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Less than 5% of prenatal thymoctes express HLA-DR before week 12 of gestation. However, the number of HLA-DR- positive cells increases during the late second and third trimesters of development. To determine the role of alpha-smooth muscle actin in fetal thymic HLA-DR signaling in different stages of development we examined and compared the immunohistochemical expression of alpha-smooth muscle actin in the myoid cells of the thymic medulla stroma in the 2 nd, and 3rd trimesters of gestation respectively, over the equivalent expression of the protein in the 1 st trimester, in relation with the appearance of HLA-DR-positive thymocytes. Our results demonstrated a quantitative difference in the second and third trimesters of development concerning the expression of alpha-smooth muscle actin in the stromal myoid cells of the thymic medulla over the equivalent expression of the protein in the first (P