HISTAMINERGIC AND NON-HISTAMINE-IMMUNOREACTIVE MAST-CELLS WITHIN THE CAT LATERAL GENICULATE COMPLEX EXAMINED WITH LIGHT AND ELECTRON-MICROSCOPY

Mast cells and their location in the cat lateral geniculate complex of the thalamus were examined by means of histamine immunohistochemistry and the mast cell stain pinacyanol erythrosinate. Brain sections from seven normal adult pigmented cats were processed for light or electro n microscopy. Histamine-containing and pinacyanol erythrosinate-staine d mast cells were widespread throughout the dorsal and ventral lateral geniculate nuclei and the surrounding regions. Mast cells were especi ally numerous rostrally in the complex and in the geniculate C laminae . The cells were found consistently in association with blood vessels, ranging from capillary size to vessels c. 150 mu m diameter, and twic e as often with arterioles as with venules. Large clusters of many mas t cells associated with single blood vessels were seen. Individual mas t cells were typically 8 mu m in diameter and somewhat oval, although multipolar and crescent-shaped cells were also seen, up to twice as lo ng. The amount of histamine labeling varied across cells. When histami ne-labeled material was secondarily stained with pinacyanol erythrosin ate, many mast cells were double labeled. In addition, there was a sma ll population of mast cells that stained only with pinacyanol erythros inate, but was otherwise identical to the histamine-immunoreactive mas t cells. Electron microscopic examination showed that the mast cells l ie on the brain side of the blood-brain barrier. Mast cells were found in close proximity to the thalamic neuropil, primarily apposed to the processes of astrocytes, but also apposed to neural elements. The dis tinctive electron-dense cytoplasmic granules in the fully granulated, mature state were largely amorphous in appearance and as large as 700 nm in diameter. Histamine was dispersed throughout some granules and c ontained within restricted areas of other granules. In degranulated ma st cells, large, irregularly shaped, electron-lucent granules were see n fused with the cell membrane on the neuropil side, as well as the lu men side of the mast cell. More mast cells were observed at the electr on microscopic level than were expected from the light level observati ons, which suggests that, despite the numbers of mast cells labeled, t hese results may still underestimate the total mast cell population pr esent in this region of the thalamus. Mast cells, by their numbers, th eir distribution and the potent chemical substances they contain, may significantly influence vascular and neural function, directly and ind irectly, in the cat lateral geniculate complex.