Animal Reproduction

1. 32 Animal Reproduction

2. Chapter 32 Animal Reproduction • Key Concepts • 32.1 Reproduction Can Be Sexual or Asexual • 32.2 Gametogenesis Produces Haploid Gametes • 32.3 Fertilization Is the Union of Sperm and Ovum

3. Chapter 32 Animal Reproduction • Key Concepts • 32.4 Human Reproduction Is Hormonally Controlled • 32.5 Humans Use a Variety of Methods to Control Fertility

4. Chapter 32 Opening Question How does “the pill” prevent contraception?

5. Concept 32.1 Reproduction Can Be Sexual or Asexual • Asexual reproduction requires no mating but does not result in genetic diversity. • Asexually reproducing species are mostly invertebrates, sessile, and live in constant environments. • Three types of asexual reproduction are budding, regeneration, and parthenogenesis.

6. Concept 32.1 Reproduction Can Be Sexual or Asexual • Budding produces new individuals that form from the bodies of older animals. • A bud grows by mitotic cell division—cells differentiate before the bud breaks away. • The bud is genetically identical to the parent. • Regeneration can replace damaged tissue or form a complete individual. • Example: Echinoderms.

7. Figure 32.1 Three Forms of Asexual Reproduction (Part 1)

8. Figure 32.1 Three Forms of Asexual Reproduction (Part 2)

9. Concept 32.1 Reproduction Can Be Sexual or Asexual • Parthenogenesis is the development of offspring from unfertilized eggs. • Parthenogenesis may determine the sex of the offspring. • In some species, females can act as males depending on cyclic states of estrogen and progesterone.

10. Figure 32.1 Three Forms of Asexual Reproduction (Part 3)

11. Concept 32.1 Reproduction Can Be Sexual or Asexual • In sexual reproduction two haploid cells, gametes, form a diploid individual. • Three fundamental steps of sexual reproduction: • Gametogenesis—making haploid gametes • Spawning or mating—getting gametes together • Fertilization—fusing gametes to form a diploid

12. Concept 32.1 Reproduction Can Be Sexual or Asexual • Sexual reproduction has a big advantage— the generation of genetic diversity. • Meiosis allows genetic diversity through crossing over between homologous chromosomes and independent assortment.

13. Concept 32.2 Gametogenesis Produces Haploid Gametes • Gametogenesis occurs in the gonads—the testes in males and ovaries in females • Male gametes,the sperm, move by flagella. • The larger gametes of females are the ova,or eggs, and are nonmotile. • Gametes are produced from germ cells—present early in development and distinct from other (somatic) cells of the body.

14. Concept 32.2 Gametogenesis Produces Haploid Gametes • Germ cells migrate to the gonads when they begin to form. • Embryonic germ cells divide by mitosis to form diploid spermatogonia in males and oogonia in females. • These multiply by mitosis, producing primary spermatocytes and primary oocytes—these enter meiosis and produce haploid gametes, sperm and ova.

15. Concept 32.2 Gametogenesis Produces Haploid Gametes • The production of sperm is spermatogenesis—of ova is oogenesis. • The first meiotic division of a primary spermatocyte results in two secondary spermatocytes. • The second division produces four haploid spermatids, which will each become a sperm.

16. Figure 32.2 Gametogenesis (Part 1)

17. Concept 32.2 Gametogenesis Produces Haploid Gametes • Oogenesis: • A primary oocyte immediately begins prophase I of meiosis. Here, development stops in many species. • The primary oocyte grows larger and acquires nutrients. • When meiosis resumes, the nucleus of the oocyte divides into two daughter cells of unequal sizes. • The cell with more cytoplasm is the secondary oocyte—the smaller one is the firstpolar body.

18. Concept 32.2 Gametogenesis Produces Haploid Gametes • The second meiotic division forms daughter cells of unequal sizes. • One is a large ootid, which differentiates to become a mature ovum. • The other forms the second polar body.

