4.1 Menstrual Cycle

Menstrual cycle

• Average menstrual cycle for an adult woman is about 28 days.

• Natural variation in the length of the menstrual cycle from about 21 to 33 days. This natural variation in the length of the menstrual cycle is due to stress, tiredness, illness, unbalanced diet or malnutrition.

• Factors that affect length of menstrual cycle include age, ethnicity and history of alcohol consumption.

1. Women aged 35 years or older had a shorter mean menstrual cycle length than women aged 35 years and below.

2. Asian women generally had a longer mean menstrual cycle length than Caucasian women.

3. Women who frequently consume alcohol had a significantly shorter mean menstrual cycle length than women who did not consume alcohol.

• Menstrual period lasts for about 5 days.

• Menopause will take place at the age of about 45-55.

4.2.1 Ovarian Cycle

• The ovaries have about 70 000 potential ova but only about 500 mature ova would be released in the lifetime of a female.

• Each potential ovum is surrounded by a layer of follicle cells and the group of cells is known as a primary follicle.

• Anterior pituitary gland secretes follicle stimulating hormone (FSH).

• Under the influence of FSH, several follicles will develop and enlarge. However, only one primary follicle may develop into a mature follicle known as the Graafian follicle which consists of a mature ovum surrounded by a large fluid-filled space known as the antrum.

• At about 14^th day from start of menstrual cycle, Graafian follicle ruptures and the mature ovum is released from the ovary into the fallopian tube. Release of a mature ovum from the ovary is known as ovulation.

Fig 5 – Structure of a Graafian follicle which consists of a potential ovum, follicle cells and a fluid filled space known as the antrum.

4.2.2 Menstrual Cycle

Days 1 to 5: Menstrual flow stage

Above: Breakdown and shedding of uterine lining through the vagina.

• Breakdown of uterine lining which then flows out of the body via the vagina.

• Anterior pituitary gland secretes follicle-stimulating hormone (FSH) into the bloodstream.

Days 6 to 13: Follicle stage

• FSH stimulates the growth and development of several follicles. However only one primary follicle will fully develop to become the Graafian follicle.

• FSH also stimulates the follicles in the ovaries to form and release oestrogen.

• Effects of oestrogen: (Important!)

NB: Oestrogen is released at two distinct periods of the menstrual cycle - 1. Growing follicles at the ovary and 2. corpus luteum

(a) Oestrogen causes the repair and growth of uterine lining to prepare for embryo implantation.

(b) High concentration of oestrogen in turn (i) inhibits FSH production, preventing the further growth and development of more follicles. (ii) stimulates the pituitary gland to secrete luteinising hormone (LH).

Day 14: Ovulation

• LH causes the Graafian follicle to repture and release the mature ovum. The process is known as ovulation.

• LH also causes the formation of the corpus luteum from the remains of the Graafian follicle. The corpus luteum appears yellow due to the release of a yellow protein known as lutein.

• The corpus luteum secretes progesterone and some oestrogen.

Day 15-28: Corpus luteum stage

Effects of progesterone: (Important!)

(a) Maintains the uterine lining causing it to thicken further and be richly supplied with blood capillaries. The thickened uterine lining prepares for the implantation of the fertilised ovum.

(b) Progesterone and oestrogen inhibit the further release of FSH and LH.

4.2.3 Fertile and infertile periods of menstrual cycle

• Fertile period of menstrual cycle – Days 11 to 16. (Ovulation occurs on Day 14.)

- No fertilisation: the uterine lining breaks down.

- If fertilisation occurs, fertilised egg becomes zygote which then develops into embryo. Embryo embeds itself in the uterine lining.

• • Remaining days of menstrual cycle are the infertile period.