Reptile Reproduction - Anatomy & Physiology
Introduction
The evolutionary development of reptiles depended primarily on them being able to discard the attachment to a watery or humid environment for survival and breeding perposes. Of the numerous features exibited by reptilia as a group, the most important in an evolutionary context are:
- Almost all reptiles are covered by a protective layer of dry skin with almost no glands. The epidermis is raised to form scales or scutes. This prevents excessive water loss, thus is essential for terrestrial existence.
- They developed a large, yolked egg, allowing the embryo sufficient food reserves to develop fully before hatching. The yolked egg is surrounded by protective extra-embryonic membranes (the amnion and allantois), serving for respiration and the storage of waste. There is also a protective calciferous shell to prevent it from drying out.
The pineal gland and the hypothalamus/pituitary gland interpret environmental stimuli into hormonal change to regulate reproduction. In temperate species, rising temperatures and increased daylight stimulates the gonads, whereas in tropical species, food availability and rainfall are more important.
Hormones of Reproduction
- The main trigger of hormones involved in reproduction is increasing light.
- Melatonin, produced by the pineal gland, is only secreted at night. Thus, production declines when the days are longer, regulating circadian rhythm.
- Melatonin stimulates the hypothalamus to produce gonadotrophin releasing hormone (GnRH).
- GnRH stimulates the anterior pituitary gland to produce leutinizing hormone (LH) and follicle stimulating hormone (FSH).
Female
- FSH stimulates follicle growth.
- LH stimulates production of sex steroid hormones, ovulation and formation of the corpus luteum.
- Oestrogen stimulates vitellogenesis of the follicles and the LH surge, triggering ovulation.
- Post ovulation, the regressing follicle becomes a corpus luteum and produces progesterone.
- Progesterone maintains pregnancy by inhibiting argenine vasotocin and prostaglandin in the uterine smooth muscle.
- When the corpus luteum regresses, argenine vasotocin induces uterine smooth muscle contraction, which is then regulated by prostaglandins.
- Argenine vasotocin has the same function as Oxytocin in mammals.