The development of hippocampal and dorsolateral pallial regions of the cerebral hemisphere in fetal rabbits. I. Fifteen millimeter stage, spongioblast morphology

Abstract

The structure of the cerebral hemisphere of 15 mm rabbit embryos was studied with aniline and Golgi stains. Three layers of successively less denselypacked cells were identified in aniline stained sections–matrix, intermediate, and marginal laminae. In the matrix lamina two types of cells were identified in Golgi impregnations: (1) neuroblasts with axons ascending to the intermediate and marginal laminae, (2) spongioblasts attached to the ventricular surface by a short proximal process. Two varieties of spongioblasts can be distinguished according to the mode of termination of long distal processes: (1) typical spongioblasts with claviform enlargements terminating at the pial surface, (2) freely‐arborizing spongioblasts with multiple short processes arborizing in the intermediate and marginal laminae. The terminal arbors of freely arborizing spongioblasts, here recognized for the first time, may represent either the growing processes of immature spongioblasts destined to reach the pial surface, or an apparatus having special relations to the embryonic vessels and nerve processes. In the intermediate lamina, pyramidal cells outline the plan of the cortex and subcortical white matter. They have apical dendrites that ascend and ramify in the marginal lamina and axons which descend and turn toward the base of the hemisphere. Neurons with ascending axons are also present, and elements with horizontal axons are located in both the marginal and intermediate laminae.