Chapter 43 Animal Reproduction

1. Chapter 43 Animal Reproduction Biology 102 Tri-County Technical College Pendleton, SC

2. Let me answer that one, Doc!!! • Asexual reproduction DOES NOT involve formation of any type (kind) of gametes (sex cells) • Sexual reproduction DOES involve the formation of gametes (sex cells) • How those gametes are gotten together is another matter and can be a real hoot!!!!

3. Types of Asexual Reproduction • Fission involves separation of parent into two or more individuals of approximately same size (sea anemone) • Budding occurs when new individual splits off from existing one (hydra and tunicates) • Release of specialized cells that grow into new adults • Freshwater sponges produce gemmules which form from aggregates of several types of cells surround by protective coat

4. Asexual, cont. • Fragmentation is breaking of body into several pieces, each of which develops into complete animal • Must be accompanied by regeneration (regrowth of lost body parts) • Sponges, cnidarians, polychaete annelids, and tunicates • Regeneration allows many animals to replace lost parts

5. Asexual Advantages??? • Allows production of many offspring in short time thereby exploiting favorable environment with maximum offspring in shortest possible time • Perpetuates successful genotypes precisely • In other words, makes clones

6. Sexual Advantages??? • Offspring have combination of genes inherited from both parents • ***Increases genetic variability in the offspring • Highly advantageous in a fluctuating environment • Sociobiology, anyone?

7. Daring to define… • Parthenogenesis is development of an egg without fertilization • Daphnia; male honeybees (drones) • Hermaphroditism: each individual has both functional male and female reproductive parts • Some self-fertilize but most mate with another individual (earthworms are classic example)

8. Daring to define, cont. • Sequential hermaphroditism describes situation where individual reverses its sex during its lifetime • Some species are protogynous (female first) • Others are protandrous (male first) • Reversal often associated with age and size

9. Male form and function… • External genitalia includes scrotum and penis (copulatory organ) • Internal reproductive organs consist of gonads (testes), accessory glands, and associated ducts • Testes comprised of highly coiled tubules (seminiferous tubules) surrounded by layers of connective tissue • “Mountain oysters,” anyone???

10. Male F & F, cont. • Interstitial cells (Leydig cells) scattered among tubules and produce testosterone and other androgens • Sperm passes from seminiferous tubules into tubules of epididymis • Sperm are stored and mature (gain motility and fertilizing power) • At ejaculation, sperm forced through vas deferens (muscular duct connecting epididymis to ejaculatory duct)

11. Male F & F III • Ejaculatory duct forms by joining of two vas deferens ducts with duct from seminal vesicles • Ejaculatory duct opens into urethra (tube that runs through penis and drains both excretory/reproductive systems) • There are 3 sets of accessory glands associated with the male system • They add their secretions to semen

12. Male F & F IV • Pair of seminal vesicles located below and behind urinary bladder and empty into ejaculatory duct • Secrete fluid containing mucus, amino acids (for coagulation), fructose, and prostaglandins • Comprise about 60% of total semen volume • Prostate gland is large gland surrounding upper portion and emptying directly into urethra

13. Male F & F V • Prostate secretes thin, milky alkaline fluid with enzymes • Balances acidity of residual urine in system • Buffers acidity of vagina/activates sperm • Bulbourethral glands are pair of small glands below prostate that empty into urethra at base of penis • Secrete clear mucus before ejaculation • Neutralize remaining acidic urine in urethra

14. Male F & F V • Head of penis (glans penis) covered with foreskin called prepuce • Is it the “y” or is it just tough being born a male? • Some male mammals possess a baculum ( a penis bone)

15. Male Visual

16. Female Form and Function • Internal reproductive organs are gonads (ovaries) and associated ducts and chambers • Involved with gamete movement/embryo development • External genitalia include clitoris/2 sets of labia • Ovaries located in abdominal cavity enclosed in tough protective capsule

17. Female Form & Function, cont. • Mesentery flanks & attaches each ovary to uterus • Each ovary contains follicles (1 egg cell surrounded by follicle cells which nourish and protect egg cell) • Follicle cells also produce estrogens • During ovulation, egg expelled from follicle • Remaining tissue forms corpus luteum which secretes progesterone (maintains uterine lining) and additional estrogen

