P311 is expressed in human visceral and vascular SM cells and myofibroblasts. All the sections have been double immunostained for P311 and SM α-actin, the latter to identify SM and myofibroblasts. (a) P311 is present in both visceral (intestine) and vascular SM (first and second panels down, respectively). P311 is not synthesized by fibroblasts (third panels), but is produced by myofibroblasts (fourth panels). (b) Immunohistochemical studies performed at an early stage of wound repair demonstrate P311 in most of the activated fibroblasts, also referred to as premyofibroblasts (SM α-actin–negative myofibroblast precursors, arrow in upper left photo). In more advanced stages of wound healing (two middle panels), P311 is detected in myofibroblasts (activated, SM α-actin–positive cells). Arrowhead in the second left photo points to a regular fibroblast present in the same field. Notice its smaller size, lack of discernible nucleoli, and negativity for P311. Once myofibroblasts disappear and the wound contains only regular fibroblasts, P311 is no longer detected (lowest panels).