Mitochondria. Coloured Scanning Electron Micro- graph (SEM) of mitochondria and smooth endoplasmic reticulum in an ovarian granulosa- lutein cell. The cell occurs in the corpus luteum of the ovary, which develops after ovulation and secretes the hormone progesterone. Mitochondria (pink) are seen sectioned, containing tubular cristae. Active secretory cells have many mitochondria which play a role in cell respiration. Membranes of smooth endoplasmic reticulum (ER, yellow) are also seen; commonly found in secretory cells, ER synthesizes lipids and transports substances around the cell. Magnification: x21, 300 at 6x7cm size. Magnification: x27, 400 at 4x5 inch size.