18. Female F & F III • Egg cell expelled into abdominal cavity near opening of oviduct • Cilia lining oviduct draw egg cell in (hopefully) uterus • Uterus (womb) is thick muscular organ that can expand to accommodate a 4-kg fetus • Inner uterine lining (endometrium) is richly supplied with blood vessels

19. Female F & F IV • Cervix is the neck of the uterus which opens into vagina • Vagina is thin-walled chamber that is repository for semen during copulation, forms birth canal, and is exit for menstrual fluids • Hymen is vascularized membrane usually covering vaginal opening from birth until ruptured by vigorous physical activity or sexual intercourse

20. Female F & F V • Vestibule: chamberlike area formed by 2 pairs of skin folds covering vaginal orifice/urethral opening • Libia minora: slender skin folds bordering vestibule • Libia majora: pair of thick, fatty ridges enclosing and protecting libia minora & vestibule

21. Female F & F VI • Clitoris: bulb of erectile tissue at front edge of vestibule which is covered by a prepuce (small hood) • Bartholin’s glands: small glands located near vaginal opening that secrete mucus into vestibule during sexual arousal, facilitates intercourse by lubricating vagina • Mammary glands: important to reproduction but NOT part of reproductive system

22. Female Visual

23. Hormones are flowing… • Androgens directly responsible for formation of primary sex characteristics (reproductive organs) & secondary sex characteristics (deepening of voice, hair growth patterns, muscle growth) • Are steroid hormones produced primarily by Leydig cells of testes • Testosterone most important androgen

24. Male hormones • Androgens are potent determinants of sexual and aggressive behavior • It’s the testosterone that’s killing the guys…but one can go to the front of the line • GnRH from hypothalamus stimulates anterior pituitary to release LH (stimulates androgen production) and FSH (acts on seminiferous tubules to > sperm production)

25. Comparing cycles • Menstrual cycle characterized by endometrium break down, sloughing off of lining, and discharge from body via vagina if fertilization has NOT occurred • From “menses” which is Latin for “months” • Uterine cycles of mammals other than humans does NOT include menstruation • Uterine lining is reabsorbed into body

26. Cycles, cont. • Ovarian cycle characterized by state of sexual receptivity called estrus (correlated with ovulation) • Female solicits male attention and may be aggressive to other females • Time for sociobiology

27. Female Hormones • Hormones coordinate menstrual and ovarian cycles • Ovulation synchronized with uterine preparation for possible implantation of embryo • FIVE hormones participate in elaborate positive and negative feedback scheme

28. Female Hormones, cont. • GnRH from hypothalamus • FHS (responsible for follicle maturation) and LH (responsible for egg release) from anterior pituitary • Estrogens (family of closely related hormones) and progesterone (female sex hormone secreted by ovaries)

29. Female Hormones Visual

30. Sync’ing the Cycles • Cycles coordinated and timed by same hormones that initiate sexual maturation • In response to FSH and LH, ovarian tissue grows and produces estrogen • Estrogen causes development of secondary sexual characteristics • Menstruation marks beginning of uterine/ovarian cycles

31. Cycles, cont. • Few days before menstruation begins, anterior pituitary > secretion of FSH and LH • Some follicles begin to mature in ovaries • Generally only one follicle continues with process • Developing follicle secretes > amounts of estrogen which causes endometrium to grow

32. Cycles III • Estrogen exerts negative feedback control on gonadotropin release by anterior pituitary (FHS & LH) during first 12 days of cycle • On day 12, estrogen exerts positive feedback on pituitary and there is surge of LH and slight surge of FSH • LH triggers mature follicle to rupture and release its egg • Also stimulates follicle to become corpus luteum which secretes estrogen and progesterone

33. Cycles IV • Estrogen and progesterone CRITICAL to continued growth and maintenance of endometrium • These sex steroids exert negative feedback on pituitary inhibiting gonadotropin release thereby preventing new follicles from beginning to mature

34. Cycles V • Egg NOT fertilized, corpus luteum degenerates on about day 26 of cycle • Without progesterone from corpus luteum, endometrium sloughs off and menstruation occurs • Decrease in circulating steriods allows hypothalamus/anterior pituitary to release GnRH, FSH, and LH and next cycle begins

