Human fetal anatomy of the posterior semimembranosus complex at the knee with special reference to the gastrocnemio-semimembranosus bursa

There is little information on the fetal anatomy of the posterior semimembranosus tendinous complex and its associated bursa. We examined histological sections (transverse or sagittal) of the right or left knee in 13 mid-term human fetuses (12–25 weeks of gestation). The medial head of the gastrocnemius provided an aponeurosis facing or attached to the muscles of the pes anserinus by 12 weeks of gestation. The peritendinous tissue of the semimembranosus provided a bursa continuous with a laterally extending plate-like tissue by 15 weeks, but sometimes the typical bursa was absent. The aponeurosis of the medial head consistently accompanied a bursa-like space (false bursa) surrounded by heterogenous structures including the popliteus and a wall of the semimembranosus bursa. Sagittal sections displayed notches on the medial head surface that received the semimembranosus and semitendinosus overriding the medial head of the gastrocnemius. In contrast to a real bursa originating from the peritendinous tissue of the semimembranosus, a false bursa without a homogeneous wall consistently develops at the origin of the medial head of the gastrocnemius. Due to mechanical stress from the tendons, the false bursa is likely to develop into a structure similar to a real bursa with a synovial lining even if the real bursa is absent in the fetus. We hypothesize that the adult gastrocnemio-semimembranosus bursa, largely or partly, originates from the fetal false bursa. Absolute resection of the false bursa is difficult because it is a mere gap between normal tissues.