Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

CHAPTER 53

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

2

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

3

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

4

Sex Determination

• Temperature-sensitive– 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

5

6

Vertebrate Fertilization

• Internal fertilization has led to three strategies for development1. Oviparity

• Fertilized eggs are deposited outside mother’s body to complete their development

2. Ovoviviparity• Fertilized eggs are kept within mother to complete

development, young obtain food from egg yolk

3. Viviparity• Young develop within mother and obtain

nourishment from her blood

7

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

8

Vertebrate Fertilization

• Amphibians– In most species

fertilization is external

– Eggs of most species develop in water

– With some interesting exceptions

9

a) Male carries tadpoles on his back

b) Froglets develop in brood pouches

c) Female carries developing larvae on her back

d) Male holds developing froglets in his vocal pouch

10

Vertebrate Fertilization

• Reptiles– Amniotic egg

• Extraembryonic membranes• Adaptation for life on land

– Most oviparous reptiles lay eggs and abandon them

• Leathery shell on egg

11

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

12

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

13

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

14

15

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

16

17

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

18

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

19

20

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

21

22

Human Male Reproductive System

• Sperm are delivered into the epididymis for storage and maturation

• From there sperm enter vas deferens

• Then to the urethra

23

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

24

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

25

26

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

27

28

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

29

30

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

31

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

body32

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

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

34

35

36

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

37

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

38

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

39

40

41

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 blood42

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

43

44