Immunohistochemical identification and characterization of a special type of desmin-producing stromal cells in human placenta and other fetal tissues

An unusual type of stromal cells has been found to be abundantly present in chorionic villi of human placenta of gestational weeks 6, 17, 35–42 and in tissues of early stages of fetal development (gestational weeks 16–21). These mesenchymal cells are loosely arranged throughout the villous interior and contain the intermediate filament (IF) proteins vimentin and desmin; however the smooth muscle (sm) markers sm-α-actin and sm-myosin are absent. Typical myoid stromal cells that are positive for both desmin and sm-α-actin also occur in this tissue but are restricted to certain dispersed cell clusters associated with blood vessels. Similar disperse desmin-positive, sm-α-actin-negative stromal cells have also been identified, although more sparsely, in the chorionic plate of the placenta and in other diverse fetal tissues such as the interstitium of the kidney, of testis and epididymis, and in cells surrounding Hassall bodies of thymus. The biological nature of these desmin-containing but sm-α-actin-negative stromal cells is discussed in relation to myoid cell differentiation. It is emphasized that despite their synthesis of considerable amounts of desmin they cannot be considered myogenic as the occurrence of desmin in the cells may represent an isolated expression of an individual IF protein gene, independent of the synthesis of other muscle proteins.