Animal Reproduction and Development

Animal Reproduction and
• Organisms must reproduce to
• Asexual reproduction - genetically
identical copies of organism – less
energy; advantage when population
needs to grow rapidly - no genetic
• Sexual reproduction increases
• Union (fertilization) of haploid cells
from 2 different parents - diploid
cell (zygote).
• Production of sex cells (gametes)
allows for various combinations of
genes through meiosis.
• Haploid egg meets haploid sperm create zygote - 2n (diploid).
• External fertilization, chemicals aid
sperm in finding egg of correct
• Internal fertilization - sperm
passes through female reproductive
tract to egg.
• Motility, viability of sperm
affected by environment of female.
• When sperm meets egg, sperm must
penetrate several layers that
surround egg.
• Humans - layers include layer of
follicle cells that surround and
support egg, and jellylike layer zona pellucida.
• Invertebrates - sperm must
penetrate jelly coat and vitelline
envelope (similar to zona pellucida).
• Enzymes found in acrosome at tip
of sperm released when sperm
contacts outer layer to digest
glycoproteins and polysaccharides
found here.
• Acrosomal process extends beyond
head of sperm to allow sperm to
contact next layer.
• Recognition of sperm and egg
proteins required at this step; if
successful, sperm and egg
membranes fuse, sperm enters egg.
• Seconds after fertilization egg
reacts to prevent fertilization by
additional sperm, membrane
potential changes, release of
calcium in egg cytoplasm, change in
egg membrane that blocks further
entry of sperm.
• Increase in calcium in cytoplasm
stimulates egg development.
• Egg pauses in 2nd meiotic division
and completes meiosis after
fertilization -polar body is
Polar body
External Development
• External development occurs
outside female’s body in water or
• Eggs of fish, amphibians fertilized
externally in water.
• Embryo develops inside egg feeding
on yolk in egg - given little parental
care, mass produced to ensure
survival of some.
• Aquatic animals release eggs and
sperm into environment where
fertilization takes place.
• Sessile (nonmotile) species often
release large numbers of gametes
at specific time of year, leaving
chance whether or not they are
• Large number increases chances of
• External fertilization cannot occur
on land.
• Very few sperm reach egg during
external fertilization, process
requires lot of eggs.
• Less care parents provide, more
eggs required to guarantee survival
of some.
• External development on land
occurs in reptiles, birds, and a few
mammals (duck-billed platypus).
• Adaptations for embryonic
development within eggs and land hard shell for protection,
extraembryonic membrane to help
provide favorable environment for
developing embryo.
• Types of extraembryonic
membranes include chorion (lines
inside of egg shell)
• Membrane moist, allows for gas
• Allantois carries out functions like
respiration and excretion; many
blood vessels - take in O2 give off
CO2, water, salt, nitrogenous
• Third membrane - amnion - encloses
amniotic fluid.
• Yolk sac encloses yolk.
• Blood vessels in yolk sac transfer
food to developing embryo.
Fig. 47.14
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Internal development
• Internal fertilization and
development occur within mother.
• Different forms including placenta
to sustain embryo.
• Nonplacental animals, development
occurs inside mother - no placenta.
• No region of exchange of materials
between blood of mother and
• Eggs relatively large - yolk supplies
developing embryo’s needs.
• Marsupials (kangaroos) nonplacental
animals - develop inside oviduct,
obtain food from yolk of egg, give
birth to live young.
• Placenta includes tissues of embryo
and mother - where exchange of
food, oxygen, waste, water take
• No direct contact between
bloodstreams of mother and
• Transport of materials - diffusion
and active transport between 2
circulatory systems.
• Eggs of placental animals small
since embryo requires small amount
of yolk for nourishment (until
placental connection develops).
• Humans have yolk sac.
• Umbilical cord attaches embryo to
placenta; composed completely of
embryonic tissue not maternal
• Umbilical cord contains umbilical
artery and vein - carry blood
into/out of placenta.
• Amnion of placental mammals
provides watery environment protects against shock.
Early development
• Fertilization to birth - single-celled
zygote must divide, grow,
differentiate into tissues and
• Begins with rapid mitotic division cleavage stage - cell does not grow
in size - rapidly divides DNA and
protein in egg cytoplasm into
smaller cells.
• Cell division starts in oviduct within
hours after fertilization.
• Early stages, mitotic division
results in 1 cell producing 2 cells,
etc - creates morula (solid ball of
• Divides to form blastula (hollow ball
of cells).
• Central cavity filled with fluid
secreted by cells (blastocoel).
• Blastula consists of outer layer
(trophoblast) - develops into
placenta; inner mass develops into
• 1st 5-10 days - dividing embryo
travels down oviduct and implants
itself into uterine wall.
• Uterus responds by increasing
blood supply - allows embryo to
receive nutrition while placenta
• Early embryo – cells totipotent have potential to develop into any
• As development continues, cells
become determined (specific fate).
• Development and differentiation of
cells affected by (partly)
environment (other cells that
surround them)
• Induction - process by which 1 cell
causes neighboring cell to take
specific developmental path.
• Further along development - less
able cell is to change fate - cell can
be taken from 1 area and placed
into another - changes fate; only
lasts for certain period of time.
• Mammals - cells of blastula not
developmentally determined as
specific tissue.
• Gastrulation, cells from blastula
form 3 germ layers - eventually give
rise to specific type of tissue.
• 3 germ layers formed - endoderm,
mesoderm, ectoderm.
• Rapid division of cells at one end of
blastula causes infolding (2-layer
• 2 germ layers – ectoderm (outside),
endoderm (inside ) -first present.
