Human Reproductive System

I.         Male Anatomy (Fig. 21.2 p. 509N)

            A.        Penis

                        1.         The external organ for depositing sperm in vagina of female.

                        2.         During sexual excitement, the nervous system causes the arteries leading to the penis to dilate, increasing blood flow. This blood flow squeezes veins leading away from the penis, restricting blood flow. As a result, erectile tissue in the penis fills with blood, causing an erection. The erection facilitates penetration into the vagina.

                        3.         The urethra is a tube inside the penis through which both sperm and urine travel.

                        4.         The ejaculatory duct regulates the movement of sperm into the urethra while the bladder contains a sphincter which regulates the passage of urine. Together, these prevent the sperm and urine from passing throught he urethra at the same time.

            B.        Testes - The male gonad which produces sperm.

            C.        Scrotum - A pouch of skin below the pelvis which contains the testes. The scrotum maintains the testes a few degrees cooler than the abdominal cavity. This is necessary for the proper production of sperm.

            D.        Seminal fluid (semen) - the male ejaculate contains a few mL of fluid and 250-500 M sperm. The fluid provides a swimming medium for the sperm cells.

                        1.         seminiferous tubules - Thin twisting tubes that are lined with cells that produce sperm. These cells are the ones which go through meiosis.

                        2.         epididymis - A tightly coiled tube attached to the outside of each testis. Sperm, produced in the epididymis, mature and are stored here until ejaculation or reabsorption by the body.

                        3.         vas deferens - This is a tube through which sperm travel from the epididymis to the urethra

                        4.         seminal vesicles

                                    a.         fructose as energy source

                                    b.         prostaglandins - chemicals secreted by cells that affect a localized area; in the female reproductive tract, these prostaglandins cause rhythmic contractions of the uterus; this increases the movement of the sperm toward the egg.

                                    c.         proteins in the seminal fluid cause the semen to coagulate once inside the female, making it easer for the uterine contractions to move the semen.

                        5.         prostate gland - produces an alkaline buffer to protect sperm from residual urine in the urethra as well as from vaginal acid.

                        6.         Bulbourethral (or Cowper’s) glands - still some speculation as to their purpose. They do secrete a mucus which may provide protection from any acidic urine residue; possibly aid in sperm movement (?); hinder other sperm (?). The bulbourethral fluid carries some sperm released before ejaculation. This is why the withdrawal method of birth control is not recommended.

                        7.         The average human male releases 250-400 M sperm per ejaculation (that’s a lot!); sperm can live for up to 72 h in the female.

II.        Hormonal Control of Male Reproductive System (Fig. 21.5 p.513N)

            A.        The hypothalamus produces GnRH which stimulates the pituitary to release FSH and LH

            B.        FSH stimulates Sertoli cells to help produce sperm. Sertoli cells release a hormone (inhibin?) which inhibits the pituitary and hypothalamus (this mechanism is not well understood).

            C.        LH stimulates interstitial cells which in turn produce testosterone

            D.        testosterone

                        1.         inhibits hypothalamus

                        2.         stimulates spermatogenesis by affecting spermatocytes

                        3.         triggers development of secondary sexual characteristics

                        4.         maturation of testes and penis

                        5.         sex drive

                        6.         facial and body hair

                        7.         lower voice

                        8.         increased muscle mass

                        9.         secretion of body oils

                        10.       Some increase in general aggression

III.       Female Anatomy (Fig. 21.6 p. 515N) - The male sexual anatomy is basically a delivery system for sperm; the female is far more complex, as it is designed to nurture and protect the growing embryo and fetus; the female contributes a long term investment of energy and effort.

            A.        Vagina

                        1.         The vagina is the repository for sperm. Sperm are deposited here during sexual intercourse after male ejaculation.

                        2.         It provides a connection between the uterus and the outside world. It is the passageway (birth canal) through which the baby is born.

                        3.         The environment is very acidic to discourage the growth of microbes.

            B.        Labia (minora and majora) - Thin folds of skin and thick, fatty layers, respectively, the labia enclose and protect the clitoris

            C.        Uterus

                        1.         myometrium - the muscular outer layer; which provides support for the fetus and later for childbirth

                        2.         endometrium - the inner layer, with many blood vessels, which nourishes the embryo; it is shed during menstruation if pregnancy does not occur

            D.        Cervix - a muscular ring which separates the vagina from the uterus. It holds the fetus in place until birth, when it dilates to allow the fetus to pass through.

