Final answer:
The endometrium is the innermost layer of the uterus that thickens during each menstrual cycle and is shed during menstruation if an egg is not fertilized.
Step-by-step explanation:
The endometrium is the innermost tissue layer of the uterus. It consists of epithelial tissue, including mucous membranes. This layer thickens during each menstrual cycle and, unless an egg is fertilized, sloughs off during the following menstrual period. If an egg is fertilized, the thickened endometrium is maintained by hormones and provides nourishment to the embryo.