# Postnatal Development of the Perirhinal and Parahippocampal Cortices: A Stereological Study in Macaque Monkeys

## TL;DR

This study examines how the perirhinal and parahippocampal cortices in macaque monkeys develop after birth, revealing differences in maturation across brain regions and layers.

## Contribution

The study provides the first detailed stereological analysis of postnatal structural development in these medial temporal lobe regions in macaques.

## Key findings

- Areas 36r and 36c mature later than areas 35, TF, and TH.
- Superficial layers in 36r and 36c mature earlier than deep layers.
- Neuron numbers decrease in area 35 and layer III of area TF between birth and 6 months.

## Abstract

The perirhinal and parahippocampal cortices are two prominent structures of the medial temporal lobe that play essential roles in memory and perceptual processes. In humans, major changes in memory capacities occur within the first 7 years of life, but the neurobiological substrates underlying these changes have long been hypothetical. Previous studies have shown that distinct regions, layers, and cells of the hippocampal formation, including the entorhinal cortex, exhibit different profiles of structural and molecular development. Here, to further understand the postnatal maturation of the medial temporal lobe, we implemented stereological techniques to characterize the structural development of the perirhinal and parahippocampal cortices in macaque monkeys. We found distinct, age‐related differences in volume, neuronal soma size, and neuron number in different layers and subdivisions. Volumetric data indicated a late maturation of areas 36r and 36c compared to areas 35, TF, and TH. There was also an earlier maturation of the superficial layers compared to the deep layers in areas 36r and 36c. We observed a transient increase in neuronal soma size at 6 months of age in several subdivisions. Additionally, we found a decrease in neuron numbers in both the perirhinal and parahippocampal cortices, but particularly in area 35 and layer III of area TF between birth and 6 months. These findings are consistent with the differential maturation of the rostral and caudal entorhinal cortex, which are interconnected with the perirhinal and parahippocampal cortices, respectively. Altogether, they support the theory that the differential maturation of distinct hippocampal circuits underlies the emergence of specific “hippocampus‐dependent” memory processes.

Volumetric data reveal a late maturation of areas 36r and 36c compared to areas 35, TF, and TH, as well as a tendency for superficial layers in areas 36r and 36c to mature earlier than the deep layers. Neuronal soma size exhibits a transient increase at 6 months of age in most subdivisions. Area 35 and layer III of area TF exhibit a decrease in neuron numbers.