Itt írjon a(z) MammalianReproduction-ról/ről

Different endocrinological aspects of mammalian reproduction

== Introduction: Presentation of Endocrinological reproduction ==

Reproduction, by definition, is the biological process by which new individual organisms (« offspring ») are produced from their parents by combining the genetic material of 2 organisms (Wikipedia). Reproduction is one of the three primary needs of mammalian survival and has a role in supporting evolution. The neuroendocrine center, located in the Hypothalamus, acts as the major regulatory center of reproductive processes by mean of the action of hypothalamic GnRH cells that are going to stimulate all the hormones necessary to the control of reproduction. In the mammalian female, those hormones function by mean of a cycle which can be very different depending of species. Endocrinological regulation is a need in reproduction as it controls sexual maturity, normal sexual cycles, pregnancy, parturition and stability of the whole sexual mechanism.

How can Endocrinology affect or improve Reproduction in some mammalian species ?

== Part III – The role of endocrinology in animal production : the dairy cow ==

Nowadays, the actual goals and roles of dairy farming is to produce more and more due to oppressing increase in population and demand. Heifers need to conceive earlier and get better production performances in average. To answer those actual questions, a perfect and well-managed knowledge of sexual endocrinological hormones is essential. This knowledge can be reinforced by the development of technologies such as Ultrasonography (monitoring follicular growth) and genomic technologies.

=== Effects of artificial endocrinology on early puberty and pregnancy in dairy heifers ===

Puberty is defined as « when ovulation is accompanied by visual signs of estrous and normal luteal function » and Pregnancy success as « correlated with percentage of heifers that reached puberty before or early in breeding season » (Perry, 2012) We also know that the optimal age for first conception in dairy cows is 2 years old ; and that can be apply by the control of the hypothalamic-pituitary axis.

Control of Puberty

Several measurements can be made in order to control puberty; blood hormone concentrations are used to determine the pubertal process and the formation of the CL (Corpus Luteum). At puberty, the cow experiences a decrease in negative feedback of estradiol on GnRH followed by an increase in LH level. That will enhance the follicular growth and result in peripubertal period helped by estradiol secretion. Some recent studies have also showned a correlation between body conditions and normal estrous cycles. Indeed, a normal estrous cycle can be influenced by the age, the BW (Body Weight), the breed and size. An interesting study has proven that Leptin, the hormone for energy expenditure, had a role in regulation of HT/HP axis (Hypothalamus-Pituitary axis). Leptin levels increase at the age of puberty, therefore increasing the total BW. « Heifers can be developed to 50 to 55% of mature body weight before breeding season ». Heifers with lower body weight (53%) were seen being cycling before the start of the breeding season in contrast to heifers with more important body weight (58%). (Perry, 2012). Indeed, we assumed that the best body conditions for heifers to cycle and get pregnant was between 55 to 65% BW. Heifers with 65% BW conceived earlier in general during the breeding season.

==== Synchronisation of estrous cycle and fertility ====

An other fabulous discovery was to improve the synchrony of estrous and fertility by controlling both the follicular development and the luteal regression. 2 methods were studied : one based on the control of the luteal function and the follicular waves to improve the pregnancy success and the other related to the hormonal induction of new follicular waves (Perry, 2012). The fisrt method consists to synchronize the follicular waves by « prolonging the lifespan of a dominant follicle » (Perry, 2012). By using Progestin, we can follow the formation of « persistent follicles ». Those modified follicles have an extended lifespan and can increase the concentration of Estradiol (E2) and the LH pulse frequency. AI (Artificial Insemination) applied right after a treatment with Progestin will have consequences in decreasing fertility and alterating the uterine environment. Progesterone in that case will create a « turnover » and therefore make possible the regression and the initiation of new follicular waves (Perry, 2012). The second method focuses on initiating ovulation or atresia in the dominant follicle in a way to generate a new follicular wave. The experiments have shown that Progesterone and Estradiol was the best combination to initiate a new follicular wave after 4 to 5 days after injection. (Perry, 2012). However, GnRH injections were half effective compared the the latest and were also dependent of the stage of the estrous cycle. Further results demonstrated that Progesterone and GnRH injections were negatively related. Indeed, P4 and E2 are important in LH release and the so called “Presynchronisation” that allows a perfect time for the oestrous to optimally respond to GnRH injection and therefore initiating of a new follicular wave. This will have a positive impact on fixed-time AI. (Perry, 2012) That method can be reinforced by the so called “Ovsynch/TAI” (Ovulation-synchronized/Timed Artificial Insemination) protocol. The specificity of this protocol is that heifers can be inseminated at any stage of the oestrous cycle, meaning that no oestrous detection is realised before. A GnRH injection is given randomly during the oestrous cycle and that will initiate a new ovulation or luteinisation of large follicles necessary for the recruitment of new follicular waves. (F. Moreira, 2000)

==== Fertility ====

Different factors can play a role in enhancing fertility. One would be the AFC (Antral Follicle Count). This is strongly correlated with pregnancy success: a decrease in AFC will enhance a decrease in P4 and LH receptors. (Perry, 2012). AFC can also influence milk production and longevity because it promotes the optimal development of mammary gland tissue. An optimal AFC is situated between 23 and 24,5 months of age (D. C. Wathes, 2007). Some programs were tested to improve fertility, and that was the case of the so called “in utero” program, based on the relationship between maternal nutrition and some elementary factors such as growth, development and future reproductive performances in offspring. Studies demonstrated that cows restricted with food for the first 110 days of gestation had their calves with fewer AFC compared to the ones supplemented with rich nutrients that participated in increased conception rates. (Perry, 2012). Some other factors are influencing the fertility but will just be listed here: change in energy and protein intake, uterine environment, genetics (age at puberty, heritability…) and stress. Fertility has decreased over the past few years. We recorded a 60% fertility in 1970 in contrast with a 40% fertility in the early 20th century. (D. C. Wathes, 2007). A poor fertility can be the main factor influencing longevity. The actual solution is a better selection in younger ages. There are several investment stages in the production of offspring. The first would be the semen; its cost, quality, fertilisation rates, etc. A study showed that failures in fertilisation were the cause of 10% of the total losses (insemination at inappropriate oestrous time, embryo losses…) (D. C. Wathes, 2007). Based on the same studies we realised that only 40% of the total AI would result in birth.

=== Revue of endocrinological improvements on fertility and production in dairy cows : the role of AMH ===

First of all, we are going to focus on the Anti-Mullerian Hormone, a hormone similar to growth factor, part of the superfamily of the TGF (Transforming Growth Factor) and which is exclusively secreted by the granulosa cells of healthy follicles. We are going to demonstrate two studies that have proven that AMH is positively related to AFC, ovarian function/ovary size, fertility, birth weight and superovulatory response. AFC, as a reminder, is known as « the average for the maximum number of antral follicles superior to 3 mm in diameter during each of the 2 or 3 consecutive follicular waves during an estrous cycle » (F. Jimenez-Krassel, 2015).