The Reinke crystal is a distinctive feature of human Leydig
cells of the adult maturation phase. This large subcellular
structure, first described in 1896 by Reinke, has been well
documented ultrastructurally by numerous investigators
during the past few decades (e.g. Fawcett & Burgos, 1960;
de Kretser, 1967; Sohval et al. 1973; Christensen, 1975;
Kerr, 1991). Although Leydig cells of many mammals
contain unusual inclusions, the Reinke crystal has long been
considered unique to the human Leydig cell. Its precise
molecular composition and significance are unknown. It is,
however, known to contain proteins (Janko & Sandberg,
1970) and has been found to increase within the cell with age
(Mori et al. 1978). The purpose of the present report is to
describe the presence of an inclusion with much similarity to
the Reinke crystal within Leydig cells of the marmoset
monkey (Callithrix jacchus).