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Chapter 46

Chapter 46. Animal Reproduction. Asexual and Sexual Reproduction. Asexual reproduction is the creation of new individuals Whose genes all come from one parent Sexual reproduction is the creation of offspring By the fusion of a male gamete (sperm) and a female gamete (egg) to form a zygote

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Chapter 46

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  1. Chapter 46 Animal Reproduction

  2. Asexual and Sexual Reproduction • Asexual reproduction is the creation of new individuals • Whose genes all come from one parent • Sexual reproduction is the creation of offspring • By the fusion of a male gamete (sperm) and a female gamete (egg) to form a zygote • Animals may reproduce exclusively asexually or sexually or they may alternate between the two

  3. Mechanisms of Asexual Reproduction • Many invertebrates reproduce asexually by fission • The separation of a parent into two or more individuals of approximately the same size

  4. Asexual Reproduction • Budding • In which new individuals arise from outgrowths of existing ones • Fragmentation • Is the breaking of the body into several pieces, some or all of which develop into complete adults • Must be accompanied by regeneration, the regrowth of lost body parts Hydra Planaria

  5. Reproductive Cycles and Patterns

  6. Reproductive Cycles and Patterns • Most animals exhibit cycles in reproductive activity • Often related to changing seasons • Reproductive cycles • Are controlled by hormones and environmental cues

  7. Parthenogenesis • Some animals reproduce by parthenogenesis • A process in which an egg develops without being fertilized • The resulting organism is haploid and may produce haploid eggs without meiosis • Male honeybees are produced by parthogenesis

  8. (a) Both lizards in this photograph are C. uniparensfemales. The one on top is playing the role of a male. Every two or three weeks during the breeding season, individuals switch sex roles. Ovarysize Ovulation Ovulation Progesterone Estrogen Hormones (b) The sexual behavior of C. uniparens is correlated with the cycle of ovulation mediated by sex hormones. As blood levels of estrogen rise, the ovaries grow, and the lizard behaves like a female. After ovulation, the estrogen level drops abruptly, and the progesterone level rises; these hormone levels correlate with male behavior. Time Behavior Female-like Male-like Female-like Male-like Parthenogenesis • Among vertebrates, several genera of fishes, amphibians, and lizards, including whiptail lizards (photo) reproduce exclusively parthenogenesis Ovulation is stimulated by mating behavior in this all female species. These lizards having evolved from species with 2 sexes still need certain sexual stimuli for maximum reproductive success There is a doubling of chromosomes after meiosis to create a diploid “zygote”

  9. Hermaphroditism • Sexual reproduction presents a special problem for certain organisms that seldom encounter a mate • One solution to the problem is hermaphroditism • In which each individual has both male and female reproductive systems • Some hermaphrodites can mate with themselves, but most must find a partner (as with mating earthworms where both donate and receive sperm)

  10. Hermaphroditism • Sequential hermaphroditism: where an individual reverses its sex during its lifetime • Some species are male-first, other species are female-first male female Caribbean bluehead wrasse. All members of this species are born female, but the oldest, largest fish complete their life as males female

  11. Mechanisms of Fertilization

  12. Eggs External Fertilization • Some species have external fertilization, in which • Eggs shed by the female are fertilized by sperm in the external environment

  13. Internal fertilization • Some species have internal fertilization • Sperm are deposited in or near the female reproductive tract, and fertilization occurs within the tract

  14. In either situation, fertilization requires critical timing • Often mediated by environmental cues, pheromones, and/or courtship behavior • Internal fertilization • Requires important behavioral interactions between male and female animals • Requires compatible copulatory organs

  15. Ensuring the Survival of Offspring • All species produce more offspring than the environment can handle • But the proportion that survives is quite small • The more of your offspring that survive to reproduce …the greater the influence of your genes on the next generation

  16. Ensuring the Survival of Offspring • The embryos of many terrestrial animals develop in shelled eggs that can withstand harsh environments • Many animals retain the embryo, which develops inside the female • Many different types of animals exhibit parental care to ensure survival of offspring Egg brooding in Giant Water Bug

  17. Gamete Production and Delivery • Animals must have systems that produce gametes to reproduce sexually • The least complex reproductive systems • Do not even contain distinct gonads, the organs that produce gametes in most animals • The most complex reproductive systems • Contain many sets of accessory tubes and glands that carry, nourish, and protect the gametes and the developing embryos

  18. 2 2 3 1 1 3 4 Genitalpore (Digestive tract) Female organs: Male organs: Uterus Yolk gland Yolk duct Sperm duct(vas deferens) Seminalvesicle Oviduct Ovary Testis Vas efferens Seminal receptacle (Excretory pore) • Many animals with relatively simple body plans possess highly complex reproductive systems

