The uterus lining, formally known as the endometrium, is a specialized layer of tissue that coats the inner cavity of the uterus, sitting atop the muscular wall (myometrium). Its function is to undergo monthly cycles of change to prepare a receptive environment for a fertilized egg. The endometrium’s appearance is dynamic, reflecting its purpose: to sustain a potential pregnancy.
The Physical Structure of the Endometrium
The endometrium is organized into two distinct layers. The deeper layer, called the basal layer, adheres closely to the underlying uterine muscle and remains relatively unchanged throughout the monthly cycle. This foundational layer contains the stem cells necessary to regenerate the upper portion of the lining after it sheds.
Above the basal layer lies the functional layer, which visibly changes and is shed during menstruation if pregnancy does not occur. The lining includes a surface of epithelial cells and a specialized, cell-rich connective tissue known as the stroma. Penetrating this tissue are numerous tubular uterine glands and a rich network of blood vessels, notably the coiled spiral arteries, which provide the blood supply needed to nourish a developing embryo.
The Dynamic Appearance During the Menstrual Cycle
The appearance of the endometrium changes across the three main phases of the menstrual cycle. During menstruation, the lining is at its thinnest (1 to 4 millimeters). This initial appearance is ragged and hemorrhagic, consisting of the remnants of the broken-down functional layer mixed with blood.
The proliferative phase begins after bleeding ends, driven by rising estrogen levels. The basal layer rapidly regenerates the functional layer, causing the lining to thicken. As the phase progresses, the endometrium builds up to about 5 to 11 millimeters, becoming multilayered and highly vascularized as new tissue and glands are constructed.
The secretory phase follows ovulation, making the lining plush and receptive. Progesterone causes the tissue to become structurally complex, reaching its maximum thickness (up to 16 to 18 millimeters). Visually, the lining is thick, highly edematous, and appears soft due to increased fluid in the stroma. The uterine glands swell and begin to secrete glycogen-rich fluid. If no fertilized egg implants, hormonal withdrawal triggers the breakdown of the spiral arteries, leading to the tissue sloughing off.
The Transformation for Implantation and Early Pregnancy
If a fertilized egg implants, the endometrium undergoes an intensive process known as decidualization instead of breaking down. This transformation converts the functional layer into a unique tissue called the decidua, which is necessary for pregnancy support.
During decidualization, the stromal cells swell dramatically, taking on a rounded, polyhedral shape, and become densely packed with stored nutrients like lipids and glycogen. This reaction creates an extremely thick, highly modified layer that is both protective and nourishing for the developing embryo.
The formation of the decidua is essential because it regulates the depth of the embryo’s invasion into the uterine wall and mediates the remodeling of the maternal blood vessels. This specialized tissue forms the maternal side of the placenta and ensures a controlled environment for the conceptus. The decidua remains in place until delivery, when it is shed along with the placenta.
Visual Indicators of Endometrial Abnormality
When the endometrium deviates from its predictable cycle, it often presents with abnormalities observable through imaging or inspection. Endometrial hyperplasia is a common deviation, described as an excessive, often homogeneous, thickening of the lining. This overgrowth can appear irregular or contain small cystic spaces, requiring medical investigation.
Another frequent abnormality is the presence of polyps, which appear as localized, irregular masses protruding from the inner surface. On ultrasound, a polyp typically presents as a single, uniform mass, sometimes demonstrating a thin vascular stalk connecting it to the uterine wall. These changes frequently disrupt the lining’s integrity and are often associated with abnormal bleeding. Submucosal fibroids can also distort the endometrial surface.