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Chapter 46

Chapter 46. Animal Reproduction. Sexual & asexual reproduction. Asexual offspring all have same genes (clones) no variation Sexual gametes (sperm & egg)  fertilization mixing of genes  variation. Parthenogenesis. Development of an unfertilized egg honey bees

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Chapter 46

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  1. Chapter 46 Animal Reproduction

  2. Sexual & asexual reproduction • Asexual • offspring all have same genes (clones) • no variation • Sexual • gametes (sperm & egg)  fertilization • mixing of genes variation

  3. Parthenogenesis • Development of an unfertilized egg • honey bees • drones = males produced through parthenogenesis  haploid • workers & queens = females produced from fertilized eggs  diploid queen worker drone

  4. Different strokes… parthenogenesis in aphids sex-change in fish

  5. Hermaphrodites • Having functional reproductive system of both sexes earthworms mating flat worm

  6. Fertilization • Joining of egg & sperm • external • usually aquatic animals • internal • usually land animals

  7. Development • External • development in eggs • fish & amphibians in water • soft eggs= exchange across membrane • birds & reptiles on land • hard-shell amniotic eggs • structures for exchange of food, O2 & waste • sharks & some snakes • live births from eggs • Internal • placenta • exchange food & waste • live birth

  8. Adaptive advantages? • What is the adaptive value of each type of sexual reproduction • number of eggs? • level of parental care • habitat?

  9. Human Reproduction: Reproductive hormones • Testosterone • from testes • Functions • Prenatal development of male genitalia • sperm production • 2° sexual characteristics • Maintenance of sex drive • Estrogen • from ovaries • functions • egg production • prepare uterus for fertilized egg • 2° sexual characteristics LH &FSH testesorovaries

  10. Sex hormone control in males Hypothalamus GnRH Pituitary FSH & LH Testes testosterone Body cells

  11. Male reproductive system • Sperm production • over 100 million produced per day! • ~2.5 million released per drop!

  12. Male reproductive system • Testes • produces sperm & hormones • Scrotum • sac that holds testicles outside of body • Epididymis • where sperm mature • Vas deferens • tubes for sperm to travel from testes to penis • Prostate, seminal vesicles, Cowper’s (bulbourethal) glands • nutrient rich fluid to feed & protect sperm • buffer to counteract acids in vagina

  13. seminiferous tubule Male reproductive system • Testes & epididymis • sperm production & maturation • Glands • seminal vesicles, prostate, bulbourethal • produce seminal fluid = nutrient-rich sperm spermatocytes

  14. Spermatogenesis Spermatogenesis occurs within each testis in small tubules called seminiferous tubules This light micrograph shows a cross section of seminiferous tubules, blood vessels and the interstitial Leydig cells. Leydig cells are responsible for the production of testosterone.

  15. Spermatogenesis Mitosis and meiosis may be taking place at any time in the spermatogonia. Mitosis replenishes numbers of cells, meiosis produces actual sperm (spermatozoa) Sertoli cells help nourish sperm cells during meiosis

  16. Spermatogenesis Testis Epididymis Germ cell (diploid) Coiled seminiferous tubules 1° spermatocyte (diploid) MEIOSIS I 2° spermatocytes (haploid) MEIOSIS II Vas deferens Spermatids (haploid) Spermatozoa Cross-section of seminiferous tubule

  17. Hormonal control of sperm production LH (Leutinizing hormone) stimulates Leydig cells to produce testosterone FSH (follicle-stimulating hormone) and testosterone stimulate meiosis in spermatogonia to produce spermatozoa

  18. Role of epididymis, seminal vesicles, and prostate in semen production Sperm cells were produced in seminiferous tubules, carried thru the lumen, and sent to the epididymis. They are stored briefly in the epididymis. On sexual arousal, millions of sperm cells are moved from the epididymis into the vas deferens. Seminal vesicles add large volume of fluid which has a high conc. of fructose for energy for the sperm cells to swim (approx. 70% of the fluid in semen is added by the seminal vesicles.) The prostrate adds more fluid: alkaline which helps the sperm survive the acidic environment in a female’s vagina.

  19. Female reproductive system

  20. Female reproductive system • Ovaries • produces eggs & hormones • Uterus • nurtures fetus; lining builds up each month • Fallopian tubes • tubes for eggs to travel from ovaries to uterus • Cervix • opening to uterus, dilates 10cm (4 inches) for birthing baby • Vagina • birth canal for birthing baby

  21. Female reproductive system

  22. Menstrual cycle Starting at puberty, human females begin a hormonal cycle called the menstrual cycle. Each cycle lasts (on average) 28 days. The purpose is to time the release of an egg (ovum) for possible fertilization and later implantation into the inner lining of the uterus (endometrium) • The implantation must occur when the endometrium is rich with blood vessels. • The breakdown of the blood vessels leads to menstrual bleeding (menstruation); a sign that no pregnancy has occurred

  23. Hormones regulatory center for the menstrual cycle. The hypothalamus produces a hormone called GnRH (gonadotrophin-releasing hormone). The target tissue of GnRH is the anterior pituitary gland, which results in the pituitary producing and secreting 2 hormones into the bloodstream: FSH and LH. The target tissues for these 2 hormones are the ovaries Hypothalamus GnRH Pituitary FSH & LH Ovaries estrogen progesterone Endometrium of uterus

