User:Lilyd/sandbox

Ovulation occurs at the ovary surface and is described as the process in which an oocyte (female germ cell) is released from the follicle. Ovulation is a non-deleterious 'inflammatory response' which is initiated by a Luteinising Hormone (LH) surge. The mechanism of ovulation varies between species. In humans the ovulation process occurs around day 14 of the menstrual cycle, this can also be referred to as 'cyclical spontaneous ovulation'. However the monthly menstruation process is typically linked to humans and primates, all other animal species ovulate by various other mechanisms.

Spontaneous ovulation is the ovulatory process in which the maturing ovarian follicles secrete ovarian steroids to generate pulsatile GnRH (the neuropeptide which controls all vertebrate reproductive function) release into the median eminence (the area which connects the hypothalamus to the anterior pituitary gland) to ultimately cause a pre-ovulatory LH surge. Spontaneously ovulating species go through menstrual cycles and are fertile at certain times based on what part of the cycle they are in. Species in which the females are spontaneous ovulators include rats, mice, guinea pigs, sheep, monkeys, and women.

Induced ovulation is the process in which the pre-ovulatory LH surge and therefore ovulation is induced by some component of coitus e.g receipt of genital stimulation. Usually spontaneous steroid-induced LH surges are not observed in induced ovulator species throughout their reproductive cycles, which indicates that GnRH release is absent or reduced due to lack of positive feedback action from steroid hormones. However, by contradiction some spontaneously ovulating species can occasionally undergo mating-induced preovulatory LH surges. Species in which the females are induced ovulators include cats, rabbits, ferrets, and camels. In 1985, Chen et al., used Bactrian camels to investigate the factor(s) that induce ovulation during breeding season. They monitored the camel ovaries for ovulation by rectal palpation following insemination of semen samples. Chen et al., concluded that in this particular camel species ovulation was induced by the seminal plasma, and not by the spermatozoa.

Evolution
Although the evolution of these two types of ovulation is not well understood, the type of ovulation that would be advantageous in some species can be understood by looking at social groups. Animals that have large, complex social groups benefit from spontaneous ovulation as only the best males get to breed with females. If there are little males suitable for breeding it makes sense to spread out the times at which females are fertile, therefore increasing the proportion in which conception occurs. This does not explain the evolution of ovulation in all species however, for example some species appear to show estrus synchronisation.

Mechanism
In spontaneous ovulators, estrogen secreted by the follicles as they grow and mature affects the release of GnRH, and therefore causes an LH surge.

Ovulation is triggered in induced ovulators by a LH surge from the anterior pituitary, which appears to be independent of GnRH release, and is induced during mating. Animals this has been recorded in include rabbits, voles, ferrets and camels. In some species such as the ferret the duration of intromission has no effect on the LH surge, whereas in other species such as the cat these are related and higher levels of LH were produced by mating multiple times. In many species, for a LH surge to occur, little intromission is required.

The pathways in which information reaches the brain and causes GnRH release are not understood well, however midbrain and brainstem noradrenergic neurones appear to be activated in response to intromission during mating. These neurones then go on to stimulate the mediobasal hypothalamic to release GnRH from the median eminence. Most experiments on GnRH and LH release have been focused on spontaneous ovulators, however there have been studies completed on some induced ovulators - rabbit and ferret. From this, it appears that norepinephrine facilitates GnRH release in the rabbit and ferret and the locus coereuleus (a nucleus in the pons of the brainstem involved with physiological responses to stress and panic) is the part of the brain involved in conveying genital-somatosensory information to the GnRH neurones. Other subsances that have similar effects include neuropeptide Y.