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Wednesday, 12 June 2013

Causes of menopause, menopause causes

Causes of menopause

The causes of menopause can be considered from complementary perspectives coming (mechanistic) (as is) or (adaptive evolutionary) perspectives end (why it happens). The last group are only hypotheses.

Proximate Perspective

Natural or physiological menopause occurs as part of the normal aging process of a woman. It is the result of the eventual depletion of almost all of the oocytes and ovarian follicles in the ovaries. This causes an increase in circulating follicle stimulating hormone (FSH) and luteinizing hormone levels (LH), because there is a decrease in the number of oocytes and follicles that respond to these hormones and estrogen production. This decrease in estrogen production leads to symptoms of pre-menopausal hot flashes, mood swings and sleeplessness. The long-term effects may include osteoporosis and vaginal atrophy.

Depletion of ovarian reserve

Titus et al. proposed an explanation for decreased ovarian reserve during aging. They found that as women age, double-strand breaks in DNA accumulate their primordial follicles. Primordial follicles are immature primary oocytes surrounded by a single layer of granulosa cells. An enzyme system is present in oocytes that normally need repair double-strand DNA breaks. This repair system called "homologous recombination repair", and is especially effective during meiosis. Meiosis is the general process by which stem cells are formed in all eukaryotes sex, and appears to be adapted to the removal of damaged efficiently in germline DNA.

Human primary oocytes are present in an intermediate bachelor meiosis called prophase I (see Oogenesis). Titus et al. Also demonstrated that the expression of four genes key DNA repair are required for homologous recombination repair during meiosis (BRCA1, MRE11, Rad51 and ATM) decrease with age in the oocytes. This age-related decline in the ability to repair the damage of double-strand DNA may account for the accumulation of damages then probably contributes to decreased ovarian reserve.

Evolutionary theories of menopause

Unlike men, women invest more in their gametes, so that a very valuable resource. Selection therefore theoretically should favor a number of eggs to be sufficient to female life. Excess investment is a waste of resources, while the lack of investment leads to reduced fitness. Human females, however, spend more than a third of your life in a post-reproductive phase. Possible evolutionary explanations for survival beyond reproductive maturation range of non-adaptation to adaptation.

Adaptive hypothesis

The high cost of investment in female offspring may lead to physiological changes that amplify susceptibility to becoming infertile. This hypothesis suggests the reproductive lifespan in humans has been optimized, but it has proven more difficult in females and thus their reproductive span is shorter. If this hypothesis were true however, age at menopause should be negatively correlated with reproductive effort and the available data do not support this.

A recent increase in female longevity due to improvements in the standard of living and social care has also been suggested. It is difficult for selection, however, in favor of aid to offspring from parents and grandparents Irrespective of living standards, adaptive responses are limited by physiological mechanisms. In other words, senescence is programmed and regulated by specific genes.
Adaptive hypotheses

The mother hypothesis

The mother hypothesis suggests that menopause has been selected to humans due to the long period of development of human offspring and high costs of reproduction so that mothers gain an advantage in reproductive fitness by redirecting their effort new offspring with a low probability of survival of existing children a greater chance of survival.

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