Transmission electron micrograph (TEM) of the ultrastructure of a luteal cell in the corpus luteum of the ovary. Luteal cells produce sex steroid hormones such as progesterone and oestrogen. Steroidogenic cells typically have smooth endoplasmic reticulum and lipid inclusions required for the synthesis of steroids. The corpus luteum serves to prepare the uterine lining if a fertilised egg is available for implantation. Magnification: x3,000 when printed at a width of 10cm.