1. The Reproductive System
Chapter 50

2. Sexual vs. Asexual Reproduction
Sexual reproduction requires meiosis to produce haploid gametes: sperms and eggs
-These are united by fertilization to produce the diploid zygote
-Develops by mitosis into a new multicellular organism

3. Sexual vs. Asexual Reproduction
In asexual reproduction genetically identical cells are produced from a single parent cell through mitosis
-Single-celled organisms utilize fission
-Cnidarians reproduce by budding

4. Novel Reproductive Strategies
Parthenogenesis = Females produce offspring from unfertilized eggs
Hermaphroditism = One individual has both testes and ovaries
-May be sequential
-Protogyny = Female-to-male change
-Protandry = Male-to-female change

5. Novel Reproductive Strategies

6. Sex Determination
In some reptiles, sex determination is environmentally controlled
In mammals, it is genetically controlled
-Embryonic gonads are indifferent
-Y chromosome converts them to testes
-A key gene is SRY
-Sex-determining region of the Y chromosome

8. Vertebrate Fertilization
Vertebrate sexual reproduction evolved in in the ocean before vertebrates colonized land
Marine bony fish use external fertilization
-Eggs and sperm are released into the water where union of free gametes occurs
Most other vertebrates use internal fertilization
-Sperm are introduced into the female reproductive tract

9. Vertebrate Fertilization
Internal fertilization has led to three strategies for development
1. Oviparity = Fertilized eggs are deposited outside mother’s body to complete their development
2. Ovoviviparity = Fertilized eggs are kept within mother to complete development, and young obtain food from egg yolk
3. Viviparity = Young develop within mother and obtain nourishment from her blood

10. Vertebrate Fertilization

11. Vertebrate Fertilization
Fishes
-In most species of bony fish, fertilization is external
-Thousands of eggs are fertilized, but only a few individuals grow to maturity
-In most species of cartilaginous fish, fertilization is internal
-Development of young is viviparous

12. Vertebrate Fertilization
Amphibians
-In most species fertilization is external
-Eggs of most species develop in water
-With some interesting exceptions
-Development is divided into embryonic, larval and adult stages

13. Vertebrate Fertilization

14. Vertebrate Fertilization (Cont.)

15. Vertebrate Fertilization (Cont.)

16. Vertebrate Fertilization
Reptiles
-Most reptiles are oviparous
-Lay amniotic eggs
-Most male reptiles use a penis to inject sperm into females
-Copulation

17. Vertebrate Fertilization
Birds
-All birds are oviparous, laying amniotic eggs
-As egg passes along oviducts, glands secrete albumin & the hard calcareous shell
-Birds are homeotherms and so incubate eggs to keep them warm

18. Vertebrate Reproductive Cycles
Female reproductive cycles involve periodic release of a mature ovum (ovulation)
Most female mammals have estrous cycles
-Females sexually receptive to males only around time of ovulation (estrus)
Primates have menstrual cycles
-Females bleed when shedding inner lining of the uterus (menstruation)
-Can copulate at any time in their cycle

19. Reproduction in Mammals
Mammals are of three types:
1. Monotremes are oviparous
-Lay eggs
2. Marsupials are viviparous
-Give birth to incompletely developed fetuses, which mature in mother’s pouch
3. Placentals are viviparous
-Retain young in uterus for long periods of development

20. Reproduction in Mammals

21. Reproduction in Mammals (Cont.)

22. Male Reproductive System
When testes form in the human embryo, they develop highly-convoluted seminiferous tubules, the sites of sperm production
-Leydig cells produce testosterone
-Convert indifferent external genitalia into penis and scrotum
Shortly before birth, the testes descend into the scrotum, because sperm need cooler temperature to develop

24. Sperm Production
In the wall of the seminiferous tubule, a spermatogonium divides by mitosis to produce two diploid cells
-One later undergoes meiosis, the other remains as a spermatogonium
The diploid cell that begins meiosis is called a primary spermatocyte
-It undergoes meiosis I, producing two haploid secondary spermatocytes

25. Sperm Production
Each secondary spermatocyte undergoes meiosis II to produce two haploid spermatids
The seminiferous tubules also contain supporting cells called Sertoli cells
-Nurse developing germ cells
-Help convert spermatids into spermatozoa (sperm) by engulfing their extra cytoplasm

27. Sperm Structure Mature sperm consist of three regions:
-Head = Contains a nucleus
-Capped by the acrosome, which facilitates penetration of the egg
-Body = Consists of many mitochondria
-Provide energy
-Tail = Consists of a flagellum
-Provides locomotion

30. Male Accessory Sex Organs
Sperm are delivered into the epididymis for storage and maturation
-From there, sperm enter another long tube, the vas deferens
-To the urethra, which empties into the penis

31. Male Accessory Sex Organs
Three glands assist in the production of semen, a complex mixture of fluids & sperm
-Seminal vesicles produce a fructose-rich fluid, which makes up 60% of semen volume
-The prostate gland contributes about 30% of the bulk of semen
-Bulbourethral glands add secretions to make up the last 10% of semen
-Also lubricate the tip of the penis

32. Penis Structure and Function
The penis consists of erectile tissue columns
-Two corpora cavernosa on dorsal side
-One corpus spongiosum on ventral side
During erection, these tissues fill with blood
-Parasympathetic nerves release nitric oxide (NO), which stimulates dilation of arteries
Ejaculation is the ejection from the penis of about 2-5 mL of semen containing an average of 300 million sperm

