Reproductive System

[In sexual reproduction new individuals are produced by the fusion of haploid gametes to form a diploid zygote. Sperm are male gametes, ova (ovum singular) are female gametes. Meiosis produces cells that are genetically distinct from each other; fertilization is the fusion of two such distinctive cells that produces a unique new combination of alleles, thus increasing variation on which natural selection can operate. Human reproduction employs internal fertilization, and depends on the integrated action of hormones, the nervous system, and the reproductive system. Gonads are sex organs that produce gametes. Male gonads are the testes, which produce sperm and male sex hormones. Female gonads are the ovaries, which produce eggs (ova) and female sex hormones.]

Identify and give functions for each of the following:

- testes (seminiferous tubules and interstitial cells) • Produce sperm.

• Produce testosterone.

- epididymis Maturation and storage of sperm

- ductus (vas) deferens Conducts sperm and stores sperm

- prostate gland Manufacture and storage of seminal fluid

- Cowper's glands Storage of seminal fluid

- seminal vesicles Manufacture and storage of seminal fluid

- penis Erection prior to sexual intercourse

- urethra Conducts semen (and urine)

 

 

 

 

 

Demonstrate a knowledge of the path of sperm from the seminiferous tubules to the urethral opening

Testes are suspended outside the abdominal cavity by the scrotum, a pouch of skin that keeps the testes close or far from the body at an optimal temperature for sperm development. Seminiferous tubules are inside each testis, and are where sperm are produced by meiosis. Once sperm form they move into the epididymis, where they mature and are stored. Sperm pass through the vas deferens and connect to a short ejaculatory duct that connects to the urethra. The urethra passes through the penis and opens to the outside.

 

[About 250 meters (850 feet) of tubules are packed into each testis. Spermatocytes inside the tubules divide by meiosis to produce spermatids that in turn develop into mature sperm. Sperm production begins at puberty at continues throughout life, with several hundred million sperm being produced each day]

List the functions of seminal fluid

At the time of ejaculation, sperm leave the penis in a fluid called seminal fluid (also called semen).

Secretions from the seminal vesicles add fructose and prostaglandins to sperm as they pass. Fructose serves as an energy source for swimming sperm. Prostaglandins are chemicals that cause the uterus in females to contract. (Some investigators now believe that uterine contraction is necessary to help propel the sperm toward the egg).

The prostate gland secretes a milky alkaline fluid. Sperm are more viable in a basic solution (pH 7.5).

The Cowper's (bulbourethral) gland secretes a mucus-like fluid that provides lubrication for intercourse. Sperm and secretions make up semen.

Identify the tail, midpiece, head, and acrosome of a mature sperm and state their functions

W- acrosome Contains acrosome enzymes which aid the sperm in reaching the surface of the egg and allow a single sperm to penetrate the egg (zone pellucida).

X- head Stores genetic material (or DNA) required to produce a new human being. (Contains 23 chromosomes)

Y- midpiece Produces ATP in mitochondria to provide energy to tail for swimming.

Z- the tail Provides the locomotion needed by the sperm to reach the egg (so that

fertilization can occur).

Describe the functions of testosterone

• Maturation of penis and testes. • Production of sperm. • Growth of body hair.

• Larynx and vocal chords enlarge. • Increased muscle strength.

• Oil and sweat glands secrete. (most effects associated with puberty)

Demonstrate a knowledge of the control of testosterone levels by the endocrine system

