The distribution of acetylcholinesterase in the hippocampal formation of the monkey.

In order to study the distribution of acetylcholinesterase (AChE) in the primate hippocampal formation, we have stained serial sections through the brains of nine macaque monkeys for AChE by two variants of the Koelle acetyl-thiocholine method. We have found a distinctive pattern of staining in the hippocampal formation which varies in intensity both from region to region, and along rostrocaudal and radial gradients within each region. In the dentate gyrus, there is intense staining of the inner one-third of the molecular layer with much lighter staining in the rest of the molecular layer except for a moderately stained band at its outer edge. In the caudal half of the dentate gyrus, the inner portion of the molecular layer is less intensely stained though there is a distinctly denser band of staining just above, and partly within, the superficial margin of the granule cell layer. The granule cells are unstained but there are AChE-positive fibers which run through the granule cell layer to the molecular layer. The hilar region of the dentate gyrus has a narrow band of heavy staining (which corresponds to an acellular layer in Nissl-stained sections) just subjacent to the granule cell layer; the remainder of the hilus, where most of the hilar cells reside, is less intensely stained and at caudal levels is almost entirely unstained. In the regio inferior of the hippocampus, there is intense staining of the stratum oriens which extends into the pyramidal cell layer; the stratum radiatum and the stratum lacunosum-moleculare are also stained and here the staining pattern shows some degree of stratification. By contrast, most of the alveus, the pyramidal cell somata, and the layer of mossy fibers (stratum lucidum) are unstained. The border region between regio inferior and regio superior of the hippocampus (field CA2 of Lorente de No, ′34) is especially heavily stained. This contrasts markedly with regio superior, which is more lightly stained than regio inferior. Stratum oriens and stratum radiatum of regio superior have a more evenly distributed pattern of staining, though the intensity of staining increases sharply at the border with the subiculum. Stratum lacunosum-moleculare is only lightly stained throughout much of the transverse extent of regio superior but there is also a conspicuous and constant patch of heavier staining at the border with the subiculum. The pyramidal cell layer of the subiculum is entirely unstained but there is a thin band of staining just above the pyramidal cell layer in the molecular layer of the subiculum which is most noticeable at about the middle of its rostrocaudal extent. The most distinctive layer of the subiculum in acetylcholinesterase preparations is the deep or polymorphic layer which, throughout its rostrocaudal extent, contains numerous stained fibers and cell bodies. The deep portion of the outer, cell dense layer of the presubiculum contains a thin plexus of AChE-positive fibers as well as lightly stained capillary profiles. The superficial portion of the principal cell layer is relatively unstained and contrasts with the fairly heavy staining of the molecular layer, which is especially pronounced in the medial half of the presubiculum. The superficial, or primary, cell layer of the parasubiculum can be divided into three subdivisions on the basis of cytoarchitectonic appearance and on acetylcholinesterase staining characteristics. The lateral and dorsal divisions of this layer stain darkly for acetylcholinesterase while the medial division is less intensely stained. The rostral portion of the entorhinal cortex, which has cytoarchitectonic features characteristic of the lateral entorhinal cortex in nonprimates, is diffusely stained with the greatest density of precipitate observed in layers III and V and the lateral half of layer I. The pattern of staining is more laminar at more caudal levels (medial entorhinal cortex) where layers I, III, IV, and V are distinctly labeled. In the lateral entorhinal cortex, the distribution of acetylcholinesterase-positive fibers in layer III takes on a patchy appearance and this pattern appears to be most closely associated with the irregular patches of cells in this layer rather than the acellular interstices. In the caudal portion of the medial entorhinal cortex, there are distinctive spheres of acetylcholinesterase-positive fibers which straddle layers I and II, and which have tightly packed clusters of layer II cells at their cores. Interestingly, these cells are distinct in appearance from the more common layer II stellate cells.