Human Reproduction and Development

Chapter 42

Modes of Reproduction1. Sexual reproduction

– Meiosis (gamete formation) followed by fertilization

– Offspring show genetic variation

2. Asexual reproduction– Mitosis– Single parent produces offspring– Offspring are genetically identical

3. Hermaphrodites

-produce eggs and sperm, some can self-fertilize

Ex.-Tapeworms…why is this an advantage?

Cost of Sexual Reproduction

• Specialized cells and structures must be formed (to deliver/receive sperm)– Reproductive timing; pheromones,

• Visual signals, special courtship, and parental behaviors can be costly

• Nurturing developing offspring, either in egg or body, requires resources from mother

Costs of Sexual Reproduction

Gamete Formation

The Reproductive System

• Gonads – primary sex organs– Testes in males– Ovaries in females

• Gonads produce gametes (sex cells) and secrete hormones– Sperm – male gametes– Ova (eggs) – female gametes

Male Reproductive System

• Accessory organs(Seminal vesicle, Prostate gland, Bulbourethral gland)

• External genitalia (penis, Scrotum: testes, epididymis)

PROSTATE GLAND EJECULATORY DUCT

URETHRA

SEMINAL VESICLE

BULBOURETHRAL GLAND

VAS DEFERENS

EPIDIDYMIS

PENIS

TESTIS

urethra

erectile tissue

urinary bladder

anus

anterior posterior

Testes• Spermatogenesis

• Seminiferous tubules– Tightly coiled structures– Function as sperm-

forming factories

• Sperm travels from testes to epididymis

• Produce testosteroneFigure 16.1

Epididymis

• Comma-shaped,

tightly coiled tube

• Function: to mature and store sperm cells (at least 20 days)

• Expels sperm with the contraction of muscles in the epididymis walls to the vas deferens

Vas Deferens• Carries sperm from epididymis to the ejaculatory duct• Vasectomy – cutting of the vas deferens at the level

of the testes to prevent transportation of sperm

PROSTATE GLAND EJECULATORY DUCT

URETHRA

SEMINAL VESICLE

BULBOURETHRAL GLAND

VAS DEFERENS

EPIDIDYMIS

PENIS

TESTIS

urethra

erectile tissue

urinary bladder

anus

anterior posterior

Urethra

• Extends from the base of the urinary bladder to the tip of the penis

• Carries both urine and sperm

• Sperm enters from the ejaculatory duct

Semen• Mixture of sperm and accessory gland

secretions

• Advantages of accessory gland secretions– Seminal Vesicles: Fructose provides energy

for sperm cells– Prostate Gland: raises pH of semen helps

neutralize the acidic environment of vagina– Bulbourethral gland: lubricating mucus clears

urethra of urine– Semen inhibits bacterial multiplication– Elements of semen enhance sperm motility

External Genitalia• Scrotum

– Divided sac of skin outside the abdomen– Maintains testes at 3°C lower than normal

body temperature to protect sperm viability

• Penis– Delivers sperm into the female reproductive

tract– Internally, three areas of spongy erectile

tissue around the urethra

Sperm Formation: Seminiferous Tubules

vas deferens

epididymis

testis

penis

seminal vesicle

prostate gland

bulbourethral gland

urethra

semini ferous tubule

Spermatogenesis• Production of sperm cells

• Begins at puberty and continues thru-out life

• In the seminiferous tubules (inside testes)

• Spermatogonium (2n) divides by mitosis to form primary spermatocyte (2n)

• Meiosis produces haploid spermatids

• Spermatids mature to become sperm

• Spermatogenesis takes 64 to 72 days

Sperm Formation (in seminferous tubule in testes)

Sertoli cell

spermato-gonium (diploid)

primary spermatocyte

MITOSIS MEIOSIS I MEIOSIS II

part of the lumen of one seminiferous tubule

immature sperm

(haploid)late

spermatidsecondary

spermatocyte

early spermatids

lumen

head midpiece with mitochondria

tail with core of microtubules

Anatomy of Mature Sperm • The only human flagellated cell

• DNA is found in the head

• Little cytoplasm

• 48-72 hour lifespan (up to 5 days)

Figure 16.5b

Testosterone Production• Produced in Testes

• Functions of testosterone– Stimulates reproductive organ development– Underlies sex drive– Causes secondary sex characteristics

• Deepening of voice• Increased hair growth • Enlargement of skeletal muscles• Thickening of bones

Regulation of Male Androgens (Sex Hormones)

Female Reproductive System

• Ovaries• Duct System

– Uterine tubes (fallopian tubes)– Uterus– Vagina– Urethra is NOT part of reproductive system

in females

• External genitalia

Female Reproductive Organs

Female Reproductive Organs

OVARY

OVIDUCT

CLITORIS

LABIUM MINOR

LABIUM MAJOR

VAGINA

anus

opening of cervix

UTERUS

MYOMETRIUM

ENDOMETRIUMurinary bladder

urethra

Ovaries• Produces eggs

• Composed of ovarian follicles (sac-like structures)

• Ovulation – egg is mature and the follicle ruptures

– Occurs every 28 days

• Ruptured follicle is transformed into a corpus luteum

Figure 16.7

Fallopian Tubes (Oviducts)

