Histochemical demonstration of zinc in the hippocampal region of the domestic pig: I. Entorhinal area, parasubiculum, and presubiculum.

A detailed description is given of the distribution of zinc in three areas of the domestic pig hippocampal region, viz., the entorhinal area, the parasubiculum, and the presubiculum. Zinc was demonstrated histochemically with use of the Neo-Timm method, a recent modification of the sulphide silver procedure. Each of the studied areas showed a distinctly stratified staining pattern, which has been correlated in detail to fields and layers defined on the basis of cyto- and fibroarchitecture, providing a combined chemo- and cytoarchitectonic map. The staining was primarily confined to the neuropil, although stained nerve cell bodies were encountered in all three parts of the hippocampal region. Two main subfields were identified in the entorhinal area that have been designated pars medialis and pars lateralis, in accordance with their topographical positions, but both the cytoarchitecture and Neo-Timm staining pattern are indicative of further subdivision. In pars medialis, the deep half of layer I, the interstices between the stellate cell bodies in layer II, and layer III were medium to heavily stained, whereas layer IV stained weakly. Layers V-VI were slightly darker than layer IV and were inseparable on the basis of the Neo-Timm staining. The staining of pars lateralis differed in many respects from that of pars medialis, the most conspicuous feature being a much lighter layer III. In the parasubiculum, the deep half of layer I together with layers II-III had the appearance of an intensely stained triangle wedged in between the entorhinal area and the presubiculum. The latter showed moderate staining of the inner half of layer I and posterior part of layer II, while layer IV was stained intensely. Layers III and V-VI exhibited only weak staining. The distribution of zinc in the pig was compared with that in the guinea pig and rat, described previously. Although many histochemical features are shared by the staining patterns of the three species, striking differences exist in the pig, the most notable being the virtually reverse staining of the entorhinal layer IV. The possible functional implications of zinc in synaptic vesicles are considered.