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A woman's womb is the place where a baby can grow and be nourished throughout a pregnancy. Between puberty and menopause, the womb prepares a suitable place for the baby each month. Every month, if the woman does not become pregnant, she sheds the old womb lining and begins to make a new one. This womb lining is known as the endometrium, and it is essential to reproduction.
The womb, or uterus, is an organ that is hollow in the middle. Its walls are made of muscle, so they can contract when necessary. Inside the womb, the surfaces are lined with a specific type of tissue called the endometrium. The endometrium, or womb lining, is actually composed of three different layers.
Next to the wall is the connective tissue layer that acts to stick the wall to the next layer, which is the epithelial basalis layer. The epithelial basalis layer is made up of column-shaped cells. On top of this layer is another known as the functionalis layer.
During the menstrual cycle, the connective tissue and the basalis layer stay attached to the womb. It is the functionalis layer that changes over the menstrual cycle. These cells can slough off the surface and out through the vagina as menstrual blood at a particular point in the cycle.
Uterine replacement of the functionalis layer of cells is essential to reproduction. When a sperm makes its way up into the fallopian tubes to fertilize an egg, the embryo needs a secure place and good access to nutrients in order to keep growing. The nutrient source is the network of blood vessels that runs through the functionalis layer. A fresh, new layer of cells on the uterine surface provides this safe place.
Each new batch of functionalis cells actually are produced by the basalis layer beneath. In the middle of cycle, the functionalis layer is as thick and hospitable as possible to any embryo. If no embryos land on the lining, then it starts to degrade.
If an embryo does secure itself to the thick and receptive lining, then the body recognizes its presence. The woman begins to build up the lining further, and it becomes the placenta. All of the directions the womb lining can go are controlled by the hormones estrogen and progesterone, which act as signaling molecules and tailor the womb's response to particular situations.