Electron and immunoelectron microscopy on healing process of the rat anterior cruciate ligament after partial transection: the roles of multipotent fibroblasts in the synovial tissue

Abstract

The healing process of the rat anterior cruciate ligament (ACL) after partial transection was examined. In sham operated samples, the synovial tissue near the infrapatellar fat pad includes an abundance of young fibroblasts which can be classified into two types: A type cells often exist along the surface of the synovial tissue and contain numerous lysosomes, while B type cells are often associated with small vessels and are actively involved in the production of fibronectin and laminin. At 1 week after transection of the ACL, B type cells frequently undergo mitotic proliferations and are eventually incorporated into the endothelium of the growing capillaries extending from the proximal remnants of the synovial tissue to the transected lesion. The transformation of B to A type cells is indicated by our electron micrographs. After 2 weeks, the replacement of the transected lesion by regenerated soft tissue becomes pronounced. After 4 weeks, B type cells in the deeper layer of the regenerated tissue are first involved in the production of type 111 and then in that of type 1 collagens as revealed by the immunocytochemistry. The present study indicates that B type cells are a kind of stem cell: they possess the ability to transform to vasoformative cells involved in a manner suggestive of vasculogenesis, to phagocytic A type cells and to the synthetic fibroblasts in the regeneration of the ACL.