The endometrium is the innermost layer of the uterus. It is a thick glandular layer of mucous membrane that plays a vital role in supporting the development of the embryo and fetus during pregnancy. The endometrium is highly vascularized, and part of it is shed during menstruation.
The uterus is a hollow organ with its walls consisting of three distinct tissue layers: the perimetrium covering the exterior, the myometrium in the middle, and the endometrium lining the interior. As the uterus is a hollow organ connected to an open body cavity, the endometrium is a mucous membrane. It consists of a deep, thick layer of areolar connective tissue and a superficial lining of simple squamous epithelial tissue. Many blood vessels permeate the areolar connective tissue to provide blood to the endometrium. The simple columnar epithelial tissue lines the lumen of the uterus and folds into long exocrine glands that penetrate deep into the endometrium and nearly reach the myometrium. Some of the surface cells are ciliated, while others are secretory cells that produce mucus.
The endometrium can be divided into two major regions: the superficial stratum functionalis, or functional layer, and the deep stratum basalis, or basal layer. During menstruation, the stratum functionalis is shed while the stratum basalis is conserved to produce a new stratum functionalis. The blood supply to these two layers is distinct, with spiral arterioles providing blood to the stratum functionalis and straight arterioles providing blood to the stratum basalis.
The endometrium plays several important roles in the female reproductive system by supporting the blastocyst, embryo, and fetus throughout pregnancy. During the secretory phase of the reproductive cycle, the endometrial glands produce a secretion rich in glycogen that fills the uterine lumen and feeds the developing blastocyst. The blastocyst reaches the surface of the endometrium around six days after fertilization, where it attaches itself in a process known as implantation.
Over the next two weeks, the blastocyst digests part of the endometrium and embeds itself within the endometrial connective tissue. It continues to feed off the secretions of the endometrium as it grows and develops into an embryo and placenta. As the placenta develops, it grows into the endometrium, which also develops larger blood vessels to form the maternal side of the placenta. In the placenta, the embryo’s blood passes near its mother’s blood to facilitate the exchange of gasses, nutrients, and wastes across a membrane.
When an egg cell, or ovum, is not fertilized during the female reproductive cycle, the endometrium is shed during the process of menstruation. Around the 28th day of the reproductive cycle, a woman’s progesterone level drops dramatically unless a blastocyst is implanted in the endometrium. This hormonal shift triggers a retraction of the spiral arterioles and a sharp decrease in blood flow to the stratum functionalis of the endometrium. Cells in the stratum functionalis die due to a lack of nutrients and oxygen and are sloughed off into the lumen of the uterus. The dead cells pass through the uterus and vagina and exit the body over the next several days, resulting in menstrual bleeding. Only the stratum basalis is left to proliferate and grow a new stratum functionalis for the next reproductive cycle.
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