50.1. How Animals Reproduce (p. 894)
A. Two Patterns of Reproduction
1. Asexual---one parent involved.
2. Sexual---two parents involved.
B. Asexual and Sexual Reproduction
1. Asexual reproduction involves only one parent, and the offspring have the same genotype as the parent.
a. Budding occurs in hydra; a new individual arises as an outgrowth (bud) of a parent individual. (Fig. 50.1)
b. Fragmentation followed by regeneration is seen among sponges, planaria, and echinoderms.
c. Parthenogenesis is common among some insects and several other animals; a modification of sexual reproduction where the egg develops into a whole organism with fertilization.
2. Sexual reproduction is a form of reproduction in which the egg of one parent is fertilized by the sperm of the other.
C. Reproductive Organs Are Primary or Accessory
1. Gonads are organs specialized to produce gametes.
a. Sponges are an exception since their collar cells give rise to sperm and eggs.
b. Hydras produce only temporary gonads in fall, when sexual reproduction occurs. (Fig. 50.1)
c. Animals in other phyla have permanent gonads.
2. There are two types of gonads: testes for sperm production and ovaries for egg production.
3. Gametes are sperm or egg cells that fuse during fertilization to form a zygote.
4. Other cells in a gonad support and nourish developing gametes or produce hormones for reproduction.
5. Accessory organs form ducts and storage areas that aid in bringing gametes together.
6. Copulation is sexual union between two animals to facilitate reception of sperm by a female.
a. Some animals, such as earthworms, lack a copulatory organ.
b. The penis is a male copulatory organ typical of terrestrial males; deposits sperm into vagina of females.
c. Aquatic animals have other types of copulatory organs or employ other strategies for delivering sperm: lobsters and crayfish modified swimmerets, cuttlefish and octopuses use an arm, and sharks have a modified pelvic fin that passes packets of sperm to the female.
d. Among terrestrial animals, most birds lack a penis and vagina; they transfer sperm from cloaca to cloaca.
D. Protecting Zygote and Embryo
1. Many aquatic animals practice external fertilization; eggs and sperm join outside of the body in the water.
2. Terrestrial animals tend to practice internal fertilization; eggs and sperm join inside of the female's body.
3. Both types of animals are usually oviparous; they deposit eggs in the external environment.
4. Eggs are produced in ovaries, and as they mature they increase in size as a result of the accumulation of yolk.
a. Yolk is stored food to be used by the developing embryo.
b. To prevent insect eggs from drying out, they are covered by a shell of several layers of protein or wax.
c. In insects, small holes are left at one end to allow entry of sperm.
5. Some insects have a special organ to store sperm for sometime so eggs can be fertilized later.
6. A larval stage is often quite different in appearance and way of life from the adult form, and is able to seek its own food to sustain itself until it becomes an adult. (Fig. 50.2) [transp. 286]
a. Metamorphosis is a dramatic change in shape and form that some animals undergo during development.
b. Incomplete metamorphosis lacks a pupal stage and the nymphs look more like the adult.
c. Larval aquatic forms can utilize a different food source than the adults.
d. Bilaterally symmetrical sea star larvae merely attach to the substrate and change into radially symmetrical adults.
e. Free-swimming barnacle larvae metamorphose into sessile adults with calcareous plates.
f. In contrast, crayfish lack a larval stage and eggs hatch into tiny juveniles with the same form as adults.
7. Reptiles and birds provide their eggs with plentiful yolk; there is no larval stage.
a. Complete development takes place within a shell containing extraembryonic membranes.
b. The chorion is the outermost membrane that lies next to the shell and functions in gas exchange.
c. The amnion forms a water-filled sac around the embryo ensuring it will not dry out.
d. The yolk sac hold yolk which nourishes the embryo.
e. The allantois holds nitrogen waste products.
f. The shelled egg frees animals from the need to reproduce in water and helps them live completely on land.
8. Birds in particular tend their eggs.
a. Newly hatched birds usually have to be fed before they develop to where they can seek food on their own.
b. The reproductive behavior of parent birds involves complex hormone and neural regulation. (Fig. 50.3)
9. Oysters and sea horses are ovoviviparous; eggs remain in body until they hatch as fully developed offspring.
10. Some animals, particularly mammals, are viviparous; embryo remains in female's body during development.
a. Nutrients needed for development are constantly supplied by the mother.
b. Viviparity represents the ultimate in caring for the zygote and the embryo.
c. Evolution of viviparity can be seen in primitive mammals.
1) Exceptions are the duckbill platypus and the spiny anteater, which are egg-laying mammals.
2) Marsupials give birth to immature offspring that finish developing within a pouch; they are fed on milk.
3) In all other mammals, development occurs in a placenta.
