Naturally occurring neuron death in the optic layers of superior colliculus of the postnatal rat.

Application of light and electron microscopic techniques to the superior colliculus of the normal rat shows that a number of neurons die within the first week after birth. Cells in the earliest recognizable stages of degeneration are characterized by an overall increase in electron density and dilation of the intracellular cisternae, although there are only minimal changes in the chromatin pattern of the nucleus. Synapses are found on these cells. A second type of degenerating cell, with more striking changes in the nucleus, also appears in the tissue but very infrequently. In later stages of degeneration, cells are reduced to a condensed chromatin mass surrounded by disrupted fragments of the rest of the cell. This 'cellular debris' is found within glial cytoplasm. The majority of these debris profiles appear in the first postnatal week and are usually most abundant in the caudal third compared to either the rostral or middle thirds of the colliculus. The results suggest that the mechanisms controlling neuron number in the superior colliculus are operative in the postnatal rat but argue against a simple relationship between the survival of a particular neuron and the total number of optic connections that neuron has received.