• Mesoderm cells move between
endoderm and ectoderm to form 3rd
layer in middle.
• Opening formed at point where
endoderm folds inward - blastopore.
• Cells of each layer differentiate
and specialize to form tissues,
organs, organ systems.
• Ectoderm - epidermis, nervous
system, sweat glands.
Fig. 47.9
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Endoderm - lining of digestive
tract, respiratory tract, parts of
liver and pancreas, bladder lining.
• Mesoderm - muscles, skeleton,
circulatory system, excretory
system, gonads, inner layer of skin
• Protostomes (annelids, arthropods,
mollusks) - future mouth forms
from blastopore (opening in
• Deuterostomes (echinoderms and
chordates) - blastopore forms anus
of mature organism, mouth forms
• Tissues organize themselves into
patterns of organs.
• Vertebrates - stage following
gastrulation - neurulation – 1st
parts of nervous system
determined from ectoderm.
• Notochord develops from
mesoderm along dorsal side of
• Notochord induces ectoderm above
it along exterior surface of embryo
to fold inward, forms neural tube of
tissue that will become nervous
system (spinal cord and brain).
• Neural tube defects (spina bifida)
occur when this does not happen
Fig. 47.11
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Human male reproductive system
• Sperm produced in testes - in
• Spermatogenesis - seminiferous
tubules in testes.
• Primary spermatocytes on exterior
of tubules produce spermatids become sperm - move into center of
tubules as they are produced.
• FSH, LH involved in
spermatogenesis (part of female
reproductive system)
• Epididymis on exterior of testes sperm finish maturation and are
• Vas deferens - duct that carries
sperm from testes to urethra
(connects bladder to exterior).
• Glands along path secrete liquid
(semen) - carries and provides
nutrients for sperm - include
seminal vesicles and prostate gland.
• Smooth muscle along ducts propel
sperm and secretions together.
• Testes produce male hormones like
testosterone - involved in
secondary sex characteristics
(facial hair, deepening of voice)
• Increased testosterone production
during puberty causes changes.
Fig. 46.8
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 46.8
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Human female reproductive
• Ovaries paired structures in lower
portion of abdominal cavity.
• Eggs produced in ovaries through
• Ovulation - eggs leave follicle,
carried into upper end of oviducts
(fallopian tubes)
• At birth, all eggs female will
ovulate during lifetime already
present in ovaries - paused in
meiotic prophase I.
• Eggs develop and ovulate at rate of
1 every 28 days, starting in puberty
ending in menopause.
• Ovaries produce female sex
hormones estrogen and
• Estrogen regulates secondary sex
characteristics in females development of mammary glands
(milk glands), widening of hips involved in menstrual cycle involves interactions of pituitary
gland, ovaries, and uterus.
Fig. 46.9
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
The menstrual cycle
• Involves hormones secreted by
hypothalamus (GnRH: gonadotropinreleasing hormone), pituitary (LH
and FSH) and ovary (estrogen and
• GnRH stimulates FSH and LH stimulate estrogen, progesterone
secretion (inhibits GnRH, LH, FSH)
• 4 stages in menstrual cycle:
follicular stage, ovulation, corpus
luteum stage, menstruation.
• Follicular stage, primary oocytes in
ovary along with follicular cells that
support oocyte form follicle.
• FSH (follicle stimulating hormone)
from anterior pituitary stimulates
follicle to mature.
• As follicle matures, produces
estrogen which promotes thickening
of uterine lining (in case of embryo)
- about 9-10 days long.
• When follicle is mature - increase
in LH secretion by pituitary causes egg to burst out of follicle
and ovary during ovulation.
• After ovulation, LH acts on
remaining follicular cells in ovary to
form corpus luteum - secretes
estrogen and progesterone to
support uterine thickening and
development of secretory tissue in
uterine lining.
• If fertilization does not occur corpus luteum degrades, uterine
lining regresses - menstruation.
Fig. 46.15
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• If fertilization does occur - embryo
goes through small number of cell
divisions in 1st few days as it moves
down oviduct into uterus.
• After few days embryo is
blastocyst (hollow ball of cells) implants in uterine lining to go
through embryonic development.
Fig. 46.16
Copyright © 2002 Pearson Education, Inc.å, publishing as Benjamin Cummings
• hCG (human chorionic gonadotropin)
- hormone produced that takes
place of LH to maintain corpus
• Corpus luteum continues to secrete
progesterone and estrogen to
maintain uterine wall.
• As pregnancy continues, placenta
develops, produces estrogen and
progesterone - corpus luteum
• If sperm in oviduct at time of
ovulation, fertilization takes place
in oviduct.
• 2 or more eggs released by ovaries
- possible that all will be fertilized
because of high number of sperm.
• 2 separate eggs fertilized by 2
sperm, dizygotic (fraternal) twins
• Twins related like any other
• Only 1 fertilized egg - identical
cells separate during early stages
of cleavage into 2+ independent
embryos -monozygotic (identical)
twins result.
• Triplets (and higher) can also occur
this way.
• 1st 3 months (1st trimester) embryo goes through organogenesis
- majority of body plan formed.
• When embryo resembles infant fetus.
• 2nd, 3rd trimesters pass fetus
grows, major organs develop.
• As fetus grows, hormones change in
intensity, uterine muscles become
more reactive to stretching
• Oxytocin (hormone that induces
labor) and reflexes caused by
pressure start labor - smooth
muscles in uterine wall contracts
against baby to push it downward
against cervix.
• Stretching of cervix allows uterine
muscles to push baby through
cervix and vagina.
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