Color enhanced transmission electron micrograph showing organelles of smooth endoplasmic reticulum, mitochondria and nucleus. Leydig cell - rat testis. Magnification 120,000x. Smooth endoplasmic reticulum, a network of tubular membranes within the cytoplasm of the cell, occurring either with a smooth surface (smooth endoplasmic reticulum) or studded with ribosomes (rough endoplasmic reticulum), involved in the transport of materials. Mitochondrion, an organelle in the cytoplasm of cells that functions in energy production. Nucleus, a specialized, usually spherical mass of protoplasm encased in a double membrane, and found in most living eukaryotic cells, directing their growth, metabolism, and reproduction, and functioning in the transmission of genetic characters. Leydig cell, any of the interstitial cells of the testes that produce androgens.