Quantitative morphology of human hippocampus early neuron development.

Previous findings in adults revealed significant hemispheric asymmetry in the size of neuronal somata in hippocampal subfield CA2 (the resistant sector) with no age-related changes. A paucity of quantitative data on the developmental status of these protected neurons has led to the investigation of their morphology in comparison to neurons in adjacent subfield CA3, bilaterally. Bilateral coronal sections from postmortem hippocampus, 24 to 76 weeks postmenstrual age (gestational age plus postnatal age), were studied. The neurons were digitized and measured on a computer. Soma size correlated positively and significantly with age in CA2 and CA3, bilaterally. CA2 somata were significantly larger (left, 34%; right, 32%) than adjacent CA3 somata. Variability in soma form or size increased appreciably with age, in both subfields, bilaterally, while variability in soma orientation was weakly correlated with brain growth. The results suggest that in early development there are similarities in hemispheric growth patterns in CA2 and CA3. Large CA2 soma size implies axonal connectivity to distantly located targets very early in development. The results have functional implications, including memory, to brain development.