30.1 Asexual and Sexual Reproduction

2. Fig. 30.1 Asexual reproduction in Euglena 30.1 Asexual and Sexual Reproduction In asexual reproduction, the offspring are genetically identical to one parent • The process begins with mitosis • Protists typically divide by fission • Cnidarians typically divide by budding

3. 30.1 Asexual and Sexual Reproduction • In sexual reproduction, a new individual is formed by the union of two gametes (egg and sperm) • A zygote is formed • Develops by mitosis into a multicellular organism • Haploid gametes are produced in the gonads

4. Different Approaches to Sex • Parthenogenesis • Offspring are produced from unfertilized eggs • Common among arthropods • Some are exclusive • Others switch! • Common also in some lizard species

5. Hamlet bass Fig. 30.2 Bluehead wrasse • Hermaphroditism • One individual has both testes and ovaries • Tapeworms and earthworms Switch sexual roles! • Hermaphroditismmay be sequential • Individuals change sex Switch sexes! • Protogyny • From female to male • Protandry • From male to female

6. Sex Determination • In some reptiles, sex is determined by environmental changes • In mammals, it is determined early in embryonic development • Embryonic gonads are indifferent • Y chromosome converts them to testes • Responsible gene is SRY • Sex-determining region of the Y chromosome

7. Fig. 30.3 Sex determination

8. 30.2 Evolution of ReproductionAmong the Vertebrates • Vertebrate sexual reproduction evolved in the ocean before vertebrates colonized land • Most marine bony fish use external fertilization • Male and female gametes are released into the water where fertilization occurs • Most other vertebrates use internal fertilization • Male gametes are introduced into the female reproductive tract

9. There are three strategies for internal fertilization • 1. Oviparity • Fertilized eggs are deposited outside mother’s body to complete their development • 2. Ovoviviparity • Fertilized eggs are retained within the mother to complete their development • Young obtain nourishment from egg yolk • 3. Viviparity • Fertilized eggs are retained within the mother to complete their development • Young obtain nourishment from mother’s blood

10. Fish and Amphibians • The eggs of most bony fish are fertilized externally • Eggs contain little yolk • Young fish must seek its food from the water surrounding it • Thousands of eggs are fertilized, but only a few of resulting individuals reach maturity • Fertilization in most cartilaginous fish is internal • Development of young is viviparous

11. Fig. 30.6 Poison arrow frog South American marsupial frog Fig. 30.7 Surinam frog Darwin’s frog • Fertilization is external in most amphibians • Eggs of most species develop in water • With some interesting exceptions Male! Female • Development is divided into embryonic, larval and adult stages Female Male!

12. Fig. 30.8 How turtles do it  Reptiles and Birds • Most reptiles are oviparous, laying amniotic eggs • Other species are ovoviviparous or viviparous • Most male reptiles use a penis to inject sperm into females • This process is called copulation

13. Fig. 30.9 • All birds are oviparous, laying amniotic eggs • As egg passes along oviduct, glands secrete albumin proteins and the hard calcareous shell • Birds are homeotherms • Incubate eggs to keep them warm

14. Mammals • Some mammals are seasonal breeders • Others have reproductive cycles • Periodic release of a mature ovum (ovulation) • Most female mammals have estrous cycles • Females sexually receptive to males only around time of ovulation (estrus) • Apes and humans have menstrual cycles • Females bleed when shedding inner lining of the uterus • Can copulate at any time in their cycle • Cats and rabbits are induced ovulators • Ovulation only after copulation due to LH secretion

15. Three types of mammals • Monotremes are oviparous • Lay eggs • Young hatchlings obtain milk by licking mammary glands (they lack nipples) • Marsupials are viviparous • Give birth to incompletely developed fetuses • Complete development in mother’s pouch • Obtain food from nipples in mammary glands • Placentals are viviparous • Retain young in uterus for long periods of development • Fetuses are nourished by the placenta

