The mucous membrane lining the uterus is called Endometrium (endo means inside, metrium refers to uterus). This lining of the uterus, participates in the formation of the placenta. Endometrium supplies nutrients needed for embryonic and fetal development. In a mature woman, the endometrium changes greatly from week to week. During period of a woman, blood and cells from the endometrium are shed and exit the body through the vagina.