by Jason Wasserman MD PhD FRCPC
May 9, 2022
The tissue that covers the inside surface of the uterus is called the endometrium. The purpose of the endometrium is to provide support for a fertilized egg to develop and grow. The surface of the endometrium is lined by epithelial cells that connect together to form glands. The tissue in between the glands is called stroma and its role is to provide support for the glands.
The endometrium is hormone-responsive and goes through regular menstrual cycles during a women’s reproductive years. Each menstrual cycle is controlled by a complex orchestra of hormones. During the menstrual cycle, the endometrium is preparing itself for the possible pregnancy by becoming thicker and richer in blood vessels.
In the first part of the menstrual cycle, the endometrium is growing under the influence of estrogen (a hormone produced by the ovaries) and is known as the proliferative phase (growing phase).
After ovulation (ovulation is when an egg is released from the ovary, pushed down the fallopian tube, and is made available to be fertilized), the endometrial changes are under the control of progesterone (another hormone released by the ovaries). If pregnancy does not occur (the egg is not fertilized), the thickened endometrial lining is shed, accompanied by bleeding (menses), and the cycle repeats.
In some situations, however, the endometrium is exposed to a prolonged influence of estrogen. That results in increased growth and crowding of the endometrial glands and can lead to endometrial hyperplasia. Some common situations that can result in prolonged estrogen exposure include polycystic ovary syndrome, obesity, eating disorders, thyroid disorders, and estrogen-only birth control pills. Women nearing menopause (perimenopause) may also experience prolonged estrogen exposure.