The squirrel monkey entorhinal cortex: architecture and medial frontal afferents.

The cytoarchitecture of the periallocortex was studied in cresyl-violet-stained frontal and sagittal sections in six adult squirrel monkeys (Saimiri sciureus). The entorhinal area, located between the sulcus semiannularis and sulcus rhinalis in the rostral parahippocampal gyrus, has been divided into a caudal-medial Area 28a and rostral-lateral Area 28b. Of the six paleocortical laminae, Layer II is the most distinctive, for in 28a it consists of a deeply-stained, dense, continuous lamina and in 28b is interrupted into cell islands. Layer IV, lamina dissecans, is thick and irregular in 28b, thin and of uniform thickness in rostral 28a, and disappears in caudal 28a intransition to the isocortex of the more caudal parahippocampal gyrus. Further observations in Fink-Heimser silver material of fiber degeneration resulting from medial prefrontal ablations in these monkeys suggest that Areas 9 and 10 project to syperficial layers (Layer II) of 28a whereas the orbital probably projects to the deeper layers (Layer V) of 28b. The topography and laminar specificity of prefrontal-entorhinal connections may have important functional consequences in terms of hippocampal input.