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Animal Reproduction Overview: Importance of Sexual and Asexual Reproduction

Explore the mechanisms of sexual and asexual reproduction in animals, from gamete production to fertilization and survival of offspring. Understand the complexities of reproductive cycles and adaptations. Discover the reproductive anatomy of various animal species.

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Animal Reproduction Overview: Importance of Sexual and Asexual Reproduction

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  1. Chapter 46 Animal Reproduction

  2. Overview: Doubling Up for Sexual Reproduction • The two earthworms in this picture are mating • Each worm produces sperm and eggs; in a few weeks, new worms will hatch from fertilized eggs • Aspects of animal form and function can be viewed broadly as adaptations contributing to reproductive success

  3. Concept 46.1: Both asexual and sexual reproduction occur in the animal kingdom • Asexual reproduction is creation of offspring whose genes all come from one parent • Sexual reproduction is creation of offspring by fusion of male gametes (sperm) and female gametes (eggs) to form zygotes

  4. Mechanisms of Asexual Reproduction • Many invertebrates reproduce asexually by fission, separation of a parent into two or more individuals of about the same size Video: Hydra Budding

  5. In budding, two new individuals arise from outgrowths of existing ones • Fragmentation is breaking of the body into pieces, some or all of which develop into adults • Fragmentation must be accompanied by regeneration, regrowth of lost body parts

  6. Reproductive Cycles and Patterns • Most animals exhibit reproductive cycles related to changing seasons • Reproductive cycles are controlled by hormones and environmental cues • Animals may reproduce asexually or sexually, or they may alternate these methods

  7. Parthenogenesis is a process in which an egg develops without being fertilized • Several genera of fishes, amphibians, and lizards reproduce only by a complex form of parthenogenesis

  8. LE 46-3 Ovary size Ovulation Ovulation Estrogen Progesterone Hormone level Time Behavior Female- like Male- like Female- like Male- like

  9. Sexual reproduction is a special problem for organisms that seldom encounter a mate • One solution is hermaphroditism, in which each individual has male and female reproductive systems • In sequential hermaphroditism, an individual reverses its sex during its lifetime

  10. Concept 46.2: Fertilization depends on mechanisms that help sperm meet eggs of the same species • The mechanisms of fertilization, the union of egg and sperm, play an important part in sexual reproduction • In external fertilization, eggs shed by the female are fertilized by sperm in the external environment Video: Hydra Releasing Sperm

  11. LE 46-5 Eggs

  12. In internal fertilization, sperm are deposited in or near the female reproductive tract, and fertilization occurs within the tract • Fertilization requires critical timing, often mediated by environmental cues, pheromones, and/or courtship behavior • Internal fertilization requires behavioral interactions and compatible copulatory organs

  13. Ensuring the Survival of Offspring • All species produce more offspring than the environment can handle, but the proportion that survives is quite small • The embryos of many terrestrial animals develop in eggs that can withstand harsh environments • Many animals retain the embryo, which develops inside the female • In many animals, parental care helps ensure survival of offspring

  14. Gamete Production and Delivery • To reproduce sexually, animals must have systems that produce gametes • The least complex systems do not contain distinct gonads, organs that produce gametes • The most complex systems contain many sets of accessory tubes and glands that carry, nourish, and protect gametes and developing embryos • Many animals with relatively simple body plans have highly complex reproductive systems

  15. LE 46-7 Genital pore (Digestive tract) Female organs: Male organs: Uterus Seminal vesicle Yolk gland Yolk duct Sperm duct (vas deferens) Oviduct Ovary Seminal receptacle Vas efferens Testis (Excretory pore)

  16. Most insects have separate sexes with complex reproductive systems

  17. LE 46-8 Accessory gland Ovary Ejaculatory duct Testis Oviduct Spermatheca Penis Vas deferens Vagina Seminal vesicle Accessory gland Male honeybee (drone) Female honeybee (queen)

  18. Concept 46.3: Reproductive organs produce and transport gametes: focus on humans

  19. Female Reproductive Anatomy • The female external reproductive structures include the clitoris and two sets of labia • The internal organs are a pair of gonads and a system of ducts and chambers that carry gametes and house the embryo and fetus Animation: Female Reproductive Anatomy

  20. LE 46-9a Oviduct Ovary Uterus (Urinary bladder) (Pubic bone) (Rectum) Cervix Urethra Shaft Vagina Clitoris Glans Bartholin’s gland Prepuce Labia minora Labia majora Vaginal opening

  21. LE 46-9b Oviduct Ovaries Follicles Corpus luteum Uterine wall Uterus Endometrium Cervix Vagina

  22. Ovaries • The female gonads, the ovaries, lie in the abdominal cavity • Each ovary is enclosed in a tough protective capsule and contains many follicles • A follicle consists of one egg surrounded by one or more layers of follicle cells

  23. Ovulation expels an egg cell from the follicle • The remaining follicular tissue grows within the ovary, forming a mass called the corpus luteum • The corpus luteum secretes hormones, depending on whether pregnancy occurs

  24. Oviducts and Uterus • The egg cell is released into the abdominal cavity near the opening of the oviduct, or fallopian tube • Cilia in the tube convey the egg to the uterus

  25. Vagina and Vulva • The vagina is a thin-walled chamber that is the repository for sperm during copulation and serves as the birth canal • The vagina opens to the outside at the vulva, which includes the hymen, vestibule, labia minora, labia majora, and clitoris

  26. Mammary Glands • The mammary glands are not part of the reproductive system but are important to mammalian reproduction • Within the glands, small sacs of epithelial tissue secrete milk

