The aim of the study was to investigate the distribution and differentiation of cell types in the stroma of human placental stem villi (SV). A total of 14 human term placental tissues were studied. Double immunolabeling was performed for desmin-vimentin, desmin-alpha-smooth actin and vimentin-alpha-smooth actin. Cytokeratin 7, proliferating cell nuclear antigen immunolabeling was also performed. Parallel tissue samples were examined by transmission electron microscopy. HSCORE was performed for the semi-quantitative analysis of distribution of cells in the stroma of SV. Vimentin-labeled cells were mostly distributed in the subtrophoblastic area. Desmin-vimentin double immunolabeling was mainly localized in the triangular area and to a lesser degree in the perivascular area and vessel walls (p=or<0.001). However, desmin-alpha smooth actin labeling was observed predominantly in the vessel wall and perivascular area. Vimentin-alpha smooth actin immunoreactivity was significantly stronger in the triangular and perivascular areas compared to the vessel walls (p=0.003). Ultrastructurally, cells in the stroma of SV were mesenchyme cells, reticulum cells, fibroblasts, myofibroblasts, smooth muscle cells, and Hofbauer cells, filamented and vacuolated cells. The differentiation of myofibroblasts in the triangular and perivascular areas may play a role in maturation of SV and villous contractility, modulation of the intervillous space and this may have effects on maternofetal placental circulation.