Ultrastructure of the common extensor tendon in tennis elbow.

In patients with tennis elbow, the common extensor tendon showed a pronounced reactive change consisting of mesenchymal cell proliferation along with aggregates of newly formed vascular channels. When studied ultrastructurally, many of the mesenchymal cells adjacent to the vascular channels were surrounded by a basal lamina, which is not normally seen around tenocytes. The cytoplasm of these cells showed features of both endothelial cells and tenocytes. It thus appeared, that the endothelial cells of the newly formed vascular channels were the source of proliferating mesenchymal cells differentiating toward tenocytes, and together they represented an intrinsic healing mechanism in the tendon.