In the eye, mast cells are present in the uveal tract, iris, ciliary body, and choroid of different animal species [
13]. In humans, mast cells are abundant in the anterior and posterior uvea but are absent in the retina [
8,
11]. Within the choroid, mast cells have a characteristic periarteriolar distribution [
13], primarily along the long posterior ciliary arteries (LPCA) and their branches [
13].
In the 1960s, Enerbäck described two morphologically distinct subpopulations of rodent mast cells, based on their specific staining characteristics and preferential tissue homing, i.e., connective tissue mast cells (CTMCs) present in the connective tissues, and mucosal mast cells (MMCs) located on the mucosae of the respiratory and gastrointestinal tracts. CTMCs could be distinguished from MMCs by red staining with safranin due to the presence of large amounts of heparin in their secretory granules. After appropriate fixation and sequential staining with Alcian blue and safranin, MMCs stain blue, being thus differentiated from CTMCs which stain with safranin and are red. Differential affinity for Alcian blue can be visualized with sequential staining, consisting of Alcian blue followed by safranin [
20]. Histochemical studies have revealed rat choroidal mast cells are of the connective tissue phenotype, containing mixed Alcian blue/safranin-positive granules. Transmission electron microscopy confirms these findings by revealing rat choroidal mast cell granules to be of uniform diameter [
13].
In humans, mast cells may be classified on the presence in their granules of high levels of tryptase but little or no chymase (MCT) in intestinal and pulmonary mucosa predominantly found at mucosal sites, or mast cells containing chymase, and little or no tryptase (MCC), and finally, mast cells containing tryptase, chymase, and carboxypeptidase (MCTC) predominantly found in the skin, lymph nodes, and lung and gut submucosa [
6]. May [
11] showed that most of the mast cells in the normal human choroid are of the MCTC variety, whereas McLeod et al. [
15] found that MCT was the predominant variety in the choroid in aged eyes with and without AMD. May [
11] identified chymase- and tryptase-positive mast cells in the human uvea and studied their association with different types of resident uveal cells. Most of the choroidal mast cells contain both chymase and tryptase, in agreement with in vitro studies of choroidal cell suspensions [
16]. In contrast, mast cells of ciliary and sphincter pupillae muscle express a specific, tryptase-positive, chymase-negative protease profile [
11]. The human iris contains only a few mast cells. In rodents, the number of iris mast cells is even lower than in primates and humans [
13]. On the other hand, in the human choroid, many mast cells are in the inner part toward the capillary layer. This difference in mast cell distribution coincides with characteristic structures of the vascular walls in both tissues. The vessels of the iris have a thick, perivascular sheath forming a barrier for larger molecules such as proteins. In contrast, the choriocapillaris is highly fenestrated, and antigenic substances can easily pass into the choroidal stroma. In this respect, the high number of mast cells in the inner choroid adjacent to the choriocapillaris might be necessary for an appropriate antigenic reaction.