Smooth endoplasmic reticulum, TEM. Coloured transmission electron micrograph (TEM) of part of a Leydig cell from the testis to show smooth (agranular) endoplasmic reticlum. Leydig cells are involved in steroid synthesis and secretion of testosterone which is essential for spermatogenesis and for maintenance of secondary sexual characteristics such as hair distribution and pitch of voice. Large amounts of agranular enoplasmic reticulum occupies the cytoplasm (red and yellow) and is characteristic of endocrine cells. Large lipid droplets and mitochondria are also present. Magnification: x11000 when printed at 10 centimetres wide.