35. Spermatogenesis • Spermatogonia are diploid cells that are precursors of sperm • Located near outer wall of seminiferous tubules, spermatogonia undergo repeated mitoses which produce large populations of potential sperm • In mature male, about 3 million spermatogonia per day differentiate into primary spermatocytes • Primary spermatocytes undergo meiosis I to produce two secondary spermatocytes which are now haploid but still have duplicated chromosomes

36. Spermatogenesis, cont. • Secondary spermatocytes undergo meiosis II • Separates sister chromatids and produces four spermatids • Spermatids mature into mature spermatozoa (sperm cells) • Involves association with Sertoli cells which transfer nutrients to spermatids

37. Spermatogenesis III • During spermatogenesis, developing sperm gradually pushed toward center of seminiferous tubule • Make their way to epididymis where they acquire motility • From spermatogonia to motile sperm = 65 to 75 days

38. Spermatogenesis Visual

39. Oogenesis • Oogenesis is development of ova (mature, unfertilized egg cells) • Begins in embryonic female when primordial germ cells undergo mitotic divisions to produce diploid oogonia • Each oogonium develops into primary oocyte by time of birth • All potential ova present in ovaries at birth

40. Oogenesis, cont. • Primary oocytes (2N) proceed to Prophase I between 3rd and 7th month of fetal development • Then enter state of suspended development until female reaches puberty • After puberty, with each ovarian cycle, FSH stimulates a follicle to enlarge & primary oocyte within completes Meiosis I to produce haploid secondary oocyte and first polar body

41. Oogenesis III • Meiosis STOPS again • LH triggers ovulation and secondary oocyte released from follicle • IF fertilization occurs, Meiosis II will occur and second polar body will separate from ovum • Female strategy appears to be QUALITY of reproductive cell whereas male strategy appears to be QUANTITY of reproductive cells

42. Oogenesis Visual

43. Comparing the Processes • In spermatogenesis all 4 products of Meiosis I and II become mature spermatozoa • In oogenesis, only one daughter cell will become single ovum, the other cells (polar bodies) will degenerate • Spermatogenesis is continual process throughout reproductive life of male

44. Comparing, cont. • All potential ova that can be produced by oogenesis are present as primary oocytes in ovaries at time of female’s birth • Spermatogenesis occurs as uninterrupted sequence (subject to brain stem influence…time for an Estesism…yeah!!! • Oogenesis has long resting periods between formation of initial steps and final production of ovum

45. What a Miracle…. • Human gestation divided into 3 trimesters (about 3 months each) • FIRST trimester is when most radical changes occur for both mother and baby • Fertilization occurs in oviduct and cleavage (cell division) begins in about 24 hours • Zygote develops into a ball of cells

46. Pregnancy, cont. • Embryo reaches uterus in 3-4 days and develops into hollow ball of cells called blastocyst (about 1 week after fertilization) • Blastocyst will implant in endometrium in next 5 days (critical thinking time) • During implantation, blastocyst bores into endometrium and endometrium begins to grow over blastocyst

47. Pregnancy III • Embryonic tissues begin to mingle with endometrium to form placenta which functions in respiratory gas exchange, nutrient transfer, and waste removal from embryo • Also main period of organogenesis (organ development) • After 8 weeks, embryo becomes a fetus • Fetus possesses ALL adult organs in rudimentary form

48. Pregnancy IV • Fetus is about 5 cm in length by end of 1st trimester • Embryo secretes hormones that signal its presence and control’s mother reproductive system • **Human chorionic gonadotropic (HCG) maintains progesterone and estrogen secretion by corpus luteum to prevent menstruation

49. Pregnancy V • High progesterone levels stimulate formation of protective mucus plug in cervix, growth of maternal part of placenta, uterus enlargement, and cessation of ovulation and menstrual cycling • SECOND TRIMESTER time of rapid growth and fetal activity • Fetus grows to about 30 cm in length

50. Pregnancy VI • Mother will feel movement (quickening) • Hormone levels stablize as HCG declines • Corpus luteum degenerates • Placenta secretes its own progesterone to maintain pregnancy • Uterus grows sufficiently for pregnancy to be obvious • THIRD TRIMESTER is time of rapid growth but decreasing fetal activity