1. HUMAN REPRODUCTION BIOLOGY 269

2. Both male and female reproductive functions controlled by hormones. We’ve already discussed some of those. Let’s review a bit and Discuss some additional hormones important in reproduction.

3. It all starts with the pituitary gland

4. Pituitary gland secretes follicle stimulating hormone (FSH): FSH stimulates follicles to develop in ovary; Follicles then produce estrogen. Estrogen: a)Stimulates development of female sexual characteristics b)Stimulates contractions of oviduct c)Stimulates endometrium of uterus to proliferate d)Inhibits secretion of more FSH from pituitary FSH stimulates seminiferous tubules of testes to produce sperm Seminiferous tubules don’t secrete any other hormones, so FSH secretion by pituitary remains constant

5. Pituitary gland secretes luteinizing hormone (LH): LH stimulates ovulation, then stimulates follicle to develop into corpus luteum; this then secretes progesterone Progesterone a)Continues to inhibit secretion of FSH from pituitary b)Keeps endometrium of uterus ready for embryo implantation c)Inhibits contraction of muscle (myometrium) of uterus LH stimulates interstitial cells of testes to secrete testosterone Testosterone a)Stimulates development of male sexual characteristics b)Does not inhibit LH secretion by pituitary, so LH remains constant

6. If fertilization occurs: The embryo begins to develop its placenta, which secretes human chorionic gonadotropin (HCG). HCG stimulates corpus luteum of ovary to continue secreting progesterone; this inhibits menstruation and inhibits the pituitary from secreting FSH, so another follicle will not start to develop If fertilization does not occur: Corpus luteum stops secreting progesterone and degenerates. As levels of progesterone fall, menstruation occurs as the myometrium (muscle) of the uterus contracts and the functional layer of its endometrium falls off. The pituitary is no longer inhibited and begins to secrete FSH again

7. The pituitary gland secretes two more hormones involved in reproduction: During pregnancy, pituitary secretes prolactin; this causes the breast to develop milk-secreting glands and begin the production of milk

8. During menstruation, pituitary secretes oxytocin, which causes myometrium (muscle) of the uterus to contract. During childbirth, pituitary secretes oxytocin which causes myometrium to contract. During breastfeeding, sucking on nipple causes pituitary to secrete oxytocin which causes smooth muscle in the breast to contract and express the milk.

9. Oxytocin levels increase sharply during early stages of attraction and sexual arousal. This starts a “positive feedback loop” which increases your receptivity even more and increases your sensitivity to sexual signals from others and to sexual stimulation. Increased levels of oxytocin also leads to feelings of both emotional and sexual attraction: “The hormone of love”. Oxytocin continues to rise during sexual activity, eventually triggering both the muscular contractions and the positive emotional responses of orgasm. For the same reason, oxytocin has also been called “The hormone of masturbation” since any sexual stimulation starts the same positive feedback loop, leading to increased sensitivity and eventually orgasm.

10. Women produce more oxytocin than men during arousal and intercourse, but also require these higher levels to get the same level of arousal and to reach orgasm. Oxytocin: significant differences between men and women Oxytocin levels fall rapidly in men after intercourse, but stay elevated much longer in women. This has been linked to women’s increased desire for a continued connection with her mate after intercourse, and to men’s relative lack of this desire. Women secrete more oxytocin in response to emotional and verbal cues; Men secrete more oxytocin in response to physical and visual cues. x