16. Duck-billed platypus Kangaroo Deer Fig. 30.10 Reproduction in mammals

17. Fig. 30.11 The male reproductive organs 30.3 Males

18. Fig. 30.13 Human sperm cell • The testis producessperm and testosterone • Enclosed in a hanging sac called the scrotum • Sperm need cooler temperature to develop • Spermatogenesis occurs in the seminiferous tubules • Sperm are then transferred to the epididymis for storage and maturation • From there to the vas deferens • To the urethra which empties through the penis Contains 23 chromosomes

19. Fig. 30.12 The testis and formation of sperm

20. Fig. 30.14 • The penis contains long cylinders of spongy tissue • These get filled with blood causing an erection Physical stimulation is required for ejaculation • 2-5 milliliters of semen are ejected • This volume contains several hundred million sperm • Plus secretions from the prostate and other glands

21. Fig. 30.15 The female reproductive organs 30.4 Females

22. At birth, a female’s ovaries contains all the oocytes she will ever produce • ~ 2 million oocytes are arrested in prophase I of the first meiotic division • At puberty, the release of FSH causes the resumption of meiosis I in a few oocytes • However, only one becomes dominant and is ovulated • Mature egg cells are called ova (singular, ovum) • This cycle is repeated about every 28 days

23. Fig. 30.16 The ovary and formation of an ovum

24. Fertilization of the egg occurs high in the Fallopian tubes (also called uterine tubes or oviducts) • The fertilized egg is now called a zygote • It is transported to the uterus • A muscular pear-shaped organ about the size of a fist • It narrows to a muscular ring called the cervix • Leads to the vagina

25. Fig. 30.17 A comparison of mammalian uteruses Primates Cats, dogs and cows Rats, mice and rabbits • Marsupials, such as opossums, have two unconnected horns, two cervices and two vaginas • Male marsupials have a forked penis!

26. The fertilized egg is pushed down the oviducts by the rhythmic contraction of its smooth muscles • The journey takes 5-7 days • The uterus is lined with a stratified epithelial membrane called the endometrium • The zygote attaches to this layer and begins embryonic development! • If the egg is not fertilized, the surface layer of the endometrium is shed during menstruation • The underlying layer generates a new surface layer during the next cycle

27. 30.5 Hormones Coordinatethe Reproductive Cycle • The female reproductive cycle is composed of two distinct phases • Follicular phase • Egg reaches maturation and is ovulated • Luteal phase • Body continues to prepare for pregnancy • A family of hormones coordinates these two phases

28. Follicular Phase • Development of the egg within the ovary • The oocyte and its surrounding mass of tissue is called the follicle • FSH secretion triggers the maturation of several follicles and resumption of meiosis in their oocytes • But only one achieves full maturity • FSH also causes the ovary to secrete estrogen • Negative feedback by estrogen, causes the hypothalamus to stop the pituitary’s FSH output

29. Luteal Phase • The body is prepared for fertilization • Hypothalamus causes the anterior pituitary to begins secreting luteinizing hormone (LH) • LH inhibits further estrogen production • It also causes the wall of the follicles to burst • Oocyte is ovulated into oviducts • LH directs the repair of the ruptured follicle, which becomes the corpus luteum

30. Pituitary gland Levels of gonadotropic hormones in blood LH FSH FSH 0 7 14 21 28 days Ovarian cycle Developing follicles Ovulation Corpus luteum Luteal phase Hormone blood levels Estradiol Progesterone 0 7 14 21 28 days Endometrial changes during menstrual cycle Menstrual phase Proliferative phase Ovulation Secretory phase 14 21 28 days 0 7 Fig. 30.18 The human menstrual cycle

31. Luteal Phase • The corpus luteum begins to secrete the hormone progesterone • Progesterone inhibits FSH • It also thickens the endometrium preparing for fertilization • If fertilization does not occur, progesterone production stops and the luteal phase ends • Thickened endometrial layer sloughs off • This causes the bleeding associated with menstruation

32. Pituitary gland Levels of gonadotropic hormones in blood LH FSH FSH 0 7 14 21 28 days Ovarian cycle Luteal regression Developing follicles Ovulation Corpus luteum Luteal phase Hormone blood levels Estradiol Progesterone 0 7 14 21 28 days Endometrial changes during menstrual cycle Menstrual phase Proliferative phase Ovulation Secretory phase Menstrual phase 14 21 28 days 0 7 Fig. 30.18 The human menstrual cycle

