diameter of the cell body measured, for each developmental stage, on 30 electron micrographs of sagittally cut Purkinje cells. The developing Purkinje cell bodies, bipolar, at first, given the presence of two processes emerging from the opposite poles of the oval perikaryon, grow progressively in size. After the 12th incubation day, they develop a branched dendritic tree, and, shortly before hatching time, the cells acquire the characteristic flask or pear-shaped configuration. On the 10th incubation day, microtubules are already detectable together with Golgi complexes and a few vesicles of rough endoplasmic reticulum; on the 14th incubation day, RER cisterns are recognizable in the supranuclear cytoplasm, later extending into the whole perikaryon, and attaining their definitive distribution by the 18th incubation day. Pinocytotic and coated vesicles, as well as subsurface cisterns are seen during the whole embryonic life. In the earliest stages of development, three distinct types of junctional contacts between Purkinje cells and surrounding axons are described, and their functional role in relation to synaptogenetic processes is discussed. Beginning with the 16th incubation day, some Purkinje neurons undergo degenerative changes similar to those described in other types of neurons of the central and peripheral nervous system.