M01.10.19 Endometrium

The endometrium is the innermost lining of the uterus, playing a crucial role in reproductive biology. Its structure and function are intricately linked to the menstrual cycle, preparing for potential implantation and shedding when pregnancy does not occur.

Structural Layers of the Endometrium

The endometrium is divided into two main layers:

1. Functional Layer (Stratum Functionalis)

• Location: Superficial layer closest to the uterine cavity.
• Composition:
  • Epithelium: Simple columnar epithelium with ciliated and secretory cells.
  • Stroma: Loose connective tissue rich in fibroblasts, immune cells, and spiral arteries.
  • Glands: Simple tubular glands that extend into the stroma and respond to hormonal changes.
• Function: Undergoes cyclical changes during the menstrual cycle and is shed during menstruation.

2. Basal Layer (Stratum Basalis)

• Location: Deep layer adjacent to the myometrium.
• Composition:
  • Dense connective tissue.
  • Glands and blood vessels remain stable throughout the cycle.
• Function: Regenerates the functional layer after menstruation.

Cyclical Changes During the Menstrual Cycle

The endometrium undergoes three distinct phases, driven by hormonal fluctuations:

1. Proliferative Phase (Days 6–14)

• Stimulus: Rising estrogen levels from ovarian follicles.
• Changes:
  • Regeneration of the functional layer.
  • Proliferation of stroma and glands.
  • Glands appear straight with narrow lumina.
  • Spiral arteries start extending toward the surface.

2. Secretory Phase (Days 15–28)

• Stimulus: Progesterone from the corpus luteum.
• Changes:
  • Glands become coiled, with wide, sacculated lumina.
  • Secretion of glycogen-rich material to nourish potential embryos.
  • Stroma becomes edematous, with increased vascularization.

3. Menstrual Phase (Days 1–5)

• Stimulus: Decline in estrogen and progesterone levels after corpus luteum regression.
• Changes:
  • The functional layer is shed.
  • Spiral arteries constrict, leading to ischemia and tissue breakdown.
  • Blood and tissue debris are expelled.

Clinical Correlation

• Endometriosis: Presence of endometrial tissue outside the uterus.
• Endometrial Hyperplasia: Overgrowth of the endometrium, often due to unopposed estrogen.
• Adenomyosis: Endometrial glands and stroma invade the myometrium.

Points to Remember

• The functional layer is hormone-responsive and sheds during menstruation, while the basal layer regenerates it.
• Endometrial glands and spiral arteries are key histological features.
• The cyclical changes of the endometrium are tightly regulated by estrogen and progesterone.

Bibliography

1. Ross, M. H., Pawlina, W. (2020). Histology: A Text and Atlas. Wolters Kluwer.
2. Gartner, L. P., Hiatt, J. L. (2021). Color Atlas and Text of Histology. Wolters Kluwer.