1. CHAPTER 53
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2. Reproductive Strategies
Most animals reproduce sexually
Requires meiosis to produce haploid gametes (sperm and eggs)
Gametes united by fertilization to produce the diploid zygote
Zygote develops by mitosis into a new multicellular organism

3. Reproductive Strategies
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 – change sex
Protogyny – female-to-male change
Protandry – male-to-female change

5. Sex Determination Temperature-sensitive Genetic
In many fish and reptiles
Has evolved many times
Genetic
XX or XY
Humans
Embryonic gonads indifferent for first 40 days
SRY gene on Y converts gonads to testes
Testosterone promotes male development

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

8. 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
Gives birth to a few, well-developed offspring

9. Vertebrate Fertilization
Amphibians
In most species fertilization is external
Eggs of most species develop in water
With some interesting exceptions

10. Male carries tadpoles on his back
Froglets develop in brood pouches
Female carries developing larvae on her back
Male holds developing froglets in his vocal pouch

11. Vertebrate Fertilization
Reptiles
Amniotic egg
Extraembryonic membranes
Adaptation for life on land
Most oviparous reptiles lay eggs and abandon them
Leathery shell on egg

12. Vertebrate Fertilization
Birds
Amniotic egg
Extraembryonic membranes
Adaptation for life on land
All birds practice internal fertilization
Hard calcareous shell on egg
Birds are endotherms
Must incubate eggs to keep them warm

13. Vertebrate Reproductive Cycles
Female reproductive cycles involve periodic release of a mature ovum (ovulation)
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

14. Reproduction in Mammals
Mammals are of three types
Monotremes are oviparous
Lay eggs
Marsupials are viviparous
Give birth to incompletely developed fetuses, which mature in mother’s pouch
Placentals are viviparous
Retain young in uterus for long period of development
Nourished by placenta from mother’s blood

16. Human Male Reproductive System
Seminiferous tubules
Sites of sperm production
Leydig cells produce testosterone
Converts indifferent external genitalia into penis and scrotum
Shortly before birth, the testes descend into the scrotum because sperm need cooler temperature to develop

18. Human Male Reproductive System
Gamete production
Spermatogonium (germ cells) divides by mitosis to produce two diploid cells
One later undergoes meiosis, the other remains as a spermatogonium
Primary spermatocyte – diploid cell that begins meiosis
Primary spermatocyte undergoes meiosis I to produce two haploid secondary spermatocytes
Each secondary spermatocyte undergoes a second meiotic division to produce 2 haploid spermatids
Total of 4 haploid spermatids produced

19. Human Male Reproductive System
Seminiferous tubules also contain supporting cells called Sertoli cells
Nurse developing germ cells
Help convert spermatids into spermatozoa (sperm) by engulfing their extra cytoplasm

21. Human Male Reproductive System
Sperm structure
Head – contains a nucleus
Capped by the acrosome, which aids in penetration of the egg
Body – consists of many mitochondria
Provide energy
Centriole acts as a basal body for the flagellum
Tail – consists of a flagellum
Provides locomotion

23. Human Male Reproductive System
Sperm are delivered into the epididymis for storage and maturation
From there sperm enter vas deferens
Then to the urethra

24. Human Male Reproductive System
Semen
Complex mixture of fluids and sperm
Seminal vesicles
Produce a fructose-rich fluid, which makes up 60% of semen volume
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

25. Human Male Reproductive System
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

27. Human Male Reproductive System
Hormonal control
Hypothalamic GnRH stimulates anterior pituitary to secrete FSH and LH
FSH stimulates the Sertoli cells to facilitate sperm development
LH stimulates the Leydig cells to secrete testosterone
Controlled by negative feedback

29. Human Female Reproductive System
Ovaries develop more slowly than the testes
Female embryo develops a clitoris and labia majora
In the absence of testosterone
Homologous structures to penis and scrotum
Ovaries contain microscopic structures called ovarian follicles
Each follicle contains a potential egg cell called a primary oocyte

31. Human Female Reproductive System
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

32. Human Female Reproductive System
Ovarian events
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 of the endometrium
Primary oocyte completes meiosis I
Produces one large secondary oocyte and one tiny polar body

33. Ovarian events Follicular phase Secondary oocyte begins meiosis II
However, its progress is arrested in metaphase II
Does not complete the second meiotic division unless it becomes fertilized in the Fallopian tube

34. Human Female Reproductive System
Ovarian events
Ovulation
Estrogen stimulates the anterior pituitary to secrete LH about midcycle
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
Fusion of the nuclei from the ovum and the sperm produces a diploid zygote

37. Human Female Reproductive System
Ovarian events
Luteal phase
LH stimulation transforms the Graafian follicle into the corpus luteum
Corpus luteum secretes estrogen and progesterone
Exert negative feedback on FSH and LH
Cause endometrium to become more vascular and glandular
Secretory phase of endometrium
Prepare uterus for embryo implantation

38. Human Female Reproductive System
Ovarian events
Luteal phase
In the absence of fertilization, the corpus luteum degenerates due to decreasing levels of FSH and LH
Built-up endometrium is sloughed off with accompanying bleeding
Menstrual phase of endometrium

39. Human Female Reproductive System
Ovarian events
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

42. 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

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