Specific lamellar structures of agranular endoplasmic reticulum in the senile mouse adrenal cortex

In the adrenal cortex of old mice, lamellar bodies of various shapes frequently appear in the zona reticularis cells. These structures consist of electron dense plates, about 20 nm in width, closely piled at regular distances of about 25 nm. Each plate displays two sheets of unit membranes enclosing a dense material and with several clear spaces between them. These paired membranes usually terminate in round saccules at both ends, but occasionally continue to the tubules of surrounding agranular endoplasmic reticulum. Saccules in several rows, in combination with a part of the lamellae, may be arranged cirularly in the cytoplasm. From these observations, it is proposed that the lamellar body is composed of a cup-shaped mass of flattened cisternae of the agranular endoplasmic reticulum and each of the cisternae is attenuated and closed except at their marginal sacculations. The concentric or horseshoe-shaped profile of the lamellar body observed in this study is accounted for by different section planes through the cup-shaped structure.