Chapter 36

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Chapter 36

Reproduction & Development

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Modes of Reproduction

• Animal reproduction takes many forms in animals.

• In sexual reproduction the fusion of haploid (n) gametes forms the zygote (2n).

• Asexual reproduction relies on mitotic cell division.

• Fission is the separation of a parent into two individuals, ie sea anemones.

• Budding creates new individuals from outgrowths of existing ones (hydra, stony corals).

• Fragmentation that leads to regeneration can produce new individuals (earthworms).

• In parthenogenesis an egg develops without fertilization into either haploid or diploid cells (whiptail lizard).

• Why have sexual reproduction?

• Hermaphroditism involves two sexes in one individual.

• Hormones control reproduction cycles.

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Fertilization of Egg and Sperm

• External fertilization requires a moist habitat and critical timing.

• Internal fertilization is more efficient and provides better protection to the embryo, but

requires more advance behaviors.

• Reproductive systems vary in the protection of gametes.

• In many female insects the spermatheca stores sperm.

• In many animals there exists a cloaca which is a common opening for multiple systems .

• Monogamy is rare in most animal

groups.

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Reproductive Organs & Gamete Transport

• The ovary is comprised of follicles and an oocyte.

• After ovulation, the corpus luteum forms which helps maintain the uterine lining during pregnancy.

• Wavelike contractions of the oviduct carry the egg to the uterus.

• Mammary glands are not part of the reproductive system yet play a role in nursing young.

• The internal male reproductive organs are the gonads and accessory

glands.

• The external organs are the scrotum and penis.

• The testes or gonads produce sperm and the scrotum regulates

temperature.

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Spermatogenesis

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Oogenesis

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Hormonal Control

• The coordinated actions of hormones of the hypothalamus, pituitary, and gonads, govern human reproduction.

• GnRH directs FSH and LH secretion which in turn regulates

gametogenesis.

• Male androgens and female

estrogen and progesterone also regulate gametogenesis.

• Sex hormones serve many functions.

• In males, FSH and LH are required for normal spermatogenesis.

• In females, hormone activity links the menstrual and ovarian cycles.

• In other mammals, the estrous cycle

reabsorbs the endometrium.

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Control by hypothalamus (a)

Hypothalamus GnRH Anterior pituitary

FSH LH

Inhibited by combination of estradiol and progesterone Stimulated by high levels of estradiol

Inhibited by low levels of estradiol

− +

−

1 0 9

7 8

5 6 4

3 2

1

(b) Pituitary gonadotropins in blood

LH

FSH FSH and LH stimulate follicle to grow

6

LH surge triggers ovulation Ovarian cycle

Growing follicle Maturing follicle

Corpu s luteum

Degenerating corpus luteum

Follicular phase Ovulation Luteal phase Estradiol secreted

by growing follicle in increasing amounts (c)

Progesterone and estradiol secreted by corpus luteum (d) Ovarian hormones

in blood

Estradiol

Peak causes LH surge (see )

Progesterone

Progesterone and estra- diol promote thickening of endometrium Estradiol level

very low

(e) Uterine (menstrual) cycle

Endometrium

Menstrual flow phase Proliferative phase Secretory phase

Days 28252015141050

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Mammalian Development

• 24 hours after conception or fertilization, the zygote begins to divide in a process called cleavage.

• By 2-3 days later the embryo arrives at the uterus.

• 1 week later cleavage produces a blastocyst.

• Several days after blastocyst formation the embryo implants into the

endometrium.

• The implanted embryo secretes hormones that signal its presence.

• hCG produced by the corpus luteum

stimulates secretion of progesterone and estrogen.

• Duration of pregnancy in placental

mammals correlates with body size and

maturity of young at birth.

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Mammalian Development

First trimester of 3 months includes the most radical changes.

• 1

^st

2-4 weeks the embryo receives nutrients from endometrium.

• The trophoblast grows outward and forms the placenta.

• Splitting of the embryo can result in monozygotic twins.

• Organogenesis takes place mainly in 1

^st

trimester.

• At 8 weeks the embryo becomes fetus Second trimester includes the deterioration

of corpus luteum and fetal activity.

Third trimester the fetus growth hits its maximum.

• Labor is regulated by local regulators and

hormones.

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Animal Development

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Initiation of Embryonic Development

• Across the animal kingdom, embryonic

development involves common stages that occur in a set order.

• Biologists use model organisms to study

development, ie sea urchin, frog, chicken, and roundworm.

• Fertilization involves: sperm dissolve the protective coating of the egg, molecules on

sperm surface bind to receptors on egg surface, changes on the egg surface prevent polyspermy.

• In sea urchins a special vesicle in the head of a sperm called the acrosome carry enzymes to aid in entry of the egg coat.

• Upon fusion of egg and sperm depolarization of the membrane acts a fast block to polyspermy.

• Upon fusion of egg and sperm nucleus chromosomes are organized by the mitotic spindle.

• After fertilization, rapid cell divisions occur during

cleavage that results in the blastula stage

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Morphogenesis

● The last two stages of embryonic stages are responsible for morphogenesis.

● In gastrulation, the blastula develops into 2 or 3 germ layers:

○ Ectoderm forms outer layer

○ Mesoderm forms middle layer

○ Endoderm forms innermost layer

● Each germ layer gives rise to a specific set of structures.

● In humans, the innermost cells will develop into the embryo, whereas the outer epithelium of the blastula forms the trophoblast for

implantation.

● By the end of gastrulation, the embryo is

attached to the uterine lining and germ layers are complete.

● In organogenesis, the 3 germ layers develop into organs.

● Apoptosis is programmed cell death which is

essential in development.

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