Two types of paracrystalline inclusions in the Leydig cell of the Chinese hamster (Cricetulus griseus).

The ultrastructure of two types of paracrystalline inclusions is described, for the first time, in the Leydig cell of the Chinese hamster (Cricetulus griseus). The subunit of both types of inclusions consists of 5-nm-wide microfilaments differing only in the arrangement and shape of the microfilaments. The electron-dense microfilaments of the type A inclusion are usually in a serrated pattern with an equidistant 5-nm interval separating them. The type B inclusion has pairs of straight microfilaments separated by a 5-nm interval. Other microfilaments course at right angles to the paired units. Both types of inclusions are absent in the newborn, rarely found in the sexually immature, and commonly found in the sexually mature and aged animals. Both inclusions are found in the cytoplasm of Leydig cells, but only the A inclusion is found in the nucleoplasm. Their functional role is not yet determined, but the inclusions are commonly found in Leydig cells whose ultrastructural characteristics indicate normal steroidogenesis.