Animal Reproduction Chapter 40

Asexual Reproduction • Asexual reproduction does not involve the fusion of sperm and egg • There are three major forms of asexual reproduction – Budding – Fission – Parthenogenesis

Asexual Reproduction • Budding – A miniature version of adult grows off of the parent’s body – Example: hydras and some sea anenomes

Asexual Reproduction • Fission – When an animal divides in two – Each piece grows into an adult by regeneration – Example: some flatworms and annelids

Asexual Reproduction • Parthenogenesis – When a female’s haploid egg develops into adult without fertilization – Example: whiptail lizards and aphids

Sexual Reproduction • Sexual reproduction requires the union of sperm and egg • This union is variously accomplished, depending on – Mobility – Aquatic vs. terrestrial breeding grounds

• Haploid gametes fuse to produce diploid offspring – Males produce small motile sperm – Females produce large non-motile eggs – In some species, hermaphrodites exist • Can produce both sperm and eggs • Example: earthworms

External Fertilization • External fertilization: release of sperm and eggs into the water, where fertilization occurs (spawning) • External fertilization – Mates synchronize their reproductive behaviors temporally and spatially – Reproductive behaviors can be reliant on • Pheromone release • Courtship rituals • Environmental cues (example: coral spawning)

Internal Fertilization • Internal fertilization: transmission of male gametes into the female – Occurs by copulation, spermatophore use

• Copulation: male directly deposits sperm • Spermatophore use: a sperm packet that is picked up by the female, or is deposited in female by the male • In most mammals, mating coincides with ovulation

Internal Fertilization • Competition may occur between animals that copulate to reproduce • Example: hundreds of male red-sided garter snakes compete to copulate with a single female, but only one succeeds

The Human Reproductive System • Humans have separate sexes, reproduce sexually, and make sex cells using paired gonads – Males produce sperm continuously – Females release an egg cell (ovulate) once per month

Puberty • The ability to reproduce begins at puberty • Puberty – Rapid growth and appearance of secondary sexual characteristics at the onset of sexual maturity – Occurs between 8 and 15 years of age

• As the result of brain maturation – The hypothalamus stimulates GnRH release • GnRH stimulates LH and FSH release from the pituitary gland • LH and FSH stimulate gamete, testosterone, and estrogen production

Puberty • In males – The penis and testes enlarge – Secondary sexual characteristics develop

• In females – Menstruation and breast development begin – Secondary sexual characteristics develop

The Testes • Includes the testes and accessory structures • Testes: male gonads – Located in scrotum: keeps temperatures optimal (cooler) for sperm production – Produce male gametes (sperm) in seminiferous tubules via spermatogenesis

The Testes • The testes include three major cell types – Interstitial cells: located between seminiferous tubules; produce testosterone – Sertoli cells: in seminiferous tubule walls; nourish developing sperm – Spermatogonia: produce sperm in seminiferous tubules via spermatogenesis

Spermatogenesis • Diploid spermatogonia divide by mitosis • Produce primary spermatocytes – Are diploid; undergo meiosis I

• Produce secondary spermatocytes – Are haploid; undergo meiosis II

• Produce spermatids – Are haploid; differentiate

• Form sperm – Functional haploid gametes

Sperm Anatomy • A sperm cell has four parts • Head – Contains nucleus (haploid set of chromosomes)

• Acrosome – A lysozyme containing enzymes to penetrate the egg

• Midpiece – Contains mitochondria; produce energy for sperm movement

• Tail – Flagellum generates swimming movements

Hormonal Control • Spermatogenesis is regulated by hormones • The hypothalamus secretes GnRH – Stimulates the anterior pituitary to secrete LH and FSH – LH stimulates interstitial cells in the testes to produce testosterone – FSH (with testosterone) stimulates Sertoli cells and spermatogonia to initiate spermatogenesis – Elevated testosterone levels and inhibin production inhibit GnRH, LH, and FSH production

Male Accessory Structures • Produce semen (sperm with glandular secretions) and conduct sperm to the outside of the body • Epididymis sperm storage • Vas deferens tube passage from testes to urethra • Glands: produce secretions to aid sperm

– Prostate gland: produces alkaline fluids; neutralize vaginal acidity – Bulbourethral glands: secretions provide lubrication for intercourse – Seminal vesicles: release fructose (sperm energy source)

