Ultrastructure of the interstitial cells of Leydig, stimulated and unstimulated.

The interstitial cells of Leydig which lie in small groups or individually between the seminiferous tubules of the testes, or at the hilum in the ovary, are known to be active in production of androgenic substances, as well as being sensitive to the influence of various trophic hormones. Among the hormones known to be produced by these cells are testosterone, dihydrotestosterone, and estradiol. Responsiveness of the function of the Leydig cells has been demonstrated with luteinizing hormone (LH), growth hormone, follicle stimulating hormone (FSH), and probably estrogen as well as prolactin. Human chorionic gonadotrophin also may have a marked effect. Attempts to correlate the cytologic appearance of Leydig cells with various states of stimulation have revealed a number of ultrastructural appearances. Since a spectrum of cellular morphology is apparent, both in the normal and in altered physiologic states, it is hazardous to ascribe a particular ultrastructural variation to the influence of a given hormonal stimulus. Nevertheless, in normal males, three types of cells can frequently be seen: fusiform cells with ovoid nuclei, small aggregates of smooth endoplasmic reticulum (SER), and variable amounts of cytoplasmic filaments, probably representing resting cells, since they are most abundant in pre-pubertal males; light cells, the most frequent type, with well developed SER, scant rough endoplasmic reticulum (RER), and mitochondria which vary in size and shape, contain abundant lipid and frequent lipochrome deposits; and dark interstitial cells which are variable in number, derive their density from stacks of tubular SER, and may represent merely an altered response to fixation or an involutional form. In addition to immature cells and normal mature cells, two additional cell types are described in various primary testicular disorders: abnormally differentiated Leydig cells with features such as grouped mitochondria, whorls of endoplasmic reticulum, absent or fragmented Reinke's crystals and paracrystalline arrays and deficiency of lipid droplets as well as masses of microfilaments; and a multivacuolated cell type characterized by swelling of cytoplasmic organelles and absence of Reinke's crystals or paracrystalline arrays. It is also clear that the interstitial cells respond in systemic diseases, are injured by alcohol ingestion, and show involutional changes in aging. While these ultrastructural changes are now well documented, there is still a need to correlate them exactly with the various stimuli which may affect testicular function.