Download Fertilization and Development

Fertilization and Development
32 weeks (8 months)
AP Biology
 Fertilization brings the haploid nuclei of

sperm and egg together, forming a diploid
zygote.
The sperm’s contact with the egg’s surface
initiates metabolic reactions in the egg that
trigger the onset of embryonic
development:
 Acrosomal Reaction
 Cortical Reaction
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The Acrosomal Reaction
 The acrosomal reaction is triggered when
the sperm meets the egg.
 The acrosome at the tip of the sperm
releases hydrolytic enzymes that digest
material surrounding the egg.
 Gamete contact and/or fusion depolarizes
the egg cell membrane and sets up a fast
block to polyspermy.
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The Cortical Reaction
 Fusion of egg and sperm also initiates the


cortical reaction:
This reaction induces a rise in Ca2+ that
stimulates cortical granules to release their
contents outside the egg.
These changes cause formation of a
fertilization envelope that functions as a
slow block to polyspermy.
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 Fertilization is followed by cleavage, a


period of rapid cell division without growth.
Cleavage partitions the cytoplasm of one
large cell into many smaller cells called
blastomeres.
The blastula is a ball of cells with a fluidfilled cavity called a blastocoel.
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(a) Fertilized egg
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(b) Four-cell stage
(c) Early blastula
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(d) Later blastula
Cleavage
 zygote  morula
solid ball stage
 blastula
hollow fluid-filled ball stage
zygote
gastrulation
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Biology
morula
blastula
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Gastrulation
 Gastrulation rearranges the cells of a

blastula into a three-layered embryo, called
a gastrula, which has a primitive gut.
The three layers produced by gastrulation
are called embryonic germ layers:
The ectoderm forms the outer layer
 The endoderm lines the digestive tract
 The mesoderm partly fills the space between
the endoderm and ectoderm.

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Gastrulation: cells change size & shape
 ectoderm (outer lining)
skin, nervous system
 endoderm (internal
lining) digestive,
respiratory, reproductive,
urinary
 mesoderm (middle
tissues) muscle, blood &
bone
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ECTODERM
Epidermis of skin and its
derivatives (including sweat
glands, hair follicles)
Epithelial lining of mouth
and anus
Cornea and lens of eye
Nervous system
Sensory receptors in
epidermis
Adrenal medulla
Tooth enamel
Epithelium of pineal and
pituitary glands
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MESODERM
Notochord
Skeletal system
Muscular system
Muscular layer of
stomach and intestine
Excretory system
Circulatory and lymphatic
systems
Reproductive system
(except germ cells)
Dermis of skin
Lining of body cavity
Adrenal cortex
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ENDODERM
Epithelial lining of
digestive tract
Epithelial lining of
respiratory system
Lining of urethra, urinary
bladder, and reproductive
system
Liver
Pancreas
Thymus
Thyroid and parathyroid
glands
Development
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Big Questions:
1. How does a multicellular organism
2.
3.
4.
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develop from a zygote?
How is the position of the parts of an
organism determined?
How does differentiation of cell type
occur in organisms?
How are genetics and development
connected? Environment?
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Development is Regulated
 The development of

an organism is
coordinated by
sequential changes in
gene expression.
Multicellular
organisms must
regulate:
Cell division
 Differentiation
 Morphogenesis

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Early Developmental Pathways
 Cytoplasmic
Determinants

unequal distribution of
mRNA and proteins
from mother
 Embryonic Induction:

Local signaling by
nearby cells send
cells down a specific
developmental path.
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Pattern Formation
 Cells need to establish their position in

the developing organism. (head/tail, right
/left etc.)
Cytoplasmic determinants and induction
lead to gradient morphogens.
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Cellular Differentiation
 Germ lines established
 Cell type already


determined
Differentiation
accomplished by the
expression of cell typespecific proteins.
There is normally no
turning back.
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Differentiation Control
 Genes must be both
turned on and turned off
at appropriate times and
locations.
 Regulated by internal
and external cues
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Internal Cues
 DNA packing, micro RNA’s and
epigenetics can switch genes ON or
OFF.
 Transcription factors must be present
and can be either stimulatory or
inhibitory.
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External Cues (outside the cell)
 External cues signal to cells, causing
cells to alter their gene expression.
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Environmental Cues
 The presence of particular molecules
and conditions in the local environment
is required for development to proceed
properly.

Ex. The role of temperature and
moisture in seed development.
Why do you water a seed
after you plant it? Hmm…
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Environmental Cues
 Changing the environment around the
cells can alter gene expression.
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Apoptosis
 “Programmed cell death”: Important
role in defining borders and openings
in the developing
organism.
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Homeobox (HOX) Genes
 A family of
related genes
that serve as
master
regulators of
animal
development
in all animals
on the planet.
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Experimental Evidence
 Mutations in normal developmental
pathways lead to malformations in
embryonic development
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Experimental Evidence
 Transplantation Experiments: Moving
regions of a developing embryo affects
normal pattern formation.
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