11. The placenta is a complex organ comprised of maternal and embryonic tissues.
a. The placenta exchanges O2, CO2, nutrients, wastes, etc. between fetal and maternal circulations.
b. Its evolution allowed developing offspring to exchange materials with the mother; made shell unnecessary.
E. Finding a Mate
1. Sexually reproducing animals have wide variety of ways of making sure the gametes find each other.
a. Aquatic animals with external fertilization are programmed to release eggs in water only at the lunar cycle.
b. The palolo worms rise to the surface to release eggs during a few hours on two or three special days.
c. Among terrestrial animals, reproductive cycles are often tied to day-length changes.
1) Photoperiod reliably indicates proper time for reproductive behavior, including long migrations.
2) Melatonin is produced by the pineal gland at night and is involved in reproductive cycles.
3) By shortening the day lengths, fall-breeding animals can be induced to breed in the spring.
2. Animals that copulate have innate courtship rituals to bring members of the opposite sex together.
a. Rituals ensure male and female are same species; also reduce aggressive tendencies between the mating pair.
b. They also promote hormonal responses that prepare the body for reproduction.
3. Reproductive success is measured by how many fertile offspring an individual contributes to the next generation.
50.2. Males Have Testes (p. 898) (Fig. 50.6)[transp. 288]
A. Male Gonads are Paired Testes.
1. Paired testes are suspended in the scrotal sacs of the scrotum. (Fig. 50.4) [transp. 287]
2. Testes began development in the abdominal cavity but descend into the scrotal sac during development.
3. If the testes do not descend, without surgery or hormonal therapy, sterility results.
4. The lower temperature of the scrotum is vital to normal sperm production.
B. Sperm
1. Sperm move from testes to penis.
2. Sperm produced in the testes mature within the epididymides.
a. These are tightly coiled tubules in which sperm undergo maturation.
b. Maturation time in the epididymis is required for the sperm to swim to the egg.
3. Once sperm have matured, they are propelled into the vasa deferentia by muscular contractions.
4. Sperm are stored in both the epididymides and the vasa deferentia.
5. When a male is sexually aroused, sperm enter the urethra, part of which extends through the penis.
6. The penis is a cylindrical copulatory organ, used to introduce spermatozoa into the female vagina.
a. It contains three columns of spongy, erectile tissue that extend down the shaft. (Fig. 50.5)
b. During sexual arousal, nervous reflexes cause an increase in arterial blood flow to the penis.
c. This increased blood flow fills and distends the erectile tissue, and the penis stiffens and increases in size.
d. These changes are called erection; failure to achieve erection is called impotency.
7. Semen seminal fluid is a thick whitish fluid that contains sperm and fluids.
a. Seminal fluid is formed by seminal vesicles, prostate gland, and bulbourethral glands.
b. Seminal vesicles lie at the base of the urinary bladder.
1) Each joins a vas deferens to form an ejaculatory duct that enters the urethra.
2) They secrete into the ejaculatory duct a thick, viscous fluid containing nutrients for use by the sperm.
c. The prostate gland is located just below the urinary bladder and surrounds upper portion of the urethra.
1) It secretes a milky, slightly alkaline solution that promotes sperm motility and viability.
2) In older men, the prostate gland may become enlarged, the urethra thereby constricting.
d. The bulbourethral glands are located below the prostate gland on either side of the urethra; release mucus secretions that provide lubrication.
8. The urethra conducts urine from the bladder during urination and also conducts semen during ejaculation.
C. Ejaculation
1. Ejaculation results in expulsion of semen; it is achieved at the peak of sexual arousal.
2. The first phase of ejaculation is emission.
a. Nerve impulses from spine trigger epididymides, vasa deferentia, prostate, and bulbourethral glands to contract.
b. Subsequent motility causes sperm to enter ejaculatory duct; seminal vesicles, prostate gland, and bulbourethral glands release their secretions.
c. Small amount of secretion from bulbourethral glands may leak from the end of penis; may provide a lubricant.
3. The second phase of ejaculation is expulsion.
a. Rhythmical contractions of muscles at base of the penis and within urethral wall expel semen in spurts.
b. Rhythmical contractions are a release from myotonia, or muscle tenseness, an important sexual response.
4. An erection lasts for a limited time and the penis returns to a flaccid state following ejaculation.
5. The refractory period follows, during which stimulation does not bring about erection.
6. Orgasm is the physiological and psychological sensations that occur at the climax of sexual stimulation.
D. Testes Produce Sperm and Hormones
1. A longitudinal section of testis show compartments called lobules, each containing one to three seminiferous tubules. (Fig. 50.6b) [transp. 288]
a. Altogether the seminiferous tubules have a combined length of about 250 meters.
b. In microscopic cross section, the tubules show cells undergoing spermatogenesis.
c. Sustentacular (Sertoli) cells support, nourish, and regulate spermatogenic cells. (Fig. 50.6c)
2. Mature sperm (spermatozoa) have three parts. (Fig. 50.6d) [transp. 288]
a. The head contains the nucleus covered by an acrosome.
