Abstract: Objective To explore the expression pattern and significance of the α-smooth muscle actin (α-SMA) in the myocardium and endocardial cushion of the outflow tract in the early human embryonic heart. Methods Serial sections of thirtytwo human embryonic hearts from Carnegie stage 10 to Carnegie stage 16 were stained immunohistochemically with antibodies against α-SMA, α-sarcomeric actin (α-SCA) and myosin heavy chain ( MHC ) to observe the α-SMA expression pattern in the myocardium and endocardial cushion during the remodeling of the outflow tract. Results During C10 to C15, the new cardiomyocytes differentiating from the pericardial splanchnic epithelium were added to the distal pole of the outflow tract. The expression of α-SMA of these cells was earlier than the expression of α-SCA and MHC. At C16, α-SMA positive cells were observed in the ridges neighboring with the myocardial cells of the outflow tract wall. The latter linked with these α-SMA positive cells by their protrusion. From C12 to C15, α-SMA positive cells gradually migrated into the endocardial cushion. At the same time, the endothelial cells of the outflow tract began to express α-SMA and differentiated toward mesenchymal cells. Mesenchymal cells of different origins aggregated to form two opposite spiral ridges. Conclusion α-SMA could be regarded as an early marker

Key words: Heart, Outflow tract, α-Smooth muscle actin, Mesenchymal cell, Outflow tract ridge, Immunohistochemistry, Human embryo