  19. Vagina 2 5 4 1 1 3 3 Accessorygland Ovary • Most insects have separate sexes with complex reproductive systems Ejaculatoryduct Testis Oviduct Spermatheca Penis Vas deferens Accessorygland Seminalvesicle (a) Male honeybee. Sperm form in the testes, pass through the sperm duct (vas deferens), and are stored in the seminal vesicle. The male ejaculates sperm along with fluidfrom the accessory glands. (Males of somespecies of insects and other arthropods haveappendages called claspers that grasp thefemale during copulation.) (b) Female honeybee. Eggs develop in the ovaries and then pass through the oviducts and into the vagina. A pair of accessory glands (only one is shown)add protective secretions to the eggs in the vagina. After mating, sperm are stored in the spermatheca, a sac connected to the vagina by a short duct.

  20. Human Reproductive System

  21. Uterus (Urinary bladder) Oviduct (Pubic bone) Ovary (Rectum) Cervix Vagina Urethra Shaft Bartholin’s gland Glans Clitoris Prepuce Labia minora Vaginal opening Labia majora Female Reproductive Anatomy

  22. Oviduct Ovaries Follicles Endometrium Uterine wall Uterus Cervix Corpus luteum Vagina Female Reproductive Anatomy

  23. 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 cell surrounded by one or more layers of follicle cells

  24. Ovulation • The process of ovulation • Expels an egg cell from the follicle • The remaining follicular tissue then grows within the ovary • To form a solid mass called the corpus luteum, which secretes hormones, depending on whether or not pregnancy occurs

  25. Oviducts • The egg cell is released into the abdominal cavity near the opening of the oviduct, or fallopian tube • Cilia in the tube help draw the ovulated egg into the tube • Cilia also convey the egg to the uterus • Fertilization usually occurs in the oviduct • The oviduct is the site of ectopic (tubal) pregnancies

  26. Oviduct Ovaries Follicles Endometrium Uterine wall Uterus Cervix Corpus luteum Vagina Uterus • A thick muscular organ • Lining is the endometrium • Cervix is the neck of the uterus that opens into the vagina

  27. Vagina and Vulva • The vagina is a thin-walled chamber • That is the repository for sperm during copulation • Serves as the birth canal through which a baby is born • The vagina opens to the outside at the vulva • Which includes the hymen, vestibule, labia minora, labia majora, and clitoris

  28. Mammary Glands • Not part of the reproductive system but are obviously important to mammalian reproduction • Within the glands small sacs of epithelial tissue produce and secrete milk • High levels of estrogen during pregnancy stimulate growth and development of mammary glands

  29. Male Reproductive Anatomy • External reproductive organs in most mammalian species are the scrotum and penis • The internal organs • gonads, which produce sperm and hormones • accessory glands

  30. (Urinary bladder) Seminal vesicle (Rectum) (Pubic bone) Erectile tissue of penis Vas deferens Ejaculatory duct Prostate gland Urethra Bulbourethral gland Glans penis Vas deferens Epididymis Testis Prepuce Scrotum Male Reproductive Anatomy

  31. (Urinarybladder) Seminal vesicle (behind bladder) Prostate gland Bulbourethral gland Urethra Erectile tissue of penis Scrotum Vas deferens Epididymis Glans penis Testis Male Reproductive Anatomy

  32. Testes • The male gonads, or testes • Consist of many highly coiled tubes (seminiferous tubules ) surrounded by several layers of connective tissue • Sperm is formed in the seminiferous tubules • Testosterone is produced in the interstitial cells (cells of Leydig)

  33. Production of normal sperm • Cannot occur at the body temperatures of most mammals • The testes of humans and many mammals • Are held outside the abdominal cavity in the scrotum, where the temperature is lower than in the abdominal cavity • A testicle not in the scrotum is a cryptorchid

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

  35. Glands • Three sets of accessory glands add secretions to the semen, the fluid that is ejaculated • A pair of seminal vesicles contributes about 60% of the total volume of semen • Prostate gland • Secretes its products directly into the urethra through several small ducts • Bulbourethral gland • Secretes a clear mucus before ejaculation that neutralizes acidic urine remaining in the urethra

  36. Semen in the Female Reproductive Tract • Once in the female reproductive tract • A number of processes, including contractions of the uterus, thinning of cervical mucus, and swimming of the spermatozoa, help move the sperm up the uterus

  37. Penis • The human penis • Is composed of three cylinders of spongy erectile tissue (distensible blood spaces) • During sexual arousal • The erectile tissue fills with blood from the arteries, causing an erection • Viagra works by increasing blood flow to the genital region

