<P> The endometrium is the inner epithelial layer, along with its mucous membrane, of the mammalian uterus . It has a basal layer and a functional layer; the functional layer thickens and then is sloughed during the menstrual cycle or estrous cycle . During pregnancy, the glands and blood vessels in the endometrium further increase in size and number . Vascular spaces fuse and become interconnected, forming the placenta, which supplies oxygen and nutrition to the embryo and fetus . The presence of commensal bacteria in the uterus and endometrium has been identified . </P> <P> The endometrium consists of a single layer of columnar epithelium plus the stroma on which it rests . The stroma is a layer of connective tissue that varies in thickness according to hormonal influences . Simple tubular uterine glands reach from the endometrial surface through to the base of the stroma, which also carries a rich blood supply of spiral arteries . In a woman of reproductive age, two layers of endometrium can be distinguished . These two layers occur only in endometrium lining the cavity of the uterus, not in the lining of the uterine (Fallopian) tubes: </P> <Ul> <Li> The functional layer is adjacent to the uterine cavity . This layer is built up after the end of menstruation during the first part of the previous menstrual cycle . Proliferation is induced by estrogen (follicular phase of menstrual cycle), and later changes in this layer are engendered by progesterone from the corpus luteum (luteal phase). It is adapted to provide an optimum environment for the implantation and growth of the embryo . This layer is completely shed during menstruation . </Li> <Li> The basal layer, adjacent to the myometrium and below the functional layer, is not shed at any time during the menstrual cycle, and from it the functional layer develops . </Li> </Ul> <Li> The functional layer is adjacent to the uterine cavity . This layer is built up after the end of menstruation during the first part of the previous menstrual cycle . Proliferation is induced by estrogen (follicular phase of menstrual cycle), and later changes in this layer are engendered by progesterone from the corpus luteum (luteal phase). It is adapted to provide an optimum environment for the implantation and growth of the embryo . This layer is completely shed during menstruation . </Li>

The layer of the myometrium that is shed during menstruation is the