induction of thymic hla-dr signaling with alpha-smooth muscle actin expression during the second and third trimesters of gestation

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