Uterus

Proliferative phase

The proliferative phase The phase when the endometrium rebuilds after menstruation. is also called the follicular phaseThe first phase of the ovarian cycle when follicles grow and estrogen levels rise.. It occurs after menstruation. Rising estrogen levels from the developing ovarian folliclesStructures in the ovaries that contain developing oocytes. stimulate this phase. As a result, the endometriumThe inner lining of the uterus that thickens during the menstrual cycle to support a potential pregn begins to regenerate. Histologically, the stratum functionale, the portion of the endometrium closer to the lumenThe inside space of a hollow organ or structure., thickens. This is due to the proliferation of simple columnar epithelial cellsThe basic structural and functional units of life., elongationThe phase of DNA replication, transcription, or translation in which the molecule is lengthened. of uterine glands, and growthAn increase in size and number of cells. of spiral arteriesBlood vessels supplying the endometrium, shedding during menstruation.. The glands are relatively straight and narrow at this stage. The myometriumThe thick muscular layer of the uterus is composed of interlacing smooth muscle fibers. It remains unchanged throughout the cycle. However, it can be seen beneath the endometrial layer.

Secretory phase

In the secretory phaseThe phase of the menstrual cycle when the endometrium is prepared for implantation., also known as the luteal phaseThe second phase of the ovarian cycle, dominated by progesterone secretion., the endometrium undergoes significant changes. Following ovulationThe release of a mature oocyte from the ovary., progesteroneA hormone that supports pregnancy and regulates the menstrual cycle. from the corpus luteum enhances its glandular structure. It becomes vascularizedTo invade a tissue with blood capillaries. in preparation for potential implantation. Histologically, the glands become coiled and dilated, filled with glycogen-rich secretions, and lined with secretory columnar cells. Spiral arteriesBlood vessels that carry oxygenated blood away from the heart (except pulmonary arteries, which carr continue to lengthen and coil further into the stratum functionale.

Menstrual phase

If implantation does not occur, the cycle enters the menstrual phaseThe phase of the cycle when the functional layer of the endometrium is shed.. The withdrawal of progesterone and estrogen leads to ischemia and breakdown of the stratum functionale. Histologically, the upper layers of the endometrium show disintegration of glandular structures. Spiral arteries rupture. This rupture results in bleeding. The stratum basaleThe deepest layer of the epidermis, where new skin cells are formed., which lies deeper, remains intact and serves as the regenerative layer for the next cycle. The shed tissue and blood constitute the menstrual flow, and the cycle begins anew with the proliferative phase.