Reptiles are the most primitive amniotes and therefore occupy a phylogenetic position of particular interest, yet their reproductive endocrinology has been the least studied of all the vertebrate groups. The seasonal spermatogenetic pattern varies greatly between species, but in general shows its most active phase during the postbreeding period. In some, spermatozoa are produced by the autumn so that the spermatogenetic cycle is completed before the start of winter, but in others, the seminiferous epithelium may advance only as far as the production of spermatocytes or spermatids when the cycle is interrupted by the hibernation period. It is completed during the succeeding spring. In addition to the spermatogenetic fluctuations, the reptilian testis also undergoes well-marked histochemical changes, both in the interstitial tissue and seminiferous tubules. These involve a seasonal accumulation of cholesterol-positive lipid in the Leydig cells and its rapid depletion at the time of maximum androgen production. This is usually succeeded by a rapid build-up to a densely sudanophilic condition which marks the terminal stages of the secretory cycle and a sudden cessation of androgen production. This is very similar to the amphibian pattern. The postnuptial regression of the seminiferous tubules is accompanied by a rapid genesis of large quantities of cholesterol-rich lipid material within the Sertoli cells. This remains until the resumption of spermatogenetic activity. It then becomes rapidly depleted concomitant with the build-up of gametogenesis. This phenomenon is common not only to the testicular cycle of reptiles, but occurs also in the seasonal events of male amphibia and birds. Its significance is not known, but in the avian testis it has been shown to have progestogenic characteristics. In the cobra Naja naja incubation of testicular material with progesterone-4-14C and chromatographic analysis, similarly indicates a blockage in the biosynthetic pathway at the progesterone stage when the seminiferous tubules are filled with postnuptial lipids. It is possible that the Sertoli cells may be involved in steroid biosynthesis. The seasonal histochemical changes in the interstitium closely parallel the in vitro capacity of cobra testis to synthesize androgens from labeled precursors. As in most vertebrates, androstenedione and testosterone are the major androgenic steroids. Production is minimal when the interstitial tissue has entered its postnuptial terminal stage with densely sudanophilic Leydig cells, but rises in late August when a new generation of interstitial cells becomes established and starts accumulating small cytoplasmic lipid droplets. A temporary decline in synthesis occurs during hibernation, but rapidly rises once the animal emerges during the early spring. © 1969.
