Female Reproductive System Menstrual Cycle

What’s the point Females release only one egg or a few eggs at one time during each reproductive cycle. Control of the female cycle is quite complex Two types of cycles occur in female mammals * Humans and many other primates have the menstrual cycle * Other mammals have the estrous cycle

Overview A woman’s first menstruation is termed menarche ( between 8-16 yrs of age) The cessation of menstrual cycles at the end of a woman’s reproductive life is termed menopause (between 40-58 yrs of age) Menstrual cycles are counted from the first day of bleeding and may be divided into several phases. The length of each phase varies from woman to woman and cycle to cycle.

Name of Phase Days Menstrual Phase 1- 4 Follicular phase 5 – 13 Ovulation 14 Luteal Phase 15-28

The Menstrual Cycle Ovaries are programmed to produce a mature egg (ovum) approx every 28 days and to prepare the uterus for implantation of an embryo if the egg becomes fertilised. To achieve this ovaries must receive the correct hormonal signals from the brain and pituitary gland.

Hormonal signals  stimulate production of female sex hormones & changes in ovaries during each cycle  stimulate changes in the lining of the uterus System is regulated and fine tuned by feedback effects of the ovarian hormones on hormone secretions from the brain and pituitary gland

The hypothalamus & Pituitary

Hormones involved Oestrogen and progesterone are produced by the ovaries Luteninizing hormone (LH) and follicle stimulating hormone (FSH) are produced by the pituitary gland. Gonadotrophin-releasing hormone (GnRH) of the hypothalamus controls the pituitary

Day one First day of menstrual blood loss Uterus begins to shed its lining and bleeding occurs Ovarian hormone secretions are at a minimum Pituitary hormone secretions increase due to a drop in ovarian hormones and stimulate activity in the ovaries

Effect of Pituitary hormones FSH stimulates growth of a few follicles in each ovary By day 10 each ovary contains several follicles in the process of maturation By mid cycle all but one of these follicles degenerate and only the ‘dominant’ follicle becomes fully mature

Follicular Phase The name given to the first half of the ovarian cycle Increased secretions of oestrogen from the developing follicles in the ovaries is released in the bloodstream Increased oestrogen levels cause the uterus lining to thicken, and become richly supplied with blood vessels

Ovulation In most normal human menstrual cycles only 1 follicle reaches full maturity to be released at ovulation Occurs on day 14 of a normal cycle A surge of LH causes the mature ‘dominant’ follicle to rupture and release it’s egg A small rise in body temperature occurs Cervical mucus changes

Luteal Phase Empty follicle left behind forms the corpus luteum in the ovary Corpus luteum produces increasing amounts of progesterone and some oestrogen These hormones act on the lining of the uterus, thickening it and preparing it for possible implantation of an embryo High progesterone and oestrogen exert negative feedback effects on the pituitary hormones

Small amounts of LH and FSH continue to maintain the function of the corpus luteum. If fertlisation doesn’t occur the corpus luteum breaks down, progesterone and oestrogen levels decrease and the uterine lining is lost. A new cycle begins

Feedback Oestrogen and Progesterone have a negative feedback effect on LH and FSH ∴ when oestrogen and progesterone increase in the bloodstream LH and FSH decrease The exception to this rule is the LH spike causing ovulation which coincides with a rise in oestrogen from developing follicles

If Fertilisation Does occur Fertilisation typically occurs in the fallopian tube within a day after ovulation Sperm must cover the distance between the vagina and the fallopian tube Sperm can take from 2 hours to as little as 30 mins to reach the secondary oocyte Of the roughly 200 million sperm in an ejaculation only 5,000 enter the fallopian tube and fewer than 50 reach the oocyte

Normally only a single sperm will accomplish fertilisation and activate the oocyte. The sperm’s acrosomal cap ruptures and the sperm and oocyte membranes begin to fuse The secondary oocyte completes meiosis II and casts out the secondary polar body The male and female nuclei come together and they immediately replicate their DNA

The zygote begins to divide and produce a blastocyst and when it reaches the uterus it implants into the uterine lining If the embryo implants into anything but the uterine lining an ectopic pregnancy will occur which can be fatal to the mother.