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Animal Reproduction

Animal Reproduction. 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.

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Animal Reproduction

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  1. Animal Reproduction

  2. 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

  3. 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

  4. 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

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

  6. 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

  7. 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

  8. 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

  9. LE 46-5 Eggs

  10. 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

  11. 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, amniotic eggs • Many animals retain the embryo, which develops inside the female, placentals • In many animals, parental care helps ensure survival of offspring, altricial

  12. 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

  13. Genital pore (Digestive tract) Female organs: Male organs: Uterus LE 46-7LE 46-8 Seminal vesicle Yolk gland Yolk duct Sperm duct (vas deferens) Oviduct Ovary Seminal receptacle Vas efferens Testis (Excretory pore)

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

  15. 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

  16. 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

  17. 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

  18. Vagina • The vagina is a thin-walled chamber that is the repository for sperm during copulation and serves as the birth canal

  19. 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

  20. 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

  21. 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

  22. 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

  23. 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

  24. 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

  25. 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 • Produce fructose for sperm energy

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

  27. 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

  28. 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

  29. 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

  30. The Reproductive Cycles of Females • In females, the secretion of hormones and the reproductive events they regulate are cyclic • FSH • LH • ESTROGEN • PROGESTERONE • GNRH

  31. 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

  32. 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

  33. Female Reproductive Cycle • 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

  34. 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

  35. The Uterine (Menstrual) Cycle • Cycle after cycle, the maturation and release of egg cells from the ovary are integrated with changes in the uterus • If an embryo has not implanted in the endometrium by the end of the secretory phase, a new menstrual flow commences

  36. Menopause • After about 450 cycles, human females undergo menopause, the cessation of ovulation and menstruation

  37. Hormonal Control of the Male Reproductive System • Testosterone and other androgens are directly responsible for the primary and secondary sex characteristics of the male • Androgen secretion and sperm production are controlled by hypothalamic and pituitary hormones Animation: Male Hormones

  38. In placental mammals, an embryo grows into a newborn in the mother’s uterus • Pregnancy, or gestation, is the condition of carrying one or more embryos in the uterus • Duration of pregnancy in other species correlates with body size and maturity of the young at birth

  39. Conception, Embryonic Development, and Birth • Conception, fertilization of an egg by a sperm, occurs in the oviduct • After fertilization, the zygote undergoes cleavage and develops into a blastocyst before implantation in the endometrium

  40. Cleavage starts Cleavage continues LE 46-15a Ovary Fertilization occurs The blastocyst implants Uterus Ovulation Endometrium From ovulation to implantation

  41. LE 46-15b Endo- metrium Inner cell mass Cavity Trophoblast Blastocyst Implantation of blastocyst

  42. First Trimester • Human gestation can be divided into three trimesters of about three months each • The first trimester is the time of most radical change for both the mother and the embryo

  43. During its first 2 to 4 weeks, the embryo obtains nutrients directly from the endometrium • Meanwhile, the outer layer of the blastocyst mingles with the endometrium and eventually forms the placenta • Blood from the embryo travels to the placenta through arteries of the umbilical cord and returns via the umbilical vein

  44. Maternal veins Maternal arteries Placenta LE 46-16 Maternal portion of placenta Umbilical cord Chorionic villus containing fetal capillaries Fetal portion of placenta (chorion) Maternal blood pools Umbilical arteries Uterus Fetal arteriole Umbilical vein Fetal venule Umbilical cord

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