The zona fasciculata of the rat adrenal cortex synthesizes and secretes glucocorticoids. As observed after aldehyde fixation, the cells in this zone contain an extensive endoplasmic reticulum (ER), a small Golgi apparatus, a moderate number of lipid droplets, and abundant mitochondria with tubulovesicular cristae. Numerous areas within the endoplasmic reticulum and mitochondrial cristae appear clear. In addition, a small percentage of mitochondria encompasses large, clear areas. After immersion of finely minced adrenal cortex in unbuffered 2% OsO4 (40–48 hr at 40°C), deposits of osmium are seen within the Golgi apparatus, the entirety of the ER, and occasionally within mitochondria. In some mitochondria, the deposits are within cristae; in others, within vacuoles; in still others, in both cristae and vacuoles. These localizations correspond best to the clear areas found in aldehyde-fixed tissue. Osmium is not deposited in lipid droplets, in bar-containing inclusions, in mitochondrial matrix inclusions, or in the peripheral, outer mitochondrial spaces. Addition of zinc-iodide to OsO4 increases the amount of Golgi apparatus and mitochondrial staining. Adrenocorticotropin (ACTH) does not affect the localization of deposits; hypophysectomy decreases mitochondrial staining. This study (a) emphasizes the necessity for electron microscopic confirmation of osmium localization when this technique is used as a Golgi apparatus stain; and (b) suggests that the ER-staining pattern may be consistent in cells actively synthesizing steroids or steroid-like compounds.

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