Final answer:
The endometrium is the innermost tissue layer of the uterus that thickens during menstrual cycles and provides nourishment to the embryo if 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. As gestation progresses, the endometrium develops into the maternal portion of the placenta.