HA12 shows an electron micrograph (EM) of portions of 3-4 adjacent interstitial cells in the testis. Parts of two nuclei are visible. The darkly stained regions in the nuclei are heterochromatin (hc). There are also numerous mitochondria and lipid droplets (lp). The most abundant organelle is the endoplasmic reticulum (ER). One of the functions of smooth ER is lipid and steroid hormone synthesis.