Leydig cell, transmission electron micrograph (TEM). Leydig cells are found in the intertubular tissue of the testis. Leydig cells synthesize and secrete androgen sex steroid hormones, chiefly testosterone, essential for sperm production and maintenance of androgen levels in peripheral blood. In addition to abundant smooth endoplasmic reticulum in the cytoplasm, human Leydig cells have large crystalloid inclusions. Known as Reinke crystals they are composed of protein subunits in hexagonal arrays but their function is unknown. Magnification: x3,000 when width printed at 10cm.