Menstrual Cycle

• Regulated by gonadotrophins released by the anterior pituitary gland
• Aims to release one follicle (ovulate) and prepare the uterus for implantation after fertilisation
• 3 phases

• Functional zone (stratum functionalis) of endometrium degenerates and is shed
• Spiral arteries are constricted = ischaemia (inadequate blood supply)
• o2 and nutrient deprived so degenerates

• Ovarian follicles rapidly grow
• Epithelial multiplication in basilar zone = restoation of uterine lining
• Uterine glands enlarge and blood vessels develop

• Endometrium thickens and becomes spongy
• uterine glands enlarge
• arteries enlongate into functional zone
• begins after ovulation and peaks approx 12 days later
• uterine cycle ends approx 14 days after ovulation (corpus luteum stops producing stimulatory hormones)

• Requires coordination of ovarian and uterine cycles
• Regulated by hypothalamus secreting GnRH
• Pulse frequency of GnRH varies throughout cycle but is constant in males

Follicular phase

• All 4 hormones relativelt low but FSH is dominant.
• Enlarging follicles secrete oestrogen which initiates repair of endometrium
• Secondary follicle releases inhibin = negative feedback on FSH
• Surge in oestrogen at end of follicular phase = inc in GnRH pulse frequency
• High oestrogen stimulates surge in LH which stimulates completion of meiosis 1, rupture of follicular wall and ovulation
• High levels of oestrogen make cervical mucus thinner and easily penetrable

Luteal phase

• Progesterone is dominant hormone, triggered by ovulation
• Oestrogen continues to develop uterus for implantation - inc blood supply and glandular secretions
• GnRH pulses = 1-4 per day (LH dominant hormone at this frequency)
• 12 days after ovulation oestrogen and progesterone drop due to degeneration of corpus luteum
• Drop in O and P = restricted blood supply to functional zone (degenerates), GnRH pulses inc, stimulating FSH which starts the cycle again