Synaptogenetic mechanisms during chick cerebellar cortex development.

Chick embryo cerebellar cortex development was analyzed in an attempt to visualize the fine ultrastructural changed involved in cortical synaptogenesis. Three major stages of synapses formation were detected during days 13 to 18 of maturation: intermembranous adherence, cytoplasmic recognition, and definitive synaptic formation period. The more primitive morphological event indicative of synaptic onset was the occurrence of external plaques; these plaques must have a negative charge since they accumulated cationized electron-dense tracers. These plaques favor the adhesions between parallel fibers and Purkinje cell dendrites. Once axonal and dendritic membranes adhered to form an adhesion contact, the dendritic half of this contact forms a pinocytic vesicle that incorporates part of the membrane of the axonic half of the contact. This particular pinosome delivers its content to different cytoplasmic organelles including the nucleus. At day 15 Purkinje cells significantly increase the number of dendritic-forming pinocytic vesicles; this phenomenon means that during this day a large amount of parallel fiber membrane is passed into the Purkinje cell cytoplasm. From day 15 to 16 postsynaptic densities begin to appear; ribosomes associated to these structures seem to be the origin of a progressively accumulated amorphous material composing the rare primitive postsynaptic densities. Some other morphological details related to these processes are given in the text.