  27. Male Reproductive Anatomy • In most mammals, the male’s external reproductive organs are the scrotum and penis • Internal organs are the gonads, which produce sperm and hormones, and accessory glands Animation: Male Reproductive Anatomy

  28. LE 46-10b (Urinary bladder) Seminal vesicle (Rectum) (Public bone) Vas deferens Erectile tissue of penis Ejaculatory duct Prostate gland Urethra Bulbourethral gland Vas deferens Glans penis Epididymis Testis Prepuce Scrotum

  29. LE 46-10a Seminal vesicle (behind bladder) (Urinary bladder) Prostate gland Bulbourethral gland Urethra Erectile tissue of penis Scrotum Vas deferens Epididymis Glans penis Testis

  30. Testes • The male gonads, or testes, consist of highly coiled tubes surrounded by connective tissue • Sperm form in these seminiferous tubules • Production of normal sperm cannot occur at the body temperatures of most mammals • The testes of many mammals are held outside the abdominal cavity in the scrotum, where the temperature is lower than in the abdominal cavity

  31. Ducts • From the seminiferous tubules of a testis, sperm pass into the coiled tubules of the epididymis • During ejaculation, sperm are propelled through the muscular vas deferens and the ejaculatory duct, and then exit the penis through the urethra

  32. Glands • Three sets of accessory glands add secretions to the semen, the fluid that is ejaculated • The two seminal vesicles contribute about 60% of the total volume of semen

  33. The prostate gland secretes its products directly into the urethra through several small ducts • The bulbourethral gland secretes a clear mucus before ejaculation that neutralizes acidic urine remaining in the urethra

  34. Semen in the Female Reproductive Tract • Within the female reproductive tract, a number of processes, including contractions of the uterus, help move the sperm up the uterus

  35. Penis • The human penis is composed of three cylinders of spongy erectile tissue • During sexual arousal, the erectile tissue fills with blood from the arteries, causing an erection

  36. Human Sexual Response • Two reactions predominate in both sexes: • Vasocongestion, the filling of tissue with blood • Myotonia, increased muscle tension • The sexual response cycle has four phases: excitement, plateau, orgasm, and resolution

  37. Concept 46.4: In humans and other mammals, a complex interplay of hormones regulates gametogenesis • The process of gametogenesis is based on meiosis, but differs in females and males • Oogenesis is development of mature ova • Spermatogenesis is production of mature sperm

  38. LE 46-11 Ovary Primary germ cell in embryo Differentiation Oogonium in ovary Oogonium Mitotic division Primary oocyte within follicle Primary oocyte Completion of meiosis I and onset of meiosis II Growing follicle Secondary oocyte First polar body Ovulation Mature follicle Entry of sperm triggers completion of meiosis II Ruptured follicle Second polar body Ovum Ovulated secondary oocyte Corpus luteum Degen- erating corpus luteum

  39. LE 46-12 Epididymis Seminiferous tubule Testis Cross section of seminiferous tubule Spermatogonium Sertoli cell nucleus Mitotic division, producing large numbers of spermatogonia Differentiation and onset of meiosis I Primary spermatocyte (in prophase of meiosis I) Meiosis I completed Secondary spermatocyte Meiosis II Lumen of Seminiferous tubule Early spermatids Spermatids (at two stages of differentiation) Differentiation Neck Sperm cells Tail Midpiece Head Plasma membrane Acrosome Nucleus Mitochondria

  40. Oogenesis differs from spermatogenesis in three major ways: • During meiotic divisions, cytokinesis is unequal, with almost all of the cytoplasm in one daughter cell, the secondary oocyte • Unlike sperm, eggs are not produced continuously throughout life • Oogenesis has long “resting” periods, whereas sperm are produced in uninterrupted sequence

  41. The Reproductive Cycles of Females • In females, the secretion of hormones and the reproductive events they regulate are cyclic

  42. Menstrual Versus Estrous Cycles • Two different types of cycles occur in females • Humans and other primates have menstrual cycles; other mammals have estrous cycles • In both cases, ovulation occurs after the endometrium has started to thicken in preparation for implantation

  43. In menstrual cycles: • The endometrium is shed from the uterus in a bleeding called menstruation • Sexual receptivity is not limited to a timeframe • In estrous cycles: • The endometrium is reabsorbed by the uterus • Sexual receptivity is limited to a “heat” period

  44. The Human Female Reproductive Cycle: A Closer Look • The female reproductive cycle is one integrated cycle involving two organs: the uterus and ovaries • Secretion of GnRH from the hypothalamus and of FSH and LH from the anterior pituitary orchestrates the female reproductive cycle • Five kinds of hormones participate in an elaborate scheme involving positive and negative feedback Animation: Ovulation Animation: Post Ovulation

  45. LE 46-13 Control by hypothalamus Inhibited by combination of estrogen and progesterone Hypothalamus Stimulated by high levels of estrogen GnRH Anterior pituitary Inhibited by low levels of estrogen FSH LH Pituitary gonadotropins in blood LH FSH FSH and LH stimulate follicle to grow LH surge triggers ovulation Ovarian cycle Degenerating corpus luteum Corpus luteum Growing follicle Mature follicle Follicular phase Ovulation Luteal phase Estrogen secreted by growing follicle in increasing amounts Progesterone and estrogen secreted by corpus luteum Ovarian hormones in blood Peak causes LH surge Progesterone Estrogen Estrogen level very low Progesterone and estro- gen promote thickening of endometrium Uterine (menstrual) cycle Endometrium Menstrual flow phase Proliferative phase Secretory phase Days 0 5 10 20 25 14 15 28

  46. The Ovarian Cycle • In the ovarian cycle, hormones stimulate follicle growth, which results in ovulation • Following ovulation, the follicular tissue left behind transforms into the corpus luteum

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