Ultrastructure and possible function of giant crystalloids in the Sertoli cell of the juvenile and adult koala (Phascolarctos cinereus).

Sertoli cells of the juvenile and adult koala testis exhibit a unique morphology due to their large nuclei and in particular, a remarkable abundance of large cytoplasmic crystalloid inclusions. Numerous crystalloid subunits in immature Sertoli cells are aggregated into distinct clusters where by assembly and union, they form large slender crystalloids consisting of an ordered substructure of filaments and tubules. Adult Sertoli cells contain large numbers of basally-positioned crystalloids up to 60 micron in length and the observations suggest a possible mechanism for their growth from collections of tubules assembled together within membrane-bound inclusions. The trunk and adluminal cytoplasm of the adult Sertoli cell also contains crystalloids, usually single, positioned between germ cells or their excess residual cytoplasm. Following sperm release, crystalloids are not shed from the seminiferous epithelium but are retained within the apical Sertoli cell cytoplasm. Although their subsequent fate could not be determined crystalloids did not show evidence of breakdown or phagocytosis by the Sertoli cell, suggesting that they may be reutilized and possibly function to stabilize the association between Sertoli cell cytoplasm and the developing germ cells.