Menstruation

• The menstrual cycle refers to the regular changes in the activity of the ovaries and the endometrium that make reproduction possible
• The endometrium is the layer of tissue lining the inside of the uterus, consisting of a functional layer subject to hormonal changes and a thin basal layer that feeds the overlying functional layer
• Menarche, the onset of the first menstrual period, usually occurs during early adolescence as part of puberty
• The menstrual cycle occurs on a monthly basis, pausing only during pregnancy until menopause when ovarian function declines and menstrual periods stop
• The monthly menstrual cycle can vary in duration from 20 to 35 days, with an average of 28 days; each menstrual cycle begins on the first day of menstruation, referred to as day one of the cycle
• Ovulation, the release of the oocyte from the ovary, usually occurs 14 days before the first day of menstruation, meaning there are usually 14 days leading up to ovulation (pre-ovulatory phase) and 14 days following ovulation (post-ovulatory phase)
• The hypothalamus and the pituitary gland control the whole menstrual cycle, secreting gonadotropin-releasing hormone (GnRH), follicle-stimulating hormone (FSH), and luteinizing hormone (LH)
• In the ovarian follicular phase, the ovarian follicles grow and compete for ovulation, with the dominant follicle continuing to grow and eventually undergoing ovulation
• The rising estrogen levels from the dominant follicle act as a positive feedback signal, stimulating the pituitary to secrete a surge of FSH and LH, which induces ovulation
• After ovulation, the remnant of the ovarian follicle becomes the corpus luteum, secreting progesterone which acts as a negative feedback signal on the pituitary, decreasing FSH and LH release
• Progesterone helps make the endometrium receptive to implantation and maintains pregnancy, leading to the secretory phase of the endometrial cycle
• The secretory phase involves spiral arteries growing and becoming coiled, uterine glands secreting more mucus, and the endometrium preparing for potential implantation
• The corpus luteum gradually degenerates into the non-functional corpus albicans, leading to decreased estrogen and progesterone levels, collapse of spiral arteries, and shedding of the functional layer of the endometrium through menstruation
• The menstrual cycle is the monthly process by which an egg matures, is released from the ovary, travels through the fallopian tube to the uterus, and then leaves the body if it has not been fertilized.