The hypothalamus produces gonadotropic releasing hormone (GnRH) which causes the anterior pituitary to produce and release two gonadotropic hormones: LH (Luteinizing Hormone) and FSH (Follicle Stimulating Hormone)[named for their function in females]. These two pituitary hormones act on the testicles: LH promotes the production of testosterone in the interstitial cells, and FSH promotes spermatogenesis (produces sperm) in the seminiferous tubules (by causing spermatogenic cells to take up testosterone). Testosterone production and spermatogenesis is regulated through a negative feedback loop between the testicles and the hypothalamus/ pituitary gland. As the level of testosterone in the blood rises to a certain level, it causes the anterior pituitary/hypothalmus to decrease its secretion of LH. As the level of testosterone in the blood falls below a certain level begins to fall the anterior pituitary increases secretion of LH, and stimulation of the interstitial cells reoccurs. Recently isolated is hormone called inhibin. Inhibin is produced by the seminiferous tubules (in addition to sperm) and exerts negative feedback control over the hypothalmus/pituitary glands to block FSH secretion.

Identify and give a function for each of the following:

- ovaries (follicles and corpus luteum) • Release egg(s). • Mature egg(s).

• Secrete estrogen. • Secrete progesterone.

- oviducts (fallopian tubes) To conduct the egg from the ovary to the uterus. Fertilization usually occurs in the tube.

- uterus The lining of the uterus is called the endometrium participates in the formation of the placenta. The uterus functions to nourish (endometrium) and contain the fetus/embryo during gestation (pregnancy) and to assist in partuition (birth).

- cervix The entrance to the uterus; part of the birth canal. Must dilate at birth.

- vagina Receives penis during sexual intercourse; part of the birth canal

- clitoris important in arousal, is a short shaft with a sensitive tip covered by a fold of skin.

Describe the functions of estrogen

Estrogen:

• Causes eggs to mature.

• Causes breasts to develop (during puberty).

• Causes endometrium to thicken.

• Causes the pelvic girdle to enlarge(during puberty).

• Causes growth of uterus and vagina (during puberty).

• Causes the onset of the menstrual cycle (during puberty).

• Causes growth of pubic and underarm hair (during puberty).

• Causes changes in fat distribution (during puberty).

 

 

Describe the sequence of events in the ovarian and uterine cycles

ovarian cycle - A longitudinal section shows there are many sac-like structures called follicles in the cortex of each ovary. Each follicle contains an immature egg. One follicle matures to produce an egg each month during a females reproductive years.

During the first 14 days of the ovarian cycle, the follicle matures and releases estrogen (follicular phase). On the 14th day, the follicle balloons out of the ovary and bursts to release the egg (ovulation) which is surrounded by a mucoprptein substance called the zona pellucida. Once a follicle has lost its egg (days 15-28), it develops into a corpus luteum (luteal phase), a structure that secretes progesterone. If pregnancy does not occur, the corpus luteum begins to degenerate after about 10 days. If pregnancy does occur the corpus luteum persists for 3 - 6 months. The follicle and corpus luteum secrete the female sex hormones estrogen and progesterone respectively.

uterine cycle - Menstrual cycles vary from between 15 and 31 days. The first day of the cycle is the first day of blood flow (day 0) known as menstruation. During menstruation the uterine lining is broken down and shed as menstrual flow. Midcycle, Estrogen and progesterone stimulate the development of the endometrium and preparation of the uterine inner lining for implantation of a zygote. If pregnancy does not occur, the corpus luteum disintegrates. The drop in hormones causes the sloughing off of the inner lining of the uterus by a series of muscle contractions of the uterus.

 

Demonstrate knowledge of the control of the ovarian and uterine cycles by hormones

