Endocrine control of reptilian viviparity

Among reptiles, viviparous reproduction is very frequent and has been described in about 1140 species of squamates. Viviparity evolved independently, by convergence, at least 90 times in different squamate lineages. Two main structures are involved in the endocrine control of reptilian viviparity: the postovulatory follicle (corpus luteum) and placenta. Some reptilian species (turtles and crocodiles) show “short lifespan” corpora lutea; consequently, like birds, egg deposition occurs shortly after ovulation. By contrast, in most of the squamates the corpora lutea have a "long lifespan" and progesterone secretion continues for a reasonable time after ovulation. This induces a relaxation of the oviducal smooth muscle and retention of the eggs within for varying lengths of embryonic development. According to the species, some days after ovulation the corpus luteum regresses and its endocrine function ceases. Following the abrupt decrease in progesterone and the increase in prostaglandins, a contraction of the oviducal muscles occurs; consequently, the eggs are laid only at an advanced stage of embryonic development. In other species, instead, the corpus luteum continues to secrete progesterone. Hence embryonic development completely continues within the oviduct and placental relationships between the uterine mucosa and the various embryonic annexes (yolk sac and/or chorioallantois) are established. During pregnancy, in some species the corpus luteum continues to produce progesterone until delivery; in other species, the corpus luteum regresses at advanced pregnancy and the production of progesterone is supported by the placenta. On the whole, the endocrine factors (sexual steroids and other signal molecules such as AVT, prostaglandins and corticosteroids) which control pregnancy in viviparous reptiles show considerable similarities to those found in eutherian mammals.