Immunohistochemical and topographic studies of dendritic cells and macrophages in human fetal cornea.

PURPOSE To investigate the distribution and phenotype of major histocompatibility complex (MHC) class II-positive dendritic cells and macrophages in normal human fetal cornea in the age range 10 to 25 weeks gestation. METHODS Peroxidase and gold immunohistochemistry were used to visualize MHC class II and macrophage antigen (S22) immunoreactive cells. Cell distributions were analyzed quantitatively, and topographic maps were produced. RESULTS Immunoreactive cells, concentrated centrally, were present at 10 weeks gestation in the corneal epithelium and stroma. Average densities increased steadily up to 25 weeks gestation. Two morphologic forms of MHC class II and S22 immunoreactive cells were observed--large, dendritiform cells and small, rounded cells with short processes. Electron microscopy revealed that most MHC class II-positive cells were morphologically consistent with previous ultrastructural descriptions of corneal Langerhans cells. Immunoreactive cells were more numerous in immunogold-labeled specimens than in peroxidase-labeled specimens of similar ages. However, quantitative analysis of both techniques revealed that S22-positive cells comprised 30% to 50% of MHC class II-positive cells. CONCLUSIONS This study provides a detailed description of heterogeneous populations of MHC class II and S22 immunoreactive cells in the human fetal cornea. In contrast to the adult cornea, which is typically devoid of MHC class II-positive cells, immunoreactive cells in the fetal cornea are concentrated centrally and increase in density up to at least 25 weeks gestation. These results indicate that reduction in Langerhans cell numbers to adult levels must occur after 25 weeks gestation. The presence of dendritic cells and macrophages in the fetal cornea has important implications for the understanding of corneal immunology.