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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 28 days long in average, though it varies from one individual to the next. The normal cycle may be as short as 21 days or as long as 40. 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 it is "estrous cycle", but that is more commonly used in reference to lower animals (e.g. mice) than humans.
Growth of endometrial tissue outside the uterus is a pathological condition known as endometriosis.
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 hormoneNote that "Hormone" is the NATO reporting name for the Soviet/Russian Kamov Ka-25 military helicopter. A hormone (from Greek horman "to set in motion") is a chemical messenger from one cell (or group of cells) to another. All multicellular organisms (incls at different points in the estrous cycle affecting other portions of the reproductive system.
Fertility in animals is often referred to as estrusEstrus (also spelled strus or heat in female mammals is the period of greatest female sexual responsiveness usually coinciding with ovulation. Most female mammals will only seek sex and are considered most fertile during their heat, also known as the mati. 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 dogThis article discusses the domestic dog. For other members of the dog family, see Canidae. The dog is a canine omnivorous mammal that has been domesticated for somewhere between 14,000 and 150,000 years. In those millennia, the dog has developed into hunds, 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 bitchThis article discusses the domestic dog. For other members of the dog family, see Canidae. The dog is a canine omnivorous mammal that has been domesticated for somewhere between 14,000 and 150,000 years. In those millennia, the dog has developed into hund, 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 horseThis article discusses ungulate mammals. For other meanings of horse see Horse (disambiguation). The Horse Equus caballus is a large ungulate mammal, one of the seven modern species of the genus Equus''. It has long played an important role in transportats. A mareA mare is a female horse. Mare is Latin for ocean A Lunar mare is an ancient basaltic lava floodplain on Earth's Moon, usually inside an impact basin. 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 activate 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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