19. Figure 32.2 Gametogenesis (Part 2)

20. Concept 32.2 Gametogenesis Produces Haploid Gametes • Few primary oocytes complete all meiotic stages—females produce far fewer gametes than males. • The average woman has about 450 menstrual cycles and releases on ovum each time, until menopause—the end of fertility. • A man produces over 100 million sperm per day.

21. Concept 32.2 Gametogenesis Produces Haploid Gametes • In hermaphroditic species such as earthworms, a single individual may produce sperm and ova simultaneously. • An anemone fish produces sperm and ova sequentially and may function as a male or a female at different times.

22. Concept 32.3 Fertilization Is the Union of Sperm and Ovum • Fertilization is the union of a haploid sperm and a haploid egg. • It creates a single diploid cell, called a zygote, which will develop into an embryo. • Fertilization involves a complex series of events.

23. Concept 32.3 Fertilization Is the Union of Sperm and Ovum • Steps in fertilization: • Recognition and binding of sperm and ovum • Activation of sperm • Plasma membranes fuse • Additional sperm entry blocked • Activation of ovum • Ovum and sperm nuclei fuse

24. Concept 32.3 Fertilization Is the Union of Sperm and Ovum • Aquatic animals bring gametes together through spawning—gamete release before external fertilization occurs. • Internal fertilization occurs when sperm is released directly into the female reproductive tract. • Internal fertilization requires accessory sex organs, such as penis and vagina. • Copulation is the joining of the male and female accessory organs.

25. Concept 32.3 Fertilization Is the Union of Sperm and Ovum • Species-specific sperm and ovum interactions are controlled by specific recognition molecules. • Ova of aquatic species release chemical attractants to cause sperm to swim toward the ovum. • Sperm must go through two protective layers to reach an ovum—a jelly coat and the vitelline envelope.

26. Figure 32.3 Fertilization of Sea Urchin Egg (Part 1)

27. Concept 32.3 Fertilization Is the Union of Sperm and Ovum • The acrosome is a membrane-enclosed structure on the sperm head. • Egg and sperm contact causes substances in the jelly coat to trigger an acrosomal reaction. • Membranes in the sperm head and acrosome break down; enzymes are released and digest the jelly coat.

28. Figure 32.3 Fertilization of Sea Urchin Egg (Part 2)

29. Concept 32.3 Fertilization Is the Union of Sperm and Ovum • An acrosomal process extends from the head of the sperm. • The acrosomal process is coated with bindin—specific recognition molecules. • Bindin acts on bindin receptors in the vitelline envelope. • Sperm and egg plasma membranes fuse to form a fertilization cone.

30. Concept 32.3 Fertilization Is the Union of Sperm and Ovum • Internal fertilization involves species-specific mating behaviors and ovum-sperm recognition mechanisms. • The ovum is surrounded by the cumulus, cells in a gelatinous matrix. • The zona pellucida, or zona, is a glycoprotein envelope beneath the cumulus.

31. Concept 32.3 Fertilization Is the Union of Sperm and Ovum • When sperm make contact with the zona, species-specific glycoproteins bind to recognition molecules on the sperm. • Binding triggers the acrosomal reaction, and enzymes digest the zona pellucida. • When sperm reaches ovum membrane other proteins facilitate membrane fusion.

32. Figure 32.4 Barriers to Mammalian Sperm

33. Concept 32.3 Fertilization Is the Union of Sperm and Ovum • Fusion and entry of a sperm into the ovum lead to: • Blocks to polyspermy—mechanisms to prevent more than one sperm from entering an ovum

34. Concept 32.3 Fertilization Is the Union of Sperm and Ovum • Fast block to polyspermy: • Transient • Caused by change in membrane potential as sodium ions (Na+) enter plasma membrane of ovum after contact with a sperm

35. Concept 32.3 Fertilization Is the Union of Sperm and Ovum • Slow block to polyspermy: • Initiated by release of calcium ions (Ca2+) • Cortical granules fuse with plasma membrane and release enzymes, dissolving bonds between vitelline envelope and ovum plasma membrane. • H2O is absorbed and the vitelline envelope rises to form a fertilization envelope. • Enzymes remove sperm-binding receptors and cause envelope to harden.

