What Happens to the Endometrium After Menstruation?

Menstruation is the regular vaginal bleeding that takes place as part of a woman's monthly cycle. Every month, the body prepares for pregnancy. If there is no pregnancy, the uterus sheds its lining, which is the endometrium. The menstrual blood contains both blood and uterine tissue. 

Periods typically begin between ages 11-14 and last until menopause, which occurs at around age 51. Periods usually last 3-5 days and can cause the following symptoms:

• Vaginal bleeding
• Pelvic or abdominal cramps
• Lower backache
• Acne
• Oily skin and scalp
• Breast pain
• Bloating
• Hunger pangs
• Changes in mood 
• Fatigue
• Headache

Endometrial healing starts almost immediately as the shedding begins and is complete by the time bleeding stops (up to 8 days). In the menstruating tissue, degrading tissue and reepithelializing sites can be detected by histology and scanning electron microscopy. Once bleeding stops (when reepithelialization is complete), the entire tissue thickens and regeneration begins. As the tissue regenerates, quick reepithelialization helps guard against bacterial invasion.

Reepithelialization can first be recognized as the migration of epithelial cells from exposed gland stumps, which can be seen by scanning electron microscopy. These cells then spread outward to meet comparable cells from other glands or those moving from any intact residual epithelium.

Reepithelialization occurs at the same time as the repair of wounds to spiral and transverse subepithelial endometrial arterioles within the stroma, which can sustain severe injuries during tissue disintegration. However, the stem or progenitor cells in the basalis layer (which is not shed) are responsible for the endometrium's complete renewal. This regeneration, which depends on estrogen activity, is fully completed by the time of ovulation. It typically occurs 14 days after the start of menstruation and 9 days after bleeding stops and full reepithelialization occurs.

Several cell types, cytokines, mediators, and the vascular system interact in a complex way to heal a wound. All injured tissues experience an influx of inflammatory cells, beginning with neutrophils that release chemokines locally to draw other leukocytes to the wound site. Together, these cells emit a variety of mediators and cytokines that encourage thrombosis, angiogenesis and reepithelialization.

The extracellular matrix is secreted by the fibroblasts and acts as scaffolding for cellular activities. Most adult tissues develop scar tissue because of the growth and extension of fibrous tissue (fibrogenesis), which is predominantly formed from stromal cells. However, there is no apparent fibrosis in the repaired endometrium. In addition, while most wounds require 4-6 weeks to heal, the endometrial lining usually does so in just 5 days.

Given that stromal cells are the leading cause of scarring in most tissues, it is expected that endometrial stromal cells generated from stem or progenitor cells during endometrial regeneration will have different programming. The rate at which the endometrial surface reepithelializes suggests that the stem or progenitor cells are activated more quickly than in other tissues. Menstrual fluid has higher levels of transforming growth factor -1 than peripheral blood, which theoretically might develop endometrial stromal cells into myofibroblasts.