The endometrium is made up mostly of mucosal tissue. It has two layers: The first layer, the stratum basal is, attaches to the layer of smooth muscle tissue of the uterus called the myometrium. This layer serves as an anchor for the endometrium within the uterus and stays relatively unchanged.
The second layer is dynamic, however. It changes in response to the monthly flux of hormones that guide the menstrual cycle. For this reason, it's called the stratum functionalist, or functional layer. It's the part of the endometrium where a fertilized egg (or blastocyst) will implant if conception takes place.
In preparation for this possibility, just before ovulation (the release of an egg from a fallopian tube), the functional layer of the endometrium goes through specific changes. Structures called uterine glands become longer and tiny blood vessels proliferate—a process called vascularization. As a result, the endometrial lining becomes thicker and enriched with blood so that it's ready to receive a fertilized egg and also support a placenta—the organ that develops during pregnancy to supply a fetus with oxygen, blood, and nutrients.