Male Accessory Structures • Urethra: conducts semen from vas deferens to tip of penis • Penis: deposits semen in female reproductive tract

The Ovaries • Ovaries: female gonads – Produce eggs (oocytes) by oogenesis – Contain follicles: each composed of supportive cells that house an oocyte; found throughout each ovary

Oogenesis • Within the ovary, a diploid oogonium divides by mitosis – Produces a primary oocyte: is diploid; undergoes meiosis I – Produces a haploid secondary oocyte and a polar body (contains discarded chromosomes): oocyte undergoes meiosis II after fertilization – Produces a haploid egg and polar body

Follicle Development • Each developing oocyte is surrounded by a layer of cells that provide nourishment and secrete female sex hormones • Together, the oocyte and these accessory cells make up a follicle • A mature ovarian follicle develops every 28 days – Occurs in response to LH and FSH secretion – Secretes estrogen which stimulates uterine development

Ovulation • A single secondary oocyte ruptures from the follicle and leaves the ovary during ovulation – The secondary oocyte enters the uterine tube – The ruptured follicle turns into the corpus luteum • Secretes estrogen and progesterone for continued uterine development

Female Accessory Structures • Uterine tubes – Fimbriae sweep up ovulated egg – Site of fertilization – Beating cilia sweep zygote toward uterus

• Uterus – Muscular site of fertilized egg (zygote) implantation and embryo development – Has two functional tissue layers • Endometrium: inner uterine lining; supplied with blood vessels; form the mother’s contribution to the placenta • Myometrium: thick smooth muscle layer; contracts during childbirth

Female Accessory Structures • Cervix – Muscular opening to uterus

• Vagina – Receptacle for penis

The Menstrual Cycle • A 28-day cycle during which – An oocyte matures – An egg is released from the ovary – The endometrium of the uterus develops to receive a zygote for implantation

• The menstrual cycle is under hormonal control

The Menstrual Cycle • GnRH (from hypothalamus) stimulates FSH and LH secretion from the anterior pituitary gland • FSH and LH stimulate follicular development – Developing follicle produces estrogen

• FSH, LH, and estrogen stimulate continued follicle growth

The Menstrual Cycle • As follicle grows, estrogen production increases – Stimulates the production of an LH and FSH surge at day 14 in menstrual cycle

• LH stimulates final growth of follicle, resulting in ovulation at day 14 – Egg released from follicle and enters uterine tube – Corpus luteum remains, produces estrogen and high levels of progesterone

The Menstrual Cycle • Estrogen and progesterone production inhibits FSH and LH production – Inhibits follicle development – Stimulates continued endometrial thickening

The Menstrual Cycle • If no pregnancy, the corpus luteum starts to degenerate 1 week after ovulation – Estrogen and progesterone production declines – Endometrium is shed during menstruation (day 28) – Low estrogen and progesterone levels result in GnRH release from hypothalamus and LH and FSH production from anterior pituitary

The Menstrual Cycle • If pregnancy occurs, developing embryo secretes chorionic gonadotrophin (CG) into female bloodstream • CG maintains corpus luteum – High estrogen and progesterone levels maintain thick endometrium and inhibit ovulation

Fertilization • During fertilization, the sperm and egg nuclei unite • Typically occurs in the uterine tubes • Hundreds of sperm reach egg – Collective acrosome enzyme activity allows one sperm to burrow through corona radiata and zona pellucida of egg – Burrowing sperm stimulates meiosis II in egg and sperm/egg nuclei fuse

Permanent Contraception • Permanent contraception can be achieved through sterilization • Sterilization: when gamete pathways are interrupted – Vasectomy: each vas deferens severed – Tubal ligation: each uterine tube severed

Temporary Contraception • There are several general approaches to temporary contraception • Preventing ovulation: if no egg is present, fertilization will not occur – Birth control pills: high doses of estrogen/progesterone inhibit LH and FSH release; ovulation won’t occur

Temporary Contraception • Barrier methods: prevent sperm and egg from meeting – Condoms and diaphragm: barrier interrupts sperm travel – Spermicides: chemicals kill sperm before fertilization – Rhythm method: no intercourse during days when egg could be fertilized; unreliable

Temporary Contraception • Preventing implantation in the uterus – Examples • Intrauterine device (IUD): plastic/metal device that prevents embryo implantation • Morning after pill: high doses of estrogen and progesterone prevent implantation