Description:
<jats:title>Abstract</jats:title><jats:p>The testicular interstitial cells of mice contain an abundant agranular endoplasmic reticulum occurring as a network of interconnected tubules. Unusual features of the reticulum are occasional extensive whorls of concentric membranes and the occurrence of bundles of parallel double‐walled tubules. The mitochondria have tubular cristae and are occasionally very large.</jats:p><jats:p>In contrast to the interstitial cells of other species that have been described, the mouse interstitial cells show a segregation of the cytoplasm into areas of dense agranular reticulum and other areas where the agranular reticulum is relatively sparse. The latter areas contain scattered cistenae of granular reticulum, many free ribosomes in clusters, mitochondria, lipid droplets and small granules. The mitochondria and lipid droplets are often encircled by cisternae of the reticulum.</jats:p><jats:p>Biochemical evidence from other laboratories, taken with the present results, indicates that the membranes of the endoplasmic reticulum in mouse interstitial cells are the site of the enzymes that mediate the synthesis of testosterone from progesterone. There is also an indication that cholesterol biosynthesis is associated with the agranular reticulum. The membranes of the agranular reticulum may also serve as a reservoir for the storage of cholesterol.</jats:p>