The menstrual cycle: basic biology

Abstract

The basic biology of the menstrual cycle is a complex, coordinated sequence of events involving the hypothalamus, anterior pituitary, ovary, and endometrium. The menstrual cycle with all its complexities can be easily perturbed by environmental factors such as stress, extreme exercise, eating disorders, and obesity. Furthermore, genetic influences such as fragile X premutations, X chromosome abnormalities, and galactose-1-phosphate uridyltransferase (GALT) point mutations (galactosemia) also contribute to perturbations of the menstrual cycle. Although not perfect, mouse models have helped to identify and confirm additional components and pathways in menstrual cycle function and dysfunction in humans.

Figure 1. General overview of the important factors in the menstrual cycle

Regulation of the menstrual cycle begins with influences at the level of the hypothalamus. The hypothalamus stimulates the anterior pituitary which stimulates the ovaries. One of the end organs for the ovarian sex hormones is the endometrium. The menstrual cycle is regulated by feed back and cross talk between these different components.

Figure 2. Anatomy of an ovarian follicle

This cartoon depicts the various cell types of an antral follicle and some of the factors secreted by each cell type. The oocyte is surrounded by cumulus granulosa cells, while the mural granulosa cells surround the antrum. The theca cells surround the entire follicle. Crosstalk between these cell types by the factors listed and others are important for ovarian folliculogenesis.

Figure 3. Stages of ovarian follicular development

Folliculogenesis requires a coordinated progression of growth of ovarian follicles. The process begins with the germ cells which are recruited to a pool of primordial follicles. The primordial follicles progress to primary and then secondary follicles. At the secondary follicle stage, theca cells are present. The early antral follicle stage is defined by the presence of antrum. The peri-ovulatory follicle stage is also known as the dominant follicle and is ready for ovulation. this stage, cumulus and mural granulosa cells are present. Once the oocyte is ovulated, the remaining granulosa cells become the corpus luteum. This cycle of folliculogenesis occurs for every single oocyte ovulated.

Figure 4. Endometrial lining throughout the menstrual cycle

The endometrial lining thickens in response to estrogen in the proliferative phase. After ovulation, the endometrium becomes decidualized in the secretory phase. The window of receptivity is 7–10 days after the LH surge, near the time of ovulation. After the cycle is finished, menstruation occurs and the cycle begins again.