IMMUNOMORPHOLOGICAL STUDIES OF MAST-CELL HETEROGENEITY, LOCATION, ANDDISTRIBUTION IN THE RAT CONJUNCTIVA

Mast cells are crucial components of immediate and some delayed-type h ypersensitivity reactions. They play a pivotal role in allergic conjuc tivitis and other immunoinflammatory disorders of the ocular surface, yet little is known of their distribution and heterogeneity in the con junctiva of potential animal models, such as the rat. In this study, m ast cell types were investigated in histologic sections and corneal-co njunctival-lid whole mounts by using toluidine blue, alcian blue-safra nin, and immunohistochemical staining methods (anti-rat mast cell prot einase [RMCP] antibodies). Quantitative analyses were performed on cor neal-conjunctival-lid whole mounts by using the optical dissector proc edure to obtain the density of mst cells per unit volume in different regions of the conjunctiva. Single and double immunohistochemical anal yses revealed that the mast cells in the conjunctiva of the limbus, fo rnices, and lid margin were strongly RMCP I+, suggesting that they wer e of the connective tissue phenotype. Mst cells containing the mucosal mast cell proteinase RMCP II were not present in the normal conjuncti va. Histochemical analysis revealed that the maturity of the connectiv e tissue mast cells, as assessed by the presence or absence of safrani n (heparin)-positive granules in their cytoplasm varied in different r egions. In the lid margin 60% to 78% of the mast cells were solely alc ian blue-positive, whereas in the fornices 68% to 78% were safranin-po sitive. In the limbus the predominant type of mst cell was either safr anin-positive or contained mixed granules. Mast cell densities were gr eatest close to the lid margin (10,000 to 12,000 cells/mm(3)), followe d by the limbus (3400 to 4800 cells/mm(3)) and were rare in the remain der of the conjunctiva (500 to 1000 cells/mm(3)), with the exception o f the region around the nictitating membrane. This study of rat conjun ctival mst cells provides essential baseline data for future studies o f the role of mast cells in models of allergic conjunctivitis.