Unit II Reproduction and Development

1. Unit IIReproduction and Development

2. The Reproductive System • unlike asexual reproduction, sexual reproduction involving the fusion of gametes creates new gene combinations – leading to genetic diversity • genetic diversity provides the basis for the evolution of species and natural selection  varied offspring, some better suited to survive • an incredible amount of diversity exists between species – many females produce thousands to millions of eggs each mating season, yet human females have only 400 000 egg cells, of which, only 400 or so ever mature to become fertilizable eggs.

3. The Reproductive System • primary reproductive organs – gonads (testes in male, ovaries in female) • responsible for production of reproductive cells – sperm or ova • secretion of sex hormones (testosterone, estrogen, progesterone) • puberty is defined as the point where reproduction is possible (in females, menstruation generally does not include ovulation for the first year)

4. The Reproductive System • puberty’s onset controlled by • 1. Hypothalamus - GnRH (gonadotropin releasing hormone) • 2. Anterior Pituitary - FSH - LH - stimulates sperm prd. - stimulates testosterone (seminiferous tubules) prd. (interstitial cells) - stimulates follicle in ovary - stimulates follicle release in ovary - stimulates formation of corpus luteum +

5. The Reproductive System • Sertoli cell inhibin negatively feeds back to pituitary in males to inhibit sperm production • in females, at the beginning of a menstrual cycle, estrogen stimulates LH secretion, whereas, after ovulation, estrogen and progesterone inhibit the pituitary and hypothalamus

6. -secondary sex characteristics (thanks to estrogen & testosterone) - development of hair (males &females) maturation of internal and external genitalia (males & females) increase in shoulder width and muscle mass (fem: wider hips) voice – larynx enlarges, vocal cords thicken and lengthen (males) skin – sebaceous gland secretion thickens and increases (acne?) mental – increased aggression, sex drive (males) distribution of fat in breasts and buttocks (females) The Reproductive System

7. The Male Reproductive System • erection is produced by the trapping of blood in the corpus cavernosa in response to stimulation of the parasympathetic nervous system – dilating the arterial supply, constricting the venous blood flow out • prepuce (foreskin) – serves a protective function (sometimes removed in circumcision) • glans – expanded tip of the penis • corpus cavernosa – erectile tissue, fills with blood producing an erection • corpus spongiosa – soft erectile tissue • urethra – carries sperm and urine (never at the same time – a sphincter controls urine excretion) penis

8. The Male Reproductive System

9. The Male Reproductive System

10. The Male Reproductive System • prostate gland – produces buffer to protect sperm from acidic environment • vas deferens (ductus deferens) – tube that carries sperm from testes to the urethra • (bladder – stores urine) • (ureter – transports urine from kidney to bladder) • seminal vesicle – sacs under the bladder that secrete fructose (food for sperm) and prostaglandins (stimulates uterine contractions which help sperm move up the uterus to the fallopian tubes), makes up 60% of total semen volume • ejaculatory duct – connects vas deferens to the urethra • bulbourethral gland (Cowper’s gland) – secrete mucous for lubrication before ejaculation – may contain enough sperm for impregnation

11. The Male Reproductive System • epididymis – coiled tube attached to the outer edge of the testis, where sperm complete development • testis – production of sperm & testosterone, each contains 250m of twisting seminiferous tubules – site of sperm production • scrotum – sac that contains testis, made of elastic skin allowing for temperature regulation by raising or lowering relative to the body (sperm develop best at 3C lower than regular body temperature) testicles

12. The Male Reproductive System

13. Male Hormone Control

14. Spermatogenesis • spermatogenesis (the production of sperm) begins at puberty and continues until death 1 spermatagonium 1 – spermatogonium 1 - 1 spermatocytes 2 - 2 spermatocytes 4 spermatids 4 mature sperm (46 chr.) mitosis (46 chr.) meiosis I (23 chr.) meiosis II (23 chr.) (23 chr.) seminiferous tubules epididymis • occurs in the seminiferous tubules – as maturation continues, sperm cells move to lumen, then to epididymis for final maturation • Sertoli (Nurse) cells in the seminiferous tubules nourish the sperm as they mature • Sperm production is stimulated by FSH and testosterone • interstitial cells of Leydig in the testes produce testosterone (stimulated by LH)

15. Spermatogenesis

16. Spermatogenesis

17. Stages of Spermatogenesis

18. Sperm Structure • over 300 000 000 produced every day! • designed for travel - the acrosome in the head of the sperm contains enzymes to penetrate the egg- nucleus containing DNA - tail – containing centriole for structure of flagellum, mid-piece contains mitochondria to provide the energy for whipping motion • semen includes sperm & secretions from supportive fluids for nourishment & protection against acidic vagina

19. Sperm Structure Nucleus (DNA) Mitochondria

21. Sperm Structure

22. Male Reproductive System • FYI • a vasectomy involves the snipping or tying of the vas deferens preventing sperm from entering the ejaculate • if testicles don’t descend, then viable sperm will not develop (fairly rare in humans, more common in dogs) • only a thin membrane separates the testes from the small intestine – if a rupture occurs, small intestine may slip through, impairing blood supply for either testis or small intestine • testicular cancer is found primarily in youngmales – easily treated with the removal of one of the testicles • prostate problems generally found in older males – enlargement leads to urinary problems (cancer of the prostate is treatedprimarily with excision)

