Endometrium

The endometrium - or uterine mucosa - is the internal wall of the uterus. During each menstrual cycle the endometrium thickens to prepare for the implantation of a fertilised ovum. If fertilisation does not occur the superficial layer of the endometrium desquamates, causing the menstrual period.

Endometriosis is defined by the presence of tissue identical to the endometrium outside of the uterine cavity.