Ultrastructure of leydig cell paracrystalline inclusions, possibly related to reinke crystals, in the normal human testis

One of two types of paracrystalline inclusions, other than Reinke crystals, was encountered in the Leydig cells of the normal mature testes of 9 out of 10 patients. The basic ultrastructure of these formations was that of parallel arrays of fine filaments (8 cases) or tubules (1 case). The filamentous structures, designated as type A paracrystals, consisted of fascicles of parallel rows of 4 or 5 dense fibrils. The latter typically contained dense granules or banded striations spaced at regular intervals. In 4 cases the inclusions were found in the cytoplasm while in 4 others they were confined to the nuclei. The tubular type of paracrystal, type B, was observed in only one patient. Here, groups of closely packed tubular inclusions occupied large portions of cytoplasm. The tubular walls were composed of closely applied, small, circular profiles. In no instance were paracrystals and Reinke crystals encountered in the same cell. A possible role of the former as precursors of the latter is discussed but there is no strong supporting evidence for this despite certain suggestive similarities in unit and subunit measurements.