33. Luteal Phase • If fertilization does occur, the corpus luteum is maintained by human chorionic gonadotropin (hCG) • hCG is a hormone produced by the embryo • It is tested for in all pregnancy tests • Two other hormones are of importance • Prolactin • Stimulates milk production • Oxytocin • Initiates milk release • Induces labor

34. 30.6 Embryonic Development • The vertebrate embryo develops in three stages • Cleavage • A hollow ball of cell forms • Gastrulation • Cells move to the interior, forming the primary tissues • Neurulation • The organs of the body form

35. Cleavage: Setting the Stage for Development • During cleavage, zygote rapidly divides into larger and larger numbers of smaller and smaller cells • A morula forms • A tightly packed mass of about 32 blastomeres • Further division results in a hollow ball of 500-2,000 cells called the blastocyst • Contains a fluid-filled cavity, the blastocoel • Inner cell mass  Forms the embryo • Trophoblast Becomes the placenta

38. Cleavage: Setting the Stage for Development • Embryo reaches the uterus on day 6 • It penetrates the endometrial lining • Initiates membrane formation • Amnion • Encloses embryo • Chorion • Forms from the trophoblast • Interacts with uterine tissue to form the placenta

39. Gastrulation: Onset of Developmental Change • Certain groups of cells move inwards from the inner cell mass at about 10-11 days after fertilization • This process of gastrulation results in the three primary germ layers • Endoderm • Ectoderm • Mesoderm

41. Neurulation: Determination of Body Architecture • In the third week, the three primary germ layers begin development into body tissues and organs • First, the notochord develops from the mesoderm • The neural tube develops from the ectoderm • The gut develops from the endoderm • On either side of the notochord blocks of tissue form • These somites give rise to muscles, vertebrae and connective tissues developing notochord • By the end of the third week, the embryo is about 2 mm (< 0.1 inches) long

42. Fig. 30.20a 30.7 Fetal Development Fourth week • Formation of body organs, or organogenesis • Critical time in development • Alcohol use may cause fetal alcohol syndrome • Embryo reaches about 5 mm

43. Second month Fig. 30.20b 30.7 Fetal Development • Great changes in morphology occur • Limbs assume adult shape • Major internal organs are evident • Embryo reaches about 25 mm

44. Third month Fig. 30.20c 30.7 Fetal Development • Development is essentially complete except for lungs and brain • Developing human is now called a fetus • It carries out primitive reflexes like sucking

45. Second trimester Fig. 30.20d 30.7 Fetal Development • A time of growth • Bone formation occurs • Hair and body are covered with fine hair called lanugo • By the end of the 6th month, the fetus is 30 cm (1 foot) long

46. Fig. 30.21 30.7 Fetal Development Third trimester • Pace of growth accelerates • Weight of fetus more than doubles • Nutrients provided by mother’s blood via the placenta • Most major nerve tracts are formed in the brain

47. Fig. 30.22 Postnatal development • Babies typically double birth weight within a few months • Different body parts grow at different rates • Allometric growth • Nerve cells produced at an average rate of > 250,000 per minute • At 6 months, neuron production ceases permanently

48. 30.8 Contraception and Sexually Transmitted Diseases • Contraception, or birth control, is the prevention of pregnancy • Several different methods are available • These differ in their effectiveness and acceptability to different couples

49. Abstinence • Simplest and most reliable way • Natural family planning, or the rhythm method • Prevention of egg maturation • Birth-control pills • Estrogen and progesterone • Shut down production of the pituitary hormones FSH and LH • Birth-control injections • Birth-control patches

50. Prevention of embryo implantation • Intrauterine devices (IUDs) • RU486 (“morning after pill”) • Sperm blockage • Condoms • Diaphragms • Sperm destruction • Spermicidal jellies • Foams • Sterilization • Vasectomy in males • Tubal ligation in females