1) Acrosome is caplike covering over anterior end of nucleus of a sperm; contains enzymes to penetrate egg.
2) A human egg is surrounded by several layers of cells and a thick membrane the sperm must penetrate.
b. Middle piece contains mitochondria wrapped around microtubules of flagellum; provides energy for movement.
c. A tail also contains microtubules as components of a flagellum; its movement propels sperm.
3. The ejaculate of a normal human male contains several hundred million sperm.
4. Fewer than 100 ever reach the vicinity of the egg; and only one sperm normally enters an egg.
E. Hormonal Regulation in Males
1. The hypothalamus has ultimate control of the testes' sexual function through gonadotropic-releasing hormone. (GnRH) (Fig. 50.7) [transp. 289]
2. There are two gonadotropic-releasing hormones, follicle-stimulating hormone (FSH) and luteinizing hormone (LH) found in both males and females.
3. In males, FSH stimulates spermatogenesis in seminiferous tubules, which also release the hormone inhibin.
4. In males, luteinizing hormone is also called interstitial cell-stimulating hormone (ICSH); it stimulates testosterone secretion by the interstitial cells of the testes.
5. Hormones of the hypothalamus-pituitary-testis system are involved in a negative feedback relationship that maintains the fairly constant production of sperm and testosterone. (Fig. 50.7) [transp. 289]
F. Testosterone Is the Male Sex Hormone
1. Testosterone is the main sex hormone in males.
2. It is essential for development of male secondary sex characteristics and for maturation of sperm.
a. It causes growth of a beard, axillary hair and pubic hair.
b. Testosterone causes the larynx and vocal cords to enlarge, causing a deeper voice.
c. It is responsible for the greater muscle strength of males; this is the reason some athletes take supplemental anabolic steroids that are testosterone or mimics (but which have serious side effects).
d. Testosterone is largely responsible for the sex drive and may contribute to aggressiveness.
e. It causes oil and sweat glands in the skin to secrete; it is largely responsible for acne and body odor.
f. Testosterone is also involved in triggering baldness if the genes are present.
50.3. Females Have Ovaries (p. 902)
A. Female Reproductive System
1. The female reproductive system includes the ovaries, the oviducts, the uterus, and the vagina.
2. Ovaries produce an egg each month; are located in the abdominal cavity. (Fig. 50.8) [transp. 290]
3. The oviducts (Fallopian tubes) extend from the ovaries to the uterus.
a. The oviducts are not attached to the ovaries.
b. Fingerlike projections called fimbriae sweep over the ovaries and waft in the egg when it erupts.
c. This is the normal site for fertilization and the embryo is slowly moved by ciliary movement to the uterus.
4. The uterus is a hollow, thick-walled muscular organ the size and shape of an inverted pear.
a. The embryo completes development by embedding itself in the uterine lining, the endometrium.
b. A small opening at the cervix of the uterus leads to the vaginal canal.
5. The vagina is a tube at a 45o angle with the small of the back.
a. The mucosal lining lies in folds and can extend.
b. It receives the penis during copulation and also serves as the birth canal.
B. The external genitalia of women are known collectively as the vulva. (Fig. 50.8b) [transp. 290]
1. Mons pubis and folds of skin called labia minora and labia majora are to side of vaginal and urethral openings.
2. At the juncture of the labia minora is the clitoris.
a. This tissue is homologous to the penis in males.
b. The clitoris has a short shaft of erectile tissue, capped by a pea-shaped glans.
c. It contains many sensory receptors that allow it to function as a sexually sensitive organ.
3. Orgasm involves the release of neuromuscular tension in the muscles of the genital area, vagina, and uterus.
C. Ovaries Produce Oocytes and Hormones
1. The ovaries produce the female sex hormones, estrogens and progesterone, during the ovarian cycle.
2. A longitudinal section through an ovary shows many cellular follicles, each containing an oocyte (egg). (Fig. 50.9) [transp. 291] [micro. slide 100]
D. Ovarian Cycle
1. A female is born with as many as two million follicles; the number is reduced to 300,000-400,000 by the time of puberty; and only a small number of follicles (about 400) ever mature.
2. As a follicle matures, it develops from a primary follicle to secondary follicle to a Graafian follicle.
3. Öogenesis is occurring; a secondary follicle contains a secondary oocyte pushed to one side of fluid-filled cavity.
4. In a Graafian follicle, follicle fills with fluid to point that follicle wall balloons out on surface of the ovary and bursts, releasing a secondary oocyte surrounded by clear membrane and follicular cells.