• Receive the ovulated oocyte

• Cilia move the oocyte towards the uterus (takes 3–4 days)

• Provide a site for fertilization

• Attaches to the uterus

• Does not physically attach to the ovary

Uterus• Hollow organ• Cervix – narrow outlet that protrudes into the vagina• Functions of the uterus

– Receives a fertilized egg– Retains the fertilized egg– Nourishes the fertilized egg

• Endometrium– Inner layer– Allows for implantation of a fertilized egg– Sloughs off if no pregnancy occurs (menses)

Vagina

• Extends from cervix to exterior of body

• Serves as the birth canal

• Receives sperm during sexual intercourse

Oogenesis

• Making eggs

• Total supply of eggs are present at birth

• Ovulation begins at puberty

• Many eggs “die off” in earlier stages of development, before ovulation occurs

• Reproductive ability ends at menopause

• Oocytes are matured in developing ovarian follicles

Oogenesis• Girl is born with 2 million primary oocytes already

in ovaries

• Each oocyte has entered meiosis I and stopped

• Meiosis resumes, with the first menstrual cycle

• Meiosis completed only after fertilization

• Unfertilized egg: lifespan 24 hours

Ovarian Cycle

primordial follicle

e Ovulation. Mature follicle ruptures and releases the secondary oocyte and the first polar body.

a Primary oocyte, not yet released from meiosis I. A cell layer is forming around it. A follicle consists of the cell layer and the oocyte.

f A corpus luteum forms from remnants of the ruptured follicle.

b A transparent and somewhat elastic layer, the zona pellucida, starts forming around the primary oocyte.

first polar body

secondary oocyte

d Mature follicle. Meiosis I is over. The secondary oocyte and first polar body are now formed.

ovary

c A fluid-filled cavity (antrum) starts forming in the follicle’s cell layer.

g The corpus luteum breaks down when the woman doesn’t get pregnant.

Oogenesis

Figure 16.10

Menstrual Cycle

• Function: produce an environment that is

hospitable to the growing embryo

• Lining of uterus builds up cyclically

• Bleeding is a by-product resulting from no

pregnancy

• The fertile period for a human female occurs

on a cyclic basis

Menstrual Cycle• Cyclic changes of the endometrium

– Regulated by cyclic production of estrogens and progesterone

• Menstrual cycle lasts about 28 days

– Day 1: first day of bleeding

– Ovulation around day 14

• Stages of the menstrual cycle– Menses: endometrium is sloughed– Follicular phase: prior to ovulation– Luteal phase: after ovulation

Figure 16.12a, b

Hormonal Control of the Ovarian and Uterine Cycles

Figure 16.12c, d

Hormonal Control of the Ovarian and Uterine Cycles

Menstrual Cycle Overviewhypothalamus

anterior pituitary

LH

midcycle peak of LH (triggers ovulation)

Blood levels of FSH (purple) and LH (lavender)

FSH LH LH

estrogens progesterone, estrogen

Blood levels of estrogens (light blue) and progesterone (dark blue)

growth of follicle

FOLLICULAR PHASE LUTEAL PHASE

menstruation

endometrium of uterus

Days of one menstrual cycle (using 28 days as the average duration)

ovary

ovulationcorpus luteum

GnRH

estrogens progesterone, estrogen

Hormone Production by the Ovaries

• Estrogens– Produced by follicle cells– Cause secondary sex characteristics

• Enlargement of accessory organs• Development of breasts• Appearance of pubic hair• Increase in fat beneath the skin• Widening and lightening of the pelvis• Onset of menses

Hormone Production by the Ovaries

• Progesterone– Produced by the corpus luteum– Production continues until LH diminishes in

the blood– Helps maintain pregnancy

Female Hormonal ControlHYPOTHALAMUS

PITUITARY

c Blood level of estrogens rises, will stimulate a surge in LH secretion

a GnRH prods anterior lobe cells to secrete FSH and LH

d Midcycle surge of LH triggers ovulation, then formation of corpus luteum

f The rise in progesterone, estrogen in blood will inhibit FSH, LH secretion during last phase of cycle

e Progesterone, estrogen secreted by corpus luteum will maintain the endometrium if pregnancy occurs

b In ovary, FSH and LH promote follicle growth and oocyte maturation, estrogen production, priming of endometrium, other reproductive events

(+)

(+)

(–)

(–)

Cycle Overview• GnRH secretion affects LH and FSH

secretion by pituitary

• LH and FSH affect follicle maturation

• Estrogen and progesterone from ovary affect uterus

Copyright ©2000 BMJ Publishing Group Ltd.

Wilcox, A. J et al. BMJ 2000;321:1259-1262

Probability of Pregnancy

What day on a regular cycle is there the highest probability of pregnancy?

Fertilization • Sperm penetrates

to egg cytoplasm

• Secondary oocyte

undergoes meiosis

II; forms mature

egg

• Egg nucleus and

sperm nucleus fuse

to form diploid

zygote

zona pellucida

follicle cell

egg nucleus

Fertilization

ovulation

oviduct

ovary

uterus

opening of cervix

vagina

Birth Control Options