The osteotendinous junction (OTJ), or enthesis, is the site of connection between tendon and bone and is also called the tendon insertion site. The unique cellular and molecular composition of the enthesis provides a gradual transition from tendinous to bone tissue. The enthesis is virtually divided into four zones: zone one, starting at the tendon side, consists of aligned collagen I fibers and decorin, and exhibits tendon properties only. The second zone contains collagen types II and III, aggrecan and decorin, resembling fibrocartilage composition. Zone three is defined as mineralized fibrocartilage and is comprised of collagen types II and X and aggrecan. Finally, zone four is composed of mineralized collagen type I and is considered to be a bone protrusion, providing a dedicated connection point. The molecular mechanisms responsible for the formation of this gradient are mostly unknown. However, it was demonstrated in the mouse forelimb model that Bmp4, produced and secreted by maturing tendon cells under Scleraxis transcriptional regulation, is responsible for initiation of bone tuberosity outgrowth at a site of tendon attachment. Other studies reveal the essential contribution of muscle contractions on the formation of bone ridge.