            E.        Ovaries

                        1.         each ovary contains about 200,000 follicles. A follicle is a structure which consists of an egg cell surrounded by cells which nourish and protect the developing egg.

                        2.          Of all these follicles, only a few hundred will ever mature and release an egg. One matures during each menstrual cycle.

                        3.         ovulation is the process of an egg cell being released from the follicle

                        4.         oogenesis - Fig 21.7 p. 516N

            F.        Oviducts (Fallopian tubes

                        1.         These tubes are the passageway through which the egg must travel from the ovaries to the uterus. Sperm, also, must swim through the oviducts to meet the egg.

                        2.         The end of the oviduct has an opening near the ovary called the fimbria. These are covered with cilia and serve to “sweep up” the egg when it is released during ovulation.

IV.      Hormonal Control of Female Reproductive System (Fig. 21.9 p. 519N) - The hormonal control of the female reproductive system attempts to coordinate two different cycles - the menstrual cycle and the uterine cycle.

            A.        The hypothalamus secretes GnRH which causes the pituitary to release FSH and LH

            B.        FSH stimulates a follicle to develop and begin producing estrogen. Low [estrogen] early in the cycle inhibits the pituitary while high [estrogen] later in the cycle stimulates both the hypothalamus and pituitary to secrete hormones.

            C.        The increase in LH stimulates the corpus luteum which then begins secreting progesterone and estrogen

            D.        These two hormones together stimulate further growth of the endometrium, strengthen the uterus, inhibit further ovulation, and prevent uterine contractions

            E.        Note: as [estrogen] and [progesterone] from the corpus luteum increase, the hypothalamus and pituitary are inhibited. The resulting decline in LH causes the corpus luteum to deteriorate. The resulting lower [estrogen] and [progesterone] allow the hypothalamus and pituitary to begin producing FSH and LH which starts the cycle again

            F.        If pregnancy does not occur, the decrease in [progesterone] triggers uterine contractions and the endometrium is shed

V.        Menstrual Cycle (Fig 21.8 p. 518N)

            A.        Flow phase - This marks the beginning of the cycle and is characterized by the shedding of the endometrium.

            B.        Follicular phase

                                    a.         development of a follicle within the ovary

                                    b.         the follicle cells produce estrogen which promotes the thickening of the endometrium. Estrogen also triggers development of the secondary sexual characteristics

                                    c.         usually only one follicle matures during each cycle

            C.        Ovulation

                                    a.         the egg bursts from follicle

                                    b.         the remaining follicle cells become the corpus luteum

                                    c.         this marks the mid-point of the cycle

            D.        Luteal phase

                                    a.         the development and maturation of corpus luteum occur during this phase

                                    b.         [estrogen] declines briefly because the follicle is gone

                                    c.         the corpus luteum begins secreting estrogen and progesterone

                                                (1)       stimulates growth of the endometrium

                                                (2)       strengthens the uterus

                                                (3)       inhibits further ovulation

                                                (4)       prevents uterine contractions

                                    d.         If pregnancy does not occur, the [progesterone] declines which triggers uterine contractions and the endometrium is shed.

VI.      Fertilization and Pregnancy

            A.        Only about 200 sperm reach the oviducts and only a few reach the egg

            B.        the fertilized egg implants itself in the uterine wall; it is important that menstruation not occur now or the zygote would be lost. *How is endometrium maintained?

            C.        The corpus luteum produces estrogen and progesterone which inhibit the release of GnRH; this means no LH is produced and the corpus luteum cannot be maintained; therefore, no progesterone is present to prevent uterine contractions and the lining would be shed. The solution is to maintain estrogen and progesterone.

            D.        The developing zygote produces HCG (human chorionic gonadotropin), a hormone which maintains the corpus luteum for about 3 months.

            E.        What then? As the placenta develops, it produces estrogen and progesterone.

VII.     Birth

            A.        High levels of estrogen trigger a sensitivity to the hormone oxytocin

            B.        Oxytocin

                        1.         is secreted by the fetus and the female pituitary

                        2.         stimulates uterine contractions. The stress of these contractions often increases oxytocin production in a positive feedback loop.

            C.        Relaxin - is a hormone produced by the placenta which causes loosening of the pelvic ligaments to facilitate the passage of the baby.

            D.        Prolactin - a hormone produced by the female pituitary that stimulates the mammary glands to produce milk; milk supplies protein, sugar, fat, and antibodies to the baby.