Leydig cell of testis. Coloured Scanning Electron Micrograph (SEM) of endoplasmic reticulum in a Leydig cell of a 14 week old human foetus. Leydig cells produce steroids in the male testis. Here, endoplasmic reticulum membranes (ER, light pink) fill most of this secretory cell. Smooth ER is at right; rough ER (at left) is dotted with ribosomes involved in protein synthesis. Synthesis of lipids and steroids occurs within the lumen of these paired ER membranes. Lipid droplets are the round blue structures. Mitochondria (yellow) produce chemical energy for the cell. Magnification:. x8,100 at 6x7cm size. x11,300 at 4x5 inch size.