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The endometrium is the uterine membrane in mammals which is augmented preparatory to fertilization, and into which a fertilized egg is implanted upon its arrival into the uterus. It is rich in blood vessels, which are soon connected to by the new embryo, forming a placenta through which the embryo, as it becomes a fetus and eventually gestates fully, receives oxygen and is nourished.

The endometrial lining is formed and destroyed periodically. Normally, the maturity of the lining coincides with the maturity of an ovum and so, when the ovum is released, there is a hospitable environment to sustain it, should it become fertilized. If no fertilized egg is detected, the endometrial lining sheds (as in humans) or is reabsorbed. The process of shedding involves the breaking down of the lining, tearing small connective blood vessels, and the loss of the tissue and blood which had constituted it through the vagina, over a series of days. This is usually accompanied by contractions of the uterus, often somewhat painful, to help expel material. When a fertilized egg is implanted, the endometrial lining does not shed, but remains and provides a blood supply for the gestating young. In humans, the cycle of building and shedding the endometrial lining is approximately 28 days, though it varies from one individual to the next and may be as short as 21 days or as long as 40, and still fall within normal parameters. The process of shedding the endometrium is referred to as menstruation, or menses. The cycle of building and then shedding is called the menstrual cycle.

Another term for the cycle is "estrous cycle," more commonly used in reference to lower animals than humans.

Growth of endometrial tissue outside the uterus is a pathological condition known as endometriosis. It can lead to serious health complications including chronic pain, taut, distended abdomen, abnormal menstruation, infertility, etc.

The endometrium and its lining develop differently in different mammals. It is more or less rich in blood supply prior to fertilization from species to species. It also forms at different rates and according to different periodic cycles. Its formation is sometimes affected by seasons, climate, stress and other factors. The endometrium itself produces certain hormones at different points in the estrous cycle affecting other portions of the reproductive system.

Fertility in animals is often referred to as estrus. Evidence of a mature endometrial lining, or other signs of fertility, are usually referred to as their being estrous, "in heat" or "in season." In dogs, for instance, a lining forms roughly once every six months and is very rich in blood. As it reaches maturity, it is oversupplied with blood, which leaks from the vagina of the bitch, signalling her fertility. When no successful breeding occurs, instead of shedding, the remaining endometrial lining is reabsorbed.

Reabsorption (of such lining as is formed) is also the case with horses. A mare forms only a scant endometrial lining if any unless fertilization occurs. In this case, there is no bleeding, though certain other changes in secretions and urinary habits give evidence of the occurrence of the cycle which, in horses, runs about every 21-13 days during breeding season. A feature of the fertility cycle of horses and other large herd animals is that it is usually affected by the seasons. The number of hours daily that light enters the eye of the animal affects the brain, which governs the release of certain precursors and hormones. When daylight hours are few, these animals "shut down," become anestrous, and do not become fertile. As the days grow longer, the longer periods of daylight cause the hormones which "turn on" the breeding cycle to be released. As it happens, this has a sort of utility for these animals in that, given a gestation period of about eleven months, it prevents them from having young when the cold of winter would militate against their survival.

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