Spatial features of the rat hippocampal vascular system.

Earlier investigations in this laboratory demonstrated paired internal transverse arteries and veins associated with the rat cornu Ammonis (CA), mapped the vessels, and suggested segmental distribution and drainage patterns. Further studies of the smaller branches and capillaries were required to resolve the presence or absence of structurally segmented or isolated capillary beds common to vertebrate forms having paired intramedullary arteries and veins. Adult Wistar rats were injected with inks to demonstrate the arterial or venous tree or both. Each of the CA3 internal transverse arteries usually supplies numerous small-diameter branches to the adjacent blade of the fascia dentata. Often a major ramus supplies CA1 before the artery branches most profusely near the deepest point of the hippocampal fissure to supply the extrahilar CA3 and portions of the area dentata. Other vessels are located in the subiculum, adjacent regions of CAl, and the area dentata. Branches of the CA3 vessels more nearly parallel the lateral hippocampal axis than either of the other two. Numerous microvascular rami distribute obliquely across the longitudinal and transverse axes. As readily observed from thick, cleared India ink tissue sections, the strata molecularelacumosum and oriens are more vascular than the stratum radiatum where the long axis of capillaries tends to be oriented parallel to the apical dendrites. Clearly a subzone of CA1 stratum pyramidale, but not CA3, is the least vascular CA region. The outer portion of stratum moleculare of the fascia dentata is more vascular than the inner third. Well-developed capillary plexuses exist adjacent to the vascular poor zone of stratum granulosum and CAl. Branches of the internal transverse veins, deep veins, and external veins drain CA3 fields. Evidence of nonexclusive pairing of the internal arteries and veins, multiple systems draining the CA3 field, and the presence of anastomosing capillary beds indicate that structurally isolated segmental microvascular organizations do not exist for CA3 of the rat hippocampus.