The Control of the Reptilian Gonad

In reptiles, there is adequate evidence to indicate environmentalcontrol of the ovarian cycle through hvpothalamic pathways andthe subsequent release of tropic hormone (s) from the anteriorpituitary. The role of the pineal-parietal complex still remainsto be elucidated. In the hypothalamus there appear to be steroidsensitive areas and both progesterone and estrogen appear tohave important feedback influences upon gonadal growth ovulation,and ovarian steroid production. Cytological studies of the reptilianpituitary indicate similar cell types to those observed in mammalianpituitaries, but thus far, two gonadotrophs cannot be identifiedwith any certainty. Chemical and biological studies of the actionof mammalian gonadotropuis suggest that in reptiles hormoneswhich are FSH-like in mammals are able to stimulate gonadaldevelopment, ovulation, and steroid biosynthesis under certainconditions. Preliminary studies of the chemistry of turtle gonadotropuishave so far demonstrated only one active principle, which ischemically similar to mammalian LH, but is far more active thanthe latter hormone in reptilian systems. Further, the hormonealso has FSH-like activity in the reptile. Thus reptilian gonadaldevelopment can be stimulated by treatment with a variety ofmammalian gonadotropins. Recent studies have indicited an importantrole for growth hormone, acting in concert with gonadotropinand estrogen in the regulation of vitellogenesis and ovariangrowth. Piolactin appears to be an antigonadal agent in reptiles,as does progesterone. The exact manner in which these hormonesexert their antigonadal action remains to be clarified, butat least one site of action is the central nervous system; othersites maybe the liver and the fat depot. Ovarian tissue from reptiles is able to synthesize and secretesteioid hormones by pathways similar to those present in mammalianovaries. Circulating levels of estrogen have not been measured,but progesterone levels in the blood show distinct changes assocntedwith pregnancy and the presence of corpora lutea in the ovary.