Tehran University of Medical Sciences Acta Medica Iranica 0044-6025 41 2 2003 06 15 73 78 Tamiolakis D Venizelos J Kotini A Skafida P Cheva A Papadopoulos N 2015 09 28 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 https://acta.tums.ac.ir/index.php/acta/article/view/2638 https://acta.tums.ac.ir/index.php/acta/article/download/2638/2620