Endocrine aspects of gestation in viviparous reptiles

In the reptilian ovary, steroid-converting enzymes are demonstrable in the granulosa and theca interna cells. Estrogens and also progestins are probably synthesized and secreted by the ovary in the preovulatory stage and synergistically stimulate the maturation of the genital tract. It is suggested that the release of the ovum during ovulation results in deactivation of the enzyme systems responsible for the conversion of progesterone to estrogens. Consequently, there is an increase of progesterone levels in the ovary and in the plasma. Within the postovulatory gonad, steroid-converting capacity is limited to the lutein and thecal cells of the corpus luteum. Most reports indicate that neither the corpus luteum nor the entire ovary is essential for embryonic development or placental transfer of amino acids. However, normal parturition is impaired by ovariectomy or deluteinization. Ovarian function is controlled by the pituitary in the preovulatory stage but, following ovulation, hypophyseal agents are not required for follicular transformation into corpora lutea or for the maintenance of the luteal structures. Hypophysectomy does not arrest gestation, although parturition may be impaired: Oxytocin and arginine vasotocin induce contractility of the genital tract musculature in vivo and in vitro. Endogenous octapeptides may normally play some role in parturition. It is suggested that the modifications which occur in the uterine mucosa during pregnancy are induced by local agents released from the fetal components of the placenta.