The ovarian cycle is hormonally regulated in two phases. The follicle secretes estrogen before ovulation; the corpus luteum secretes both estrogen and progesterone after ovulation. Hormones from the hypothalamus and anterior pituitary control the ovarian cycle. The ovarian cycle covers events in the ovary; the uterine (menstrual) cycle occurs in the uterus. Menstrual cycles vary between individuals from 15 to 31 days. The first day of the cycle is the first day of blood flow (day 0) known as menstruation. During menstruation the uterine lining is broken down and shed as menstrual flow. FSH (primarily) and LH are secreted on day 0, beginning both the menstrual cycle and the ovarian cycle. Days 1 - 14, FSH (primarily) and LH stimulate the maturation of a single follicle in one of the ovaries. The maturing follicle secretes estrogen. Rising levels of estrogen in the blood trigger further secretion of LH (positive feedback on the hypothalmus), which stimulates follicle maturation and ovulation (day 14, or midcycle). LH stimulates the remaining follicle cells to form the corpus luteum, which produces both estrogen and progesterone. Estrogen and progesterone stimulate the development of the endometrium and preparation of the uterine inner lining for implantation of a zygote. If pregnancy does not occur, the drop in FSH and LH cause the corpus luteum to disintegrate. The drop in hormones also causes the sloughing off of the inner lining of the uterus by a series of muscle contractions of the uterus (menses).

Summary table of the ovarian cycle

What happens on day 14?

• Ovulation occurs on Day 14.

What causes this event to occur?

• A surge of LH is thought to be responsible.

What causes Phase 2 to end?

• Degeneration of the corpus luteum. • All hormones are at their lowest levels.

• Negative feedback of LH stops its production.

Describe the effects of implantation (pregnancy) on the ovarian cycle.

• Degeneration of the corpus luteum is prevented.

• More progesterone is produced.

• No new follicles mature.

 

Demonstrate knowledge of a positive feedback mechanism involving oxytocin

Oxytocin.

• Causes expression of milk by mammary glands. When a breast is suckled, nerve endings in the areola (pigmented area around the nipple) are stimulated. The signal travels to the hypothalmus which causes oxytocin to be released from the posterior pituitary. When this hormone arrives at the breasts it causes the lobules to contract so that milk flows into the ducts.

• Causes the smooth muscles of the uterus to contract [during parturition - labour and expulsion of the fetus] and these contractions increase the release of more oxytocin from the posterior pituitary (positive feedback). [Can be given to induce parturition]

Describe the hormonal changes that occur as a result of implantation

• Embryonic membrane produces HCG (human chorionic

gonadotropic) hormone.

• HCG maintains corpus luteum in the secretory phase; therefore,

it continues to secrete progesterone (in even larger amounts).

• After its formation, the placenta continues HCG production.

• Negative feedback results in decreased FSH (follicle-stimulating

hormone) and LH production.

• Placenta secretes estrogen and progesterone and the corpus

luteum degenerates by the end of the first trimester.

Describe the effects of implantation (pregnancy) on the ovarian cycle.

• Degeneration of the corpus luteum is prevented (HCG).

• More progesterone is produced (corpus luteum and placenta).

• No new follicles mature (decreased FSH and increased estrogen and progesterone).

• More estrogen and progesterone from the placenta also help to maintain the lining of the uterus so that the corpus luteum is not needed

[Sexual Responses]

Humans do not have a mating season , females are sexually receptive to the male at all times of the year. There are four stages in mating: arousal, plateau, orgasm, and resolution. During male arousal, blood flows into the three shafts of spongy erectile tissue inside the penis, causing it to become elongated and erect. The female arousal has the swelling of the areas around the vagina, erection of the clitoris and

nipples, and secretion of lubricating fluids in the vagina. After insertion of the penis into the vagina, pelvic thrusts by both partners stimulate sensory receptors in the penis, vaginal walls, and clitoris. The sperm leave the epididymis and secretions of glands form the semen. Orgasm involves contractions of muscles of the penis (male) or vagina (female) and waves of pleasurable sensations.

Resolution reverses the previous phases: muscles relax, breathing slows, the penis returns to its normal size.

LH (luteinizing hormone):

• A surge of LH causes ovulation in females.

• Promotes testosterone production in males.

• Leads to luteal phase in females (ovarian cycle).

• Causes development of the corpus luteum in females.

Follicle stimulating hormone:

• Promotes the development of the follicle.

• Produces sperm.

• Causes spermatogenic cells to take up testosterone.

• Maturation of the egg.