34. Male Reproductive Hormones
The hypothalamus secretes gonadotropin-releasing hormone (GnRH)
-GnRH stimulates the anterior pituitary to produce two gonadotropins
-These regulate testicular function

35. Male Reproductive Hormones
Luteinizing hormone (LH)
-Stimulates Leydig cells to secrete testosterone
Follicle-stimulating hormone (FSH)
-Stimulates Sertoli cells to facilitate sperm development, and to secrete inhibin
Testosterone and inhibin exert feedback inhibition on LH and FSH, respectively

37. Female Reproductive System
Ovaries develop more slowly than the testes
-Female embryo develops a clitoris and labia majora
-Ovaries contain microscopic structures called ovarian follicles
-Each follicle contains a potential egg cell called a primary oocyte and smaller granulosa cells

39. The Menstrual Cycle At puberty, granulosa cells secrete estrogen
-Triggers menarche, first menstrual cycle
-Stimulates secondary sexual characteristics
At birth, ovaries contain about 1 million follicles
-Each contains a primary oocyte that is arrested in prophase of meiosis I
The human menstrual cycle lasts ~ 1 month
-Divided into follicular phase, ovulation and luteal phase

40. The Menstrual Cycle Follicular phase
-Several follicles are stimulated to grow under FSH stimulation
-But only one achieves full maturity as a tertiary, or Graafian, follicle
-Estrogen causes growth of the endometrium
-Proliferative phase

41. The Menstrual Cycle

42. The Menstrual Cycle Follicular phase
-The primary oocyte completes meiosis I
-Produces one large secondary oocyte and one tiny polar body
-The secondary oocyte begins meiosis II
-However, its progress is arrested in metaphase II

43. The Menstrual Cycle Ovulation
-Estrogen stimulates the anterior pituitary to secrete LH about midcycle
-The LH surge causes the Graafian follicle to burst, releasing its secondary oocyte into the Fallopian tube
-If oocyte is not fertilized, it disintegrates
-If it is fertilized, it completes meiosis II, forming a mature ovum and second polar body

44. The Menstrual Cycle

45. The Menstrual Cycle (Cont.)

46. The Menstrual Cycle Luteal phase
-LH stimulation transforms the Graafian follicle into the corpus luteum
-Secretes estrogen and progesterone
-Exert negative feedback on FSH and LH
-Cause endometrium to become more vascular and glandular
-Secretory phase

47. The Menstrual Cycle Luteal phase
-In the absence of fertilization, the corpus luteum degenerates due to decreasing levels of FSH and LH
-The built-up endometrium is sloughed off with accompanying bleeding
-Menstrual phase

48. The Menstrual Cycle Luteal phase
-If ovulated oocyte is fertilized, the corpus luteum is maintained by human chorionic gonadotropin (hCG)
-Keeps high levels of estrogen and progesterone, and so prevents menstruation until placenta takes over
-hCG is produced by the embryo
-Tested for in all pregnancy tests

51. The Estrous Cycle
Menstruation is absent in mammals with estrous cycles
-The endometrium is cyclically shed, however without bleeding
The estrous cycle is divided into four phases:
-Proestrus, estrus, metestrus, and diestrus

52. Female Accessory Sex Organs
Fallopian tubes (uterine tubes or oviducts)
Uterus = Muscular pear-shaped organ
Cervix = Narrow opening
Vagina = Initially covered by a membrane called the hymen
-Bartholin’s glands secrete lubricant that facilitates penetration by penis
During sexual arousal, external genitalia become engorged with blood

53. Female Accessory Sex Organs
Mammals other than primates have more complex female reproductive tracts

54. Female Accessory Sex Organs (Cont.)
Mammals other than primates have more complex female reproductive tracts

55. Female Accessory Sex Organs
Mammals other than primates have more complex female reproductive tracts

56. Contraception
Throughout history, people have attempted to control reproduction without avoiding sexual intercourse
The prevention of pregnancy or giving birth is known as birth control
-Methods of birth control that act prior to implantation are usually termed contraception

57. Contraception Abstinence = No sexual intercourse at all
-Most reliable way to avoid pregnancy
Sperm blockage = Prevention of sperm entry into the uterus
-Condoms, cervical caps & diaphragms
Sperm destruction = Elimination of sperm after ejaculation
-Douches and spermicidal jelly

58. Contraception Prevention of ovulation
-Birth control pills or oral contraception
-Contain analogues of progesterone, sometimes with estrogens
-Hormone-containing capsules can also be implanted beneath the skin
Prevention of embryo implantation
-Intrauterine devices (IUD)
-“Morning-after pill”, or Plan B

59. Contraception

60. Contraception (Cont.)

61. Contraception Sterilization -Vasectomy in males
-Tubal ligation in females

62. Infertility
Infertility is the inability to conceive after one year of contraception-free sexual intercourse
Female infertility
-Can occur due to a failure at any stage from oocyte production to embryo implantation
-Pelvic inflammatory disease (PID)
-Endometriosis
-Premature ovarian failure (POF)
-Hormonal imbalances

63. Infertility Male infertility
-Can occur due to a reduced number, viability, or motility of sperm in the ejaculate
-Infection
-Hormonal imbalances
-Autoimmunity to sperm
-Damage to vas deferens or seminiferous tubules

64. Treatment of Infertility
1. Hormonal treatment
-To produce high levels of FSH and LH
-Clomiphine (Clomid)
2. Assistive reproductive technology
-Artificial insemination
-Gametic intrafallopian transfer (GIFT)
-In vitro fertilization (IVF)
-Intracytoplasmic sperm injection (ICSI)