  24. Effects of FSH and LH on the ovaries To increase the production and secretion of estrogen by the follicle cells of the ovary Estrogen enters the bloodstream Its target tissue is the endometrium of the uterus Result is growth of lining (increase in blood vessels) Stimulates egg development in ovaries

  25. Egg maturation in ovary • A spike in levels of FSH and LH leads to ovulation (release of oocyte from the follicle) • The outer ring of follicle cells begin to divide and fill in the ‘wound’ area left by ovulation which forms a corpus luteum. The corpus luteum produces progesterone for 10-12 days after ovulation. • Progesterone (hormone) maintains the thickened endometrium • If no pregnancy, the corpus luteum eventually breaks down = less estrogen and progesterone

  26. Menstrual cycle … as estrogen and progesterone levels fall, the highly vascular endometrium can no longer be maintained. The capillaries begin to rupture and menstruation begins. The drop in progesterone and estrogen also signals the hypothalamus to begin secreting GnRH and another menstrual cycle begins. LH FSH ovulation = egg release egg development corpus luteum estrogen progesterone lining of uterus days 0 7 14 21 28

  27. Oogenesis In ovaries, cells called oogonia go thru mitosis repeatedly to build up numbers of cells. Then those oogonia grow into larger cells (primary oocytes) and start meiosis, but the process stops at Prophase I, until puberty. Then FSH periodically stimulates a follicle to grow and induces its primary oocyte to complete meiosis I and start meiosis II ovulation Meiosis 2 completed triggered by fertilization

  28. Comparison of spermatogenesis and oogenesis Millions of sperm produced every day 4 gametes for every cell that begins meiosis Very small Occurs in testis Released during ejaculation Haploid nuclei Continues throughout all of life, starting at puberty One egg is ovulated per menstrual cycle One gamete is produced for each cell that begins meiosis (plus polar bodies) Eggs are ‘large’ Occurs in ovaries Released during ovulation Haploid nuclei Ovulation starts at puberty, occurs with each menstrual cycle, then stops at menopause Spermatogenesis Oogenesis

  29. Fertilization

  30. Fertilization Enzymes from acrosome are released at the zona pellucida. Sperm head enters egg, plasma membranes fuse together. This initiates the cortical reaction.

  31. Fertilization and the cortical reaction As soon as the sperm reaches the nucleus, the cortical granules fuse with the cell membrane and release their enzymes to the outside, making it impermeable to any other sperm cells.

  32. Early human embryonic development: Cleavage • Fertilization triggers the zygote to start repeated mitotic divisions as it moves down the Fallopian tube toward the uterus • By the time the embryo reaches the uterus, it is approx. 100 cells and is ready to implant into the endometrium of the uterus. At this point, it’s a hollow ball of cells called a blastocyst.

  33. A few days later, the embryo imbeds itself in the uterine lining. At this point, a woman's body begins to secrete a pregnancy hormone known as HCG. This hormone maintains production of progesterone from the ovary, a crucial factor in maintaining a pregnancy.

  34. HCG specifics HCG = Human Chorionic Gonadotrophin. It enters the bloodstream of the mother its target tissue is the corpus luteum. HCG acts to maintain the secretory functions of this gland beyond the length of time typical of a normal menstrual cycle. The corpus luteum continues to make both estrogen and progesterone and the endometrium is maintained. Later on in pregnancy, the placenta will take over estrogen and progesterone production.

  35. Organogenesis Umbilical blood vessels Mammalian embryo Chorion Bird embryo Amnion Yolk sac Allantois Fetal blood vessels Placenta Maternal blood vessels

  36. Placenta • Materials exchange across membranes

  37. Role of the Placenta When fully formed, 2 fetal blood vessels within the umbilical cord carry fetal blood to the placenta. The blood is deoxygenated and carries waste products (CO2, urea, hormones). This fetal blood exchanges materials (not blood) with the maternal bloodstream. Another fetal blood vessel returns the ‘clean’ blood to the fetus (has oxygen and nutrients, hormones) At no time do the 2 bloods actually mix When the corpus luteum stops producing estrogen and progesterone, the placenta starts producing and secreting them.

  38. Role of the amniotic fluid Amniotic sac extends all the way around the fetus The fluid within it is called amniotic fluid Several functions: Provides cushioning effect if something happens to mom’s abdomen • Provides safe environment where fetus has free movement and can exercise its developing muscles and skeleton • Provides temperature stability

  39. Human fetal development 4 weeks 7 weeks

  40. Human fetal development 10 weeks

  41. Human fetal development 12 weeks 20 weeks

  42. Human fetal development • 24 weeks (6 months; 2nd trimester) fetus is covered with fine, downy hair called lanugo. Its skin is protected by a waxy material called vernix

  43. Human fetal development • 30 weeks (7.5 months) umbilical cord

  44. Getting crowded in there!! • 32 weeks (8 months) The fetus sleeps 90-95% of the day & sometimes experiences REM sleep, an indication of dreaming

  45. Hormone induction Birth positive feedback

  46. Birth (36 weeks) Intestine Placenta Umbilical cord Wall of uterus Bladder Cervix Vagina

  47. The end of the journey!

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