Endometrial Histology in the Menstrual Cycle The endometrial lining of the uterus undergoes dynamic changes throughout the menstrual cycle in response to hormonal fluctuations. These changes can be divided into distinct phases, each characterized by specific histological features. Understanding these phases is crucial for diagnosing menstrual disorders and assessing reproductive health. 1. Menstrual Phase (Days 1-5):  Histological Features: The menstrual phase marks the shedding of the functional layer of the endometrium. Histologically, the endometrial lining appears fragmented with extensive necrosis and sloughing of the superficial layers. The stroma is edematous, and there is often the presence of blood and debris within the endometrial cavity.  Glandular Activity: The endometrial glands are disrupted and may appear disorganized or irregular. The glands are often dilated, and their secretions are mixed with blood and cellular debris. 2. Proliferative Phase (Days 6-14):  Histological Features: Following menstruation, the proliferative phase is characterized by the repair and regeneration of the endometrium. The endometrial lining undergoes rapid proliferation under the influence of estrogen.  Glandular and Stromal Changes: The endometrial glands are simple and tubular, lined by columnar epithelium. They proliferate and elongate, and their secretions are minimal. The stroma shows increased cellularity with newly formed connective tissue and fibroblasts.  Vascular Changes: The blood vessels in the endometrium also proliferate, forming new capillary networks to support the growing endometrial tissue. 3. Secretory Phase (Days 15-28):  Histological Features: This phase is driven by progesterone from the corpus luteum. The endometrial lining becomes thicker and more glandular, preparing for potential implantation of an embryo.  Glandular Activity: The endometrial glands become more tortuous and show increased secretory activity. The epithelial cells of the glands are filled with secretory granules, and the glands may appear swollen and distended with secretions.  Stromal Changes: The stroma becomes more edematous and rich in glycogen. Decidualization begins, with stromal cells undergoing transformation to decidual cells, which provide a supportive environment for embryo implantation.  Vascular Changes: The spiral arteries in the endometrium become more prominent and coiled, providing increased blood flow to the endometrial lining. 4. End of Cycle and Menstruation:  Histological Transition: If implantation does not occur, the corpus luteum regresses, leading to a decrease in progesterone levels. This hormonal drop triggers the breakdown of the endometrial lining, leading to the menstrual phase.  Cellular Changes: There is apoptosis of the endometrial stromal cells, and the glands undergo necrosis, leading to the shedding of the endometrial lining. Conclusion: The histology of the endometrium reflects the physiological changes that occur throughout the menstrual cycle, influenced primarily by hormonal variations. The menstrual phase involves shedding of the functional layer, the proliferative phase focuses on repair and regeneration, and the secretory phase prepares the lining for potential implantation. Understanding these histological changes is essential for diagnosing and managing various gynecological conditions and menstrual disorders.