Ultrastructure of Leydig cells in Down's syndrome.

Hormonal and ultrastructural examinations of the testis were performed in a patient with Down's syndrome. Plasma testosterone was abnormally low, and circulating levels of LH and FSH were higher than the normal range. The administration of HCG resulted in a low response of plasma testosterone. Electron microscopy revealed proliferation of smooth endoplasmic reticulum, an increase in paracrystalline inclusions and the absence of Reinke's crystals in Leydig cells.