What is Endometrium

The endometrium is the inner membrane of the mammalian uterus. Female endometrium is a very important part with the uterus the most important female reproductive organ.

1. Functions of Endometrium

The endometrium functions as a lining for the uterus, preventing adhesions between the opposed walls of the myometrium, thereby maintaining the patency of the uterine cavity. During the menstrual cycle or estrous cycle, the endometrium grows to a thick, blood vessel-rich, glandular tissue layer. This represents an optimal environment for the implantation of a blastocyst upon its arrival in the uterus. The endometrium is central, echogenic (detectable using ultrasound scanners), and has an average thickness of 6.7mm.

During pregnancy, the glands and blood vessels in the endometrium further increase in size and number. Vascular spaces fuse and become interconnected, forming the placenta, which supplies oxygen and nutrition to the embryo and fetus.

2. Uterine Endometrial Cycle

Cyclical changes in the endometrium prepare for implantation in the event of fertilization and necessitate menstruation in the absence of fertilization.

The endometrium is divided into two portions. The functionalis undergoes changes throughout the menstrual cycle and is shed during menstruation while the basalis remains constant during the menstrual cycle and regenerates the functionalis each month.

The uterine endometrial cycle can be divided into three phases: the follicular or proliferative phase, the luteal or secretory phase, and the menstrual phase.

1) Follicular Phase

The follicular, or proliferative phase, spans from the end of the menstruation until ovulation. Increasing levels of estrogen induce proliferation of the functionalis from stem cells of the basalis, proliferation of endometrial glands, and proliferation of stromal connective tissue. Endometrial glands are elongated with narrow lumens and their epithelial cells contain some glycogen. Glycogen, however, is not secreted during the follicular phase. Spiral arteries elongate and span the length of the endometrium.

2) Luteal Phase

The luteal, or secretory phase, begins at ovulation and lasts until the menstrual phase of the next cycle. At the beginning of the luteal phase, progesterone induces the endometrial glands to secrete glycogen, mucus, and other substances. These glands become tortuous and have large lumens due to increased secretory activity. The spiral arteries extend into the superficial layer of the endometrium. In the absence of fertilization by day 23 of the menstrual cycle, the corpus luteum begins to degenerate and consequently ovarian hormone levels decrease. As estrogen and progesterone levels decrease, the endometrium undergoes involution. Days 25-26 of the menstrual cycle, endothelin and thromboxin begin to mediate vasoconstriction of the spiral arteries. The resulting ischemia may cause some early menstrual cramps. By day 28 of the menstrual cycle, intense vasoconstriction and subsequent ischemia cause mass apoptosis of the functionalis.

3) Menstrual Phase

The menstrual phase begins as the spiral arteries rupture secondary to ischemia, releasing blood into the uterus, and the apoptosed endometrium is sloughed off and usually lasts four days. During this period, the functionalis is completely shed. Arterial and venous blood, remnants of endometrial stroma and glands, leukocytes, and red blood cells are all present in the menstrual flow.