Secretory phase

A temporary endocrine gland, the corpus luteum, develops in the ruptured follicle in response to stimulation from the luteinising hormone (LH) secreted by the anterior lobe of the pituitary gland. The corpus luteum secretes the hormone progesterone which together with oestrogen causes the lining of the uterus (endometrium) to become thicker and richly supplied with blood in preparation for pregnancy.

Ovum

Ovary

Fallopian tube

Uterus

Endometrium

Ovum

Uterus

Endometrium

Ovary

Fallopian tube

Menstrual fluid

Days 1-6 Menstruation

Menstrual fluid

Ovum

Days 1-6 Menstruation

Ovum

Days 15-28 Secretory phase

Days 7-13 Proliferative phase

Days 15-28 Secretory phase

Days 7-13 Proliferative phase

Ovum

Ovum

Day 14 Ovulation

Fig 9.3 The menstrual cycle

After ovulation, the ovum can only be fertilised during the next eight to 24 hours. If fertilisation does occur, the fertilised ovum becomes attached to the endometrium and the corpus luteum continues to secrete progesterone. Pregnancy then begins. The corpus luteum continues to secrete progesterone until the fourth month of pregnancy by which time the placenta has taken over this function.

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