  38. Human Sexual Response • Two types of physiological reactions predominate in both sexes • Vasocongestion, the filling of tissue with blood • Myotonia, increased muscle tension • The sexual response cycle can be divided into four phases • Excitement, plateau, orgasm, and resolution

  39. Gametogenesis • The process of gametogenesis • Is based on meiosis, but differs in females and males

  40. Ovary Primary germ cell in embryo Differentiation Oogonium in ovary Oogonium 2n Mitotic division Primary oocyte within follicle Primary oocyte, arrested in prophase of meiosis I (present at birth) 2n Growing follicle Completion of meiosis I and onset of meiosis II n First polar body Secondary oocyte, arrested at meta- phase of meiosis II n Ovulation Mature follicle Entry of sperm triggers completion of meiosis II n n Ruptured follicle Ovum Ovulated secondary oocyte Corpus luteum Degenerating corpus luteum Oogenesis • Oogenesis is the development of mature ova

  41. Spermatogenesis • The production of mature sperm Epididymis Seminiferous tubule Testis Cross section of seminiferous tubule Spermatogonium 2n Mitotic division, producing large numbers of spermatogonia Sertoli cell nucleus Differentiation andonset of meiosis I Primary spermatocyte (in prophase of meiosis I) 2n Meiosis I completed n n Secondary spermatocyte Meiosis II Lumen of Seminiferous tubule Early spermatids Spermatids (at two stages of differentiation) n n n n Differentiation (Sertoli cells provide nutrients) Sperm cells Neck n n n n Head Midpiece Tail Plasma membrane Mitochondria Nucleus Acrosome

  42. Oogenesis differs from spermatogenesis • First • during the meiotic divisions of oogenesis cytokinesis is unequal, with almost all the cytoplasm monopolized by a single daughter cell, the secondary oocyte • Second, • sperm are produced continuously throughout a male’s life which is not the case in oogenesis • Third, • oogenesis has long “resting” periods, while spermatogenesis produces sperm in uninterrupted sequence

  43. The Reproductive Cycles of Females • The secretion of hormones and the reproductive events they regulate are cyclic

  44. 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 at a time in the cycle after the endometrium has started to thicken in preparation for implantation

  45. Menstrual Versus Estrous Cycles • In menstrual cycles • The endometrium is shed from the uterus in a bleeding called menstruation • Sexual receptivity is not limited to a specific timeframe • In estrous cycles • The endometrium is reabsorbed by the uterus • Sexual receptivity is limited to a “heat” period • In estrous animals that bleed (dogs & cats), it is due to high estrogen that accompanies receptive period

  46. The Human Female Reproductive Cycle • The female reproductive cycle • Is one integrated cycle involving two organs, the uterus and ovaries • Cyclic secretion of GnRH from the hypothalamus • And of FSH and LH from the anterior pituitary orchestrates the female reproductive cycle

  47. Control by hypothalamus Inhibited by combination of estrogen and progesterone Hypothalamus Stimulated by high levels of estrogen GnRH (a) Anterior pituitary Inhibited by low levels of estrogen LH FSH 1 Pituitary gonadotropins in blood 2 (b) 6 LH FSH FSH and LH stimulate follicle to grow LH surge triggers ovulation Ovarian cycle 3 (c) 7 8 Corpus luteum Degenerating corpus luteum Growing follicle Mature follicle Ovulation Luteal phase Follicular phase Progesterone and estrogen secreted by corpus luteum Estrogen secreted by growing follicle in increasing amounts Peak causes LH surge 4 Ovarian hormones in blood (d) 5 Progesterone Estrogen 10 Progesterone and estro- gen promote thickening of endometrium Estrogen level very low 9 Uterine (menstrual) cycle (e) • Endometrium Secretory phase Menstrual flow phase Proliferative phase 25 28 5 14 15 0 20 10 Days Reproductive cycle of the human female Refer back to this figure to integrate your thinking of ovarian and uterine events

  48. The Ovarian Cycle • The female has a set of primary follicles containing primary oocytes that have become arrested in early meiosis • With each cycle a small # of primary follicles are stimulated by FSH to grow and mature (generally one follicle completes maturation) • In the Follicular Phase of the ovarian cycle • FSH stimulates follicle growth • The growing follicle secretes ever increasing amounts of estrogen (which ultimately leads to the LH surge)

  49. Ovulation • A surge in LH levels results in ovulation

  50. The Ovarian Cycle • In the Luteal Phase of the ovarian cycle • The follicular tissue left behind transforms into the corpus luteum • The corpus luteum secretes progesterone and estrogen • Over time the corpus luteum regresses and sex hormone production declines. This leads to menstruation

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