Degranulation of mast cells in the rat thalamus

Brain mast cells are selectively concentrated in the thalamus of many mamma lian species. We here describe by light and electron microscopy in the norm al thalamus of adult rats the features of mast cell degranulation, which in dicate an active release of the mediators stored in their intracellular gra nules. The state of activity of thalamic mast cells in basal conditions was found to range from the release of a few granules to a massive degranulati on, and the latter process was much less frequent than a partial degranulat ion. Mast cells were subdivided in three categories (fully granulated, part ially or massively degranulated) on the basis of their cytoplasmic features revealed by acidic toluidine blue staining; the fully granulated cells wer e found to represent only 23% of thalamic mast cells. This strategy of eval uation could be of help in the comparison of the functional correlates of m ast cells in different conditions and experimental paradigms. However, we a lso demonstrated with image analysis a continuum of the variation of staini ng intensity of granulated and degranulating mast cells, without a sharp su bdivision into different categories. Therefore our results reveal that the vast majority of mast cells are active in the thalamus in basal conditions, and that image analysis can provide an objective index of the activity of these cells.