The menstrual cycle  Menstruation: the condition in female when the lining of the uterus breaks down and blood with cells pass out of the vagina during the period.  Between the ages of years a woman’s periods stop as her menstrual cycles have stopped which is called a menopause.

Stages in the menstrual cycle  Girls are born with many potential egg cells in their ovaries.  Each potential egg is surrounded by a small group of cells and together they form a follicle.  At puberty, some of the follicles start to develop. This development involves the egg dividing by meiosis which reduces the number of chromosomes in the nucleus by half.  Each month one or a few follicles start to develop.  A follicle starts to develop in the ovary at the beginning of the cycle as menstruation finishes.  After 2 weeks, the follicle bursts releasing the egg (ovulation).

Stages in the menstrual cycle  The follicle cells left behind in the ovary form the yellow body which remains for the next 2 weeks.  If implantation does not occur, it will then decrease in size. If implantation occurs it remains active during pregnancy.  If fertilisation occurs, the fertilised egg divides to form a ball of cells which implants into the thick uterus lining and pregnancy occur.  If fertilisation does not occur, the egg dies and passes out of the vagina and the yellow body in the ovary breaks down. The thick lining of the uterus breaks down and is lost during menstruation.  If a pregnancy does occur, the embryo releases a hormone that stimulates the yellow body to remain active which in turn stimulates the lining of the uterus to thicken and ensures that menstruation will not occur.

Control of the menstrual cycle  Hormones from the pituitary gland control the cycles.  A follicle stimulating hormone (FSH) stimulates a follicle to develop. The egg cell starts to divide inside the follicle and getting bigger.  Cells in the follicle start to secrete oestrogen that causes the thickening of the lining of the uterus in preparation to receive a fertilised egg and prevents any more eggs developing. Oestrogen also passes to the pituitary gland to stops from making any more FSH.

Control of the menstrual cycle  Oestrogen also stimulates the production of Luteinising Hormone (LH).  LH travels to the ovary and stimulates it to do an ovulation and stimulate the making of a yellow body.  The yellow body starts to make the hormone progesterone to thicken the lining of the uterus.  If pregnancy occurs, both oestrogen and progesterone continue to be produced.

Summary  During a menstrual cycle a follicle develops inside the ovary and thicken the lining of the uterus. If implantation does not occur, the lining of the uterus and vagina breakdown (menstruation).  The pituitary hormone FSH stimulates the development of a follicle inside the ovary and the secretion of oestrogen.  Oestrogen stimulates the thickening of the uterus lining, stop the FSH production and causes the pituitary to release LH.

Summary  LH stimulates ovulation and the formation of the yellow body from the remains of the follicle.  The yellow body secretes progesterone which maintains the uterus lining.  Oestrogen and progesterone will make sure that the lining of the uterus stay thick and stop the woman’s menstruation starting again (pregnancy)