36. Concept 32.3 Fertilization Is the Union of Sperm and Ovum • In mammals sperm entry does not cause membrane depolarization. • Ca2+ influx occurs, and cortical granules fuse with the ovum plasma membrane. • No fertilization envelope forms, but cortical granule enzymes destroy the sperm-binding molecules in the zona pellucida.

37. Concept 32.3 Fertilization Is the Union of Sperm and Ovum • Fertilized ova may be released into the environment or retained: • Oviparity—egg laying: • Oviparous animals lay eggs in the environment, and embryos develop outside the mother’s body. • Viviparity—live bearing: • Viviparous animals retain the embryo in the mother’s body during early development.

38. Concept 32.3 Fertilization Is the Union of Sperm and Ovum • Viviparity differs in mammals as they have a specialized female reproductive tract: • Uterus(or womb)—holds the embryo • Placenta—develops in the uterus and enables exchange of nutrients and waste

39. Concept 32.4 Human Reproduction Is Hormonally Controlled • Sperm are produced in the paired male gonads, or testes. • The testes are located in the scrotum, outside of the body, to maintain optimal temperature for sperm production. • Semen is made up of sperm and other fluids and molecules.

40. Figure 32.5 Reproductive Organs of the Human Male (Part 1)

41. Figure 32.5 Reproductive Organs of the Human Male (Part 2)

42. Concept 32.4 Human Reproduction Is Hormonally Controlled • Spermatogenesis occurs in the seminiferous tubules in each testis. • Between the tubules are Leydig cells, which produce testosterone. • Spermatogonia reside in the outermost regions of the tubules, near Sertoli cells, which provide nutrients.

43. Figure 32.6 Spermatogenesis Takes Place in the Seminiferous Tubules (Part 1)

44. Figure 32.6 Spermatogenesis Takes Place in the Seminiferous Tubules (Part 2)

45. Figure 32.6 Spermatogenesis Takes Place in the Seminiferous Tubules (Part 3)

46. Concept 32.4 Human Reproduction Is Hormonally Controlled • Immature sperm cells are shed in the lumen of the seminiferous tubule. • They move into the epididymis, mature, and become motile. • Sperm travel in the vas deferens which joins with the semen-carrying ejaculatory duct. • This joins the urethra, the common final duct, at the base of the penis and opens to the outside at the tip of the penis.

47. Concept 32.4 Human Reproduction Is Hormonally Controlled • Besides sperm, semen contains seminal fluids—the products of several accessory glands: • The paired seminal vesicles, the prostate gland, and the bulbourethral glands • The prostate gland produces a fluid that is alkaline and reduces acidity in male and female reproductive tracts. • The fluid also contains enzymes to thicken semen and later to dissolve it.

48. Concept 32.4 Human Reproduction Is Hormonally Controlled • The bulbourethral glands produce an alkaline secretion that: • Neutralizes acidity in the urethra • Provides lubrication and facilitates sperm movement during climax • These secretions precede climax yet carry residual sperm capable of fertilization. • Pregnancy can occur even when the penis is withdrawn prior to ejaculation (coitus interruptus).

49. Concept 32.4 Human Reproduction Is Hormonally Controlled • Male copulatory organ is the penis. • The sensitive tip of the penis, the glans penis,is covered by theforeskin—removal of the foreskin is circumcision. • Sexual stimulation triggers the nervous system to produce penile erection. • Nitric oxide (NO) acts on blood vessels by stimulating production of cGMP. • cGMP causes dilation of blood vessels so that spongy erectile tissue fills with blood.

50. Concept 32.4 Human Reproduction Is Hormonally Controlled • At the climax of copulation, semen is ejaculated through the vasa deferentia and urethra. • Ejaculation is accompanied by feelings of intense pleasure called orgasm. • After ejaculation NO release decreases and enzymes break down cGMP—blood vessels are no longer compressed, and erection declines.