Histogenesis of the superior colliculus of the albino rat: a tritiated thymidine study

The pattern of generation of neurons in the albino rat superior colliculus has been studied in adult and fetal material. Neurons are generated between embryonic days 12 to 17, with rostrolateral colliculus in advance of caudomedial parts. More of the cells contributing to the deeper layers are generated early, while more of the later generated cells are located superficially. The cells of individual laminae are not formed on specific days as in the cortex, nor are the complicated gradients described previously for the chick optic tectum evident. While the largest cells (found deep in the colliculus) are among the first formed and the small marginal cells among the last, there is in general a broad range of cell size being generated at any one time. The observed patterns are consistent with the concept of simultaneous production of several cell types from the ventricular epithelium on any given day. Studies of material at short times after [3H]thymidine injection allow correlation of the time of arrival of cells in their appropriate layer with time of arrival of afferents. In addition they suggest that factors controlling the final placement of cells in the mature nervous system is a very complex process and may involve some form of intercellular recognition.