5. Ovulation is rupture of the ovarian (Graafian) follicle with discharge of a secondary oocyte into pelvic cavity.
6. At ovulation, the secondary oocyte has not completed a second meiotic cell division until fertilization occurs.
7. Meanwhile, the empty follicle is developing into corpus luteum; if pregnancy does not occur, the corpus luteum begins to degenerate in about 10 days.
8. The ovarian cycle is under the control of the gonadotropic hormones, follicle-stimulating hormone (FSH) and luteinizing hormone (LH). (Fig. 50.10) [transp. 292] (Table 50.3)
9. Gonadotropic hormones are not present in constant amounts but are secreted at different rates during the cycle.
10. During follicular phase, FSH promotes the development of a follicle that secretes estrogen.
11. As estrogen level in blood rises, it exerts feedback control over anterior pituitary secretion of FSH; the follicular phase comes to an end.
12. Estrogen levels in blood rise, causing hypothalamus to secret more GnRH; this causes surge in LH secretion.
13. The LH spike triggers ovulation. (Fig. 50.11)
14. The luteal phase is the second half of the ovarian cycle, following ovulation.
a. LH promotes the development of the corpus luteum, which secretes large amounts of progesterone.
b. Progesterone causes the endometrium to build up.
c. As blood level of progesterone rises, negative feedback to anterior pituitary's secretion of LH causes corpus luteum to degenerate.
d. The luteal phase ends with menstruation.
E. Uterine Cycle
1. Estrogens and progesterone affect endometrium of uterus to cause a cycle of events known as the uterine cycle. (Table 50.3) (Fig. 50.11)
2. A uterine cycle averaging 28 days is divided into four sections.
a. During days 1-5, there are low levels of estrogen and progesterone in the body, causing menstruation.
1) Menstruation is periodic shedding of tissue and blood from endometrium; lining disintegrates and blood vessels rupture.
2) The flow of blood and tissues, known as menses, passes out the vagina.
b. During days 6-13, increased production of estrogens by an ovarian follicle causes the endometrium to thicken and to become vascular and glandular (the proliferative phase).
c. Ovulation usually occurs on day 14 of the 28-day cycle.
d. During days 15-28, increased production of progesterone by the corpus luteum causes endometrium to double in thickness and uterine glands to mature, producing a thick mucoid secretion (secretory phase).
1) The endometrium is now prepared to receive the developing embryo.
2) If no pregnancy occurs, progesterone and estrogen levels decline and corpus luteum degenerates.
3) With low levels of progesterone, the uterine lining also begins to degenerate.
4) During menstruation, anterior pituitary begins increasing production of FSH; a new follicle begins to mature.
3. The ovarian cycle controls the uterine cycle. (Table 50.3)
F. Events Following Fertilization
1. If fertilization occurs, the embryo begins development while it travels down the oviduct to the uterus.
2. The embryo becomes embedded in the endometrium several days following fertilization.
3. The placenta develops from both maternal and embryonic tissues.
a. The placenta functions to exchange gases and nutrients between the fetal and maternal circulation.
b. There is normally no mixing of the blood itself.
4. Initially, the placenta produces human chorionic gonadotropin (HCG) which maintains the corpus luteum.
5. The corpus luteum is maintained by HCG until placenta produces its own progesterone and estrogens.
6. Progesterone and estrogens have two effects at this stage:
a. they shut down the anterior pituitary so that no new follicles mature; and
b. they maintain the lining of the uterus so that the corpus luteum is not needed.
7. There is no menstruation during pregnancy.
G. Estrogens and Progesterone Are Female Sex Hormones
1. Estrogens maintains the secondary sex characteristics of females.
2. There is less body and facial hair; more fat accumulates beneath the skin, providing a more rounded appearance.
3. The pelvic girdle enlarges and the pelvic cavity is larger; therefore, women have wider hips.
4. Both estrogen and progesterone are required for breast development.
H. Breasts Produce Milk
1. The female breast contains one or two dozen lobules, each with a mammary duct. (Fig. 50.12)
2. This mammary duct begins at the nipple and divides into numerous other ducts, which end in the alveoli, which are blind sacs lined by milk-producing cells in the lactating breast.
3. In the nonlactating breast, the ducts far outnumber the alveoli.
4. The hormone prolactin is needed for lactation (milk production) to begin.
5. Production of prolactin is suppressed by the feedback inhibition that estrogens and progesterone have on the anterior pituitary during pregnancy.
6. It takes a couple of days after delivery of a baby for milk production to begin.
7. Breasts produce a watery, yellowish white fluid (colostrum) similar to milk but containing more protein, less fat and is rich in Ig A antibodies that provide some immunities to the newborn.