Animal Reproduction

1. Animal Reproduction

2. Sexual Reproduction • Sexual reproduction confers the production of genetic diversity. • Sexual reproduction requires the joining of two haploid cells into one, which becomes a diploid individual. • These haploid cells, or gametes, are produced by meiosis. • Two events in meiosis contribute to genetic diversity: crossing over of homologous chromosomes and independent assortment of the chromosomes.

3. Sexual Reproduction • The three fundamental steps of sexual reproduction are: • Gametogenesis (producing sex cells) • Mating (getting sex cells together) • Fertilization (fusion of sex cells) • Gametogenesis and fertilization are fairly similar in different groups of animals. • Mating behaviors, however, show incredible diversity.

4. Sexual Reproduction • The gonads (testes and ovaries) are the sites of gametogenesis. • Male gametes (sperm) move by beating flagella; female gametes (eggs or ova) are nonmotile.

5. Sexual Reproduction • Spermatogenesis involves two meiotic divisions to produce four haploid spermatids. • Through further development, spermatids become compact, streamlined, and motile.

6. Sexual Reproduction • Oogenesis produces eggs through two meiotic divisions. • The primary oocyte enters prophase of the first meiotic division, and then its development is arrested (for as long as 50 years). • When it resumes meiosis, the primary oocyte completes the first division, resulting in two cells of unequal size, the secondaryoocyte and first polar body. • A second period of arrested development occurs at this point. The egg may be expelled from the ovary in this condition.

7. Sexual Reproduction • With the second meiotic division, one daughter cell becomes the mature ovum, with a second polar body produced. • The mature ovum is thus large and well-provisioned

8. Sexual Reproduction • Fertilization is the union of haploid sperm and haploid egg to produce a single diploid cell, the zygote. It involves a complex series of events: • The sperm and egg recognize each other. • The sperm is activated so that it can gain access to the plasma membrane of the egg. • The plasma membranes of the sperm and egg fuse. • The egg blocks entry of additional sperm. • The egg is stimulated to start development. • The egg and sperm nuclei fuse.

9. Sexual Reproduction • The mammalian egg is surrounded by a thick layer called the cumulus. Beneath that is a protein envelope called the zona. • A species-specific glycoprotein in the zona binds to the head of the sperm, triggering a reaction and releasing enzymes that digest a path through the zona.

10. Sexual Reproduction • The first response to fertilization are blocks to polyspermy: If more than one sperm enters the egg, the resulting embryo is unlikely to survive. Human egg about to be fertilized.

11. Sexual Reproduction • External fertilization requires an aquatic habitat in which the gametes are released for transport. • Timing and amount of release are important, as well as synchronization of male and female reproductive activities. • Behavior, such as congregation of potential mates in a suitable environment, may play a vital role in bringing gametes together. Eggs laid by a fish. Whelk laying eggs. Green anemone releasing sperm.

12. Sexual Reproduction

13. Sexual Reproduction • To enable internal fertilization, organisms have many accessory sex organs, such as the penis (a tube for depositing sperm in the female reproductive tract). • The physical joining of the male and female accessory sex organs is called copulation.

14. Sexual Reproduction • Species that have separate and distinct male and female individuals are dioecious (“two houses”). • Species that have male and female reproductive systems in the same body are monoecious, or hermaphroditic. • An example is the earthworm, which is both male and female at the same time. • Some animals are male at one time in the life cycle but female at another. Male and female moths mate on a cocoon. Earthworms copulating; each functions as both male and female.

15. Sexual Reproduction • All mammals have internal fertilization. • Two patterns of embryo care and nurture are present in animals: oviparity (egg bearing) and viviparity (live bearing).

16. Sexual Reproduction Eggs laid by insect on underside of leaf. • Oviparous animals lay eggs stocked with abundant nutrients outside the mother’s body in the environment. • Insects, reptiles, and birds protect the eggs from desiccation with tough, waterproof membranes or shells. • In some oviparous animals, parents protect the eggs, but until hatching, the embryos are entirely dependent on nutrients stored in the egg. Eggs of a robin. Eggs of a turtle.

17. Sexual Reproduction

18. Sexual Reproduction • Viviparous animals retain the embryo within the mother’s body during early embryo development. • Mammals (except monotremes) are viviparous, and there are examples of viviparity in all other vertebrate groups except crocodiles, turtles, and birds. • Viviparous mammals have a uterus that holds the embryo and interacts with it to produce a placenta. Aphid giving birth.

19. The Human Reproductive System • Semen, a mixture of fluids and molecules to support sperm and facilitate fertilization, is the product of the male reproductive system. • Sperm is produced in the testes, which are located in a sac called the scrotum outside the body cavity.

20. The Human Reproductive System • The location of the scrotum ensures that the optimal temperature for spermatogenesis (slightly below body temperature) is maintained – muscles control this distance. • Spermatogenesis takes place in the seminiferous tubules.

21. Sexual Reproduction

22. The Human Reproductive System • The nucleus in what will become the head of the mammalian sperm becomes compact, and the surrounding cytoplasm is lost. • A flagellum develops, and the mitochondria become a condensed midpiece that will provide energy for motility. • A cap, or acrosome, forms over the nucleus in the head and contains digestive enzymes to penetrate the protective layers of the egg.