23. How Viagra Works • Sperm Production • Vasectomies • Male Reproductive System Video

24. Female Reproductive System • in the female, the urinary and reproductive duct systems are completely separate • clitoris, labia major & minor, mons pubis – external genitalia (vulva) • clitoris homologous to male penis • uterus (womb) – pear shaped, where fertilized ovum embeds in endometrium, lining is shed during menstruation • ovary – store and produce ova, generally one ovum is produced monthly (alternating ovaries) each egg is ~1mm large (largest cell produced by the human body)

25. Female Reproductive System • oviduct (fallopian tube) – tube carrying mature ova to the uterus, usually the site of fertilization. Ova are swept in by fimbria at open ends. • cervix (cervical canal) – a muscular band that prevents the fetus from delivering prematurely – will dilate during birth • vagina – connects the uterus with the outside world – site of deposition of sperm, birth canal, protects the rest of the female reproductive system from bacterial invasion (highly acidic)

26. Female Reproductive System

27. Female Hormone Control

28. Female Hormone Levels

31. Oogenesis and Ovulation • the development of the ova shares many similarities with sperm production • initial development occurs in the ovary (seminiferous tubule) • final development occurs in the fallopian tube (epididymis) • there are also startling differences • women are born with all of the ova-producing cells they will ever have • every spermatogonia will produce 4 sperm, whereas 1 oogonia produces one ova (the rest of the potential ova become polar bodies, sacrificing themselves to provide more cytoplasm for the one ovum produced)

33. Oogenesis and Ovulation before birth! monthly 1 – oogonium 1 - 1 oocytes 1 - 2oocyte 1 – mature ovum (46 chr.) (46 chr.) (23 chr.) (23 chr.) 1– polar body 3 – polar bodies ovary oviduct • in the development of a mature ova, one (usually) primary oocyte and the surrounding granulosa cells will develop monthly in response to FSH • one of the 2 cells produced dies (polar body), fluid-filled cavity forms around oocyte • mature follicle ruptures (ovulation)

34. Oogenesis and Ovulation • oocyte completes maturation in oviduct (producing 3 polar bodies and one ovum in total) if fertilization occurs (meiosis II is completed) • after ovulation, the follicular cells in the ovary collapse forming the corpus luteum which secretes progesterone and some estrogen, necessary for pregnancy. • If implantation doesn’t occur, the corpus luteum breaks down forming a “scar” termed the corpus albicans and the oocyte will disintegrate and be reabsorbed by the body

35. Oogenesis and Ovulation

36. Oogenesis and Ovulation

37. Oogenesis and Ovulation

38. Female Reproductive System • FYI • a female baby houses all of the reproductive potential of an adult – about 2 000 000 eggs. About 300 000 of these survive to puberty, about 450 of them will develop (the rest provide support for the 450) • a tubal ligation (tying or cutting of the fallopian tubes) is one method of “permanent” birth control • an egg can live for 2-3 days, a sperm up to 3 days. An egg’s voyage to the uterus takes 3-5 days • cervical cancer is tested with a PAP smear, sampling cells, looking for signs of irregular cells

39. The Menstrual Cycle • Menstruation – periodic discharge of blood and fluid from the uterus, caused by low levels of estrogen and progesterone at the end of the monthly cycle (hormone withdrawal) • Menstrual cycle – repetitive sequence of shedding the uterine lining (endometrium), development and release of the egg (ovulation), and replacement of the endometrium. This cycle continues from puberty to menopause.

40. The Menstrual Cycle • Can be divided into four phases • Flow (menstrual) phase (day 1-5) • Lasts an average of five days • Marked by the shedding of the endometrial lining • Triggered by a decrease in ovarian hormones (particularly progesterone)  stimulates FSH and LH from pituitary • Corpus luteum degenerates forming corpus albicans • Low hormone levels thought to be responsible for headaches, cramps, nausea, mood changes

41. The Menstrual Cycle • Follicular phase (day 6-13) • Lasts until ovulation • Governed by estrogen released by the developing follicle (which is stimulated by FSH) • Endometrial lining thickens (estrogen) • FSH increases then is lowered (negative feedback from estrogen) • LH production increases (stimulated by estrogen) • Follicle matures

42. The Menstrual Cycle • Ovulation (day 14) • Estrogen & LH levels peak • Follicle erupts, ovum is released into fallopian tube • Female’s temperature peaks (may be used as a contraceptive method or to help in impregnation)

43. The Menstrual Cycle • Luteal phase (day 15-28) • corpus luteum develops • Lasts until menstruation • Governed by progesterone released by the corpus luteum (which is stimulated by LH) • Further ovulation & uterine contractions are inhibited (progesterone) • Endometrial lining continues to thicken, preparing for pregnancy (progesterone & estrogen from cl) • Progesterone and estrogen inhibit LH release (-ve feedback)  corpus luteum starts to degenerate  breakdown of endometrium

44. FYI • Endometriosis, a sometimes debilitating disease is caused by endometrial lining that should be forming only in the uterus to form inside the stomach cavity (and is still shed monthly) • Birth control pills work by preventing ovulation (progesterone prevents ovulation) • Both males and females have both androgens and estrogens – the relative quantities determine the degree of secondary sex characteristics displayed • Ectopic pregnancies result from implantation in the fallopian tube instead of the uterus. The pregnancy will rupture the fallopian tube if allowed to continue.