The human hippocampus observed by scanning electron microscopy (SEM): the dentate gyrus is made of an array of the neuronal lamellae.

Abstract

Scanning electron microscopy (SEM) was employed to clarify the three dimensional structure of the human hippocampus.The polymorphic layer was L-shaped in coronal histological sections. The superior limb and lateral two thirds of the inferior limb formed a continuous plate. This plate consisted of L-shaped bars that were fused side by side with borders that were, although incompletely, demarcated by the stratum granulosum. The medial one third of the inferior limb was independent part of these L-shaped bars and took part in formation of the dentes. There were 40 to 50 dentes, and each had segmental blood vessels. Thus, the polymorphic layer was organized on a segmental plan, 40- 50 in number, arrayed along the antero-posterior axis.CA4 was surrounded by the L-shaped polymorphic layer and also had the superior and inferior crura. The medial end of the inferior crus was enveloped by the medial one third of the polymorphic tissue and was completely independent from its neighbors. Therefore, CA4, too, may be segmentally constructed following the same plan as the bars of the polymorphic layer. These observations suggest that, first, three major components of the hippocampus, the stratum granulosum, polymorphic layer, and CA4, are constructed based on the same lamellar unit in the dentate gyrus, and, second, the individual lamellae appear as distinct bars in the medial one third, but form a plate in the lateral two thirds of these structures. There were 7 to 12 pyramidal hillocks, organized of the central process and its covering, in the subiculum. Pyramidal cells showed clear polarity in the hillock; the cell apex oriented to the central process and the base to the periphery. The axon emitted from the cell base and formed the alveus. Pyramidal hillocks caused slight waves of the stratum pyramidale on the lateral border of the hippocampus but did not affect the superior surface. Functional aspects of the segmental arrangement of neuronal units along the antero-posterior axis and their medio-lateral diversity were discussed in terms of the three-synapse pathway in the hippocampus.