23. The Human Reproductive System • From the lumen of the tubules, sperm move to the epididymis, a storage sac where they mature. • The epididymis connects to the urethra via the vas deferens. • The urethra is the common duct for urinary and reproductive systems. • The components of semen come from several accessory glands. Human vas deferens Cross section of penis from human infant

24. The Human Reproductive System • Paired seminal vesicles produce about two-thirds of the volume of semen, consisting of mucus, protein, and fructose as an energy force for the sperm. • The prostate gland produces the thin, milky fluid that makes up the rest of the volume of semen. • Prostate fluid makes the uterine environment more hospitable to sperm and converts the semen, by enzyme action, into a gelatinous mass. Human prostate tissue

25. The Human Reproductive System • The penis is a tubular shaft. • A fold of skin called the foreskin covers the skin; the cultural practice of circumcision removes a portion of the foreskin. • Erections occur when the sexually aroused male’s autonomic nervous system causes penis blood vessel dilation. • This swells the spongy, erectile tissue and compresses the blood flow from the penis.

26. The Human Reproductive System • At the climax of copulation, semen is propelled through the vas deferens and urethra in two steps. • During emission, contractions of smooth muscles move semen into the urethra. • During ejaculation, contractions of the muscles at the base of the penis force semen through the urethra and out of the penis.

27. The Human Reproductive System • In females, the mature egg is released into the body cavity and is swept into the end of the oviduct (Fallopian tube), where fertilization takes place. • Cilia propel the egg toward the uterus, a muscular, thick-walled cavity. • The opening at the bottom of the uterus is the cervix, which leads into the vagina. • Sperm are deposited in the vagina, and the fetus must pass • through it during birth.

28. Sexual Reproduction

29. The Human Reproductive System • The external opening of the vagina has two sets of folded skin, the labia majora and labia minora, which also surround the urethra. • At the tip of the labia minora is the clitoris that is highly sensitive to sexual stimulation. • Both the labia minora and clitoris become engorged with blood during sexual stimulation.

30. The Human Reproductive System • To achieve fertilization, sperm swim up the vagina. • The sperm then pass through the cervix and most of the oviduct to the egg (secondary oocyte) in the upper oviduct. • Fertilization stimulates completion of the second meiotic division, after which egg and sperm (both haploid) fuse to produce the diploid zygote. • Still in the oviduct, the zygote divides to become a blastocyst and continues down the oviduct. • In the uterus, the blastocyst attaches to thewall lining called the endometrium. Human fallopian tube Human endometrium

31. The Human Reproductive System • Stimulated by estrogen, the endometrium develops new blood vessels to cradle the blastocyst. • The blastocyst burrows in (implantation), interacting with the wall to form the placenta. • The placenta is the organ of exchange of nutrients and waste products between the embryo’s and the mother’s blood. • If the blastocyst fails to arrive or embed, the endometrium regresses and is sloughed off in the subsequent monthly menstrual period.

32. The Human Reproductive System • The female reproductive cycle actually consists of two linked cycles: an ovarian cycle that produces eggs and hormones and a uterine cycle that prepares the endometrium for the arrival of a blastocyst. • The ovarian cycle repeats about every 28 days. • The end of fertility (menopause) occurs at about age 50. Ovary of 2-year-old girl; follicles fill the ovary at birth.

33. The Human Reproductive System • Follicles (cells surrounding the primary oocyte) nourishes the growing egg with the nutrients and proteins it will use if fertilized. • After two weeks, ovulation occurs, a process in which the follicle ruptures and the egg is released.

34. The Human Reproductive System • After ovulation, the follicle continues to proliferate and forms an endocrine mass, called the corpus luteum, which produces estrogen and progesterone for another two weeks, then degenerates if a blastocyst is not implanted in the uterus. Follicles in the ovary Corpus leuteum from human ovary

35. The Human Reproductive System • The uterine cycle parallels the ovarian cycle and involves the buildup, then breakdown, of the endometrium. • About five days into the ovarian cycle, the endometrium builds in preparation for the blastocyst. • About five days after ovulation, the uterus is maximally prepared and stays that way for another nine days. • If the blastocyst does not arrive by then, the endometrium breaks down and sloughs off during menstruation. • Most other female mammals do not menstruate, but instead resorb the uterine lining.

36. Figure 43.13 The Ovarian and Uterine Cycle (Part 1) Menstruation marks the start of each uterine and ovarian cycle. FSH and LH levels from the pituitary increase and follicles begin maturing to produce estrogen.

37. Figure 43.13 The Ovarian and Uterine Cycle (Part 2)

38. The Human Reproductive System • After implantation in the uterus, a blastocyst begins to secrete human chorionic gonadotropin (hCG), which keeps the corpus luteum functional. • The presence of this hormone is the basis for pregnancy testing. • Tissues derived from the blastocyst also begin to produce estrogen and progesterone. • Continued high levels of estrogen and progesterone prevent the pituitary from secreting gonadotropins; thus, the ovarian cycle ceases for the duration of the pregnancy. • This same mechanism is the basis for birth control pills.

39. Human Sexual Behavior • The only sure methods of preventing conception and pregnancy are complete abstinence from sexual intercourse and surgical removal of gonads. • Various forms of birth control have also been developed. • These birth control methods either block gametogenesis or block embryo development, and vary enormously in effectiveness and acceptability.

40. Human Sexual Behavior • Copulation is one of the most intimate types of contact between two individuals. • It is not surprising that disease parasites use sexual contact between their hosts as their means of transmission. • These organisms cause sexually transmitted diseases (STDs) and include viruses, bacteria, yeasts, and protozoans. • Human STDs have been around since ancient times and are even today serious public health problems.