Download BIOSC 041

Processes and Stages of Development
BIOSC 041
Reproduction & Development
Reference: Lecture; Chapters 46 & 47
The Processes of Development
v Development
§  A series of progressive changes in shape, form, function
§  Begins with fertilization of egg with sperm à zygote
§  Continues throughout an organism’s life cycle
v Development consists of
§  Growth
§  Determination
§  Differentiation
§  Morphogenesis
v The processes of development
v Influence of cytoplasmic segregation and induction in
cell determination
v Stages of development
1.  Cytoplasmic determinants (egg and sperm)
2.  Fertilization
3.  Cleavage: repackaging the cytoplasm
4.  Gastrulation: producing the body plan
5.  Neurulation: initiating the nervous system
6.  Extraembryonic membranes
The Processes of Development
v Growth
§  Occurs by cell division and/or expansion
§  Starts with zygote
§  Repeated mitotic cell divisions increase cell #
§  May or may not increase actual size of embryo
The Processes of Development
The Processes of Development
v The fertilized egg is totipotent
§  it can give rise to all other cell types of the organism
v Morphogenesis
§  The shaping of the multicellular body and its organs
§  Results from pattern formation
§  Organization of differentiated tissues into specific
structures (e.g., limbs, organs)
§  Influenced by signal transduction pathways that control:
§  Cell movement within developing embryo (animals)
§  Apoptosis (programmed cell death)
§  Ultimately under genetic control
v Determination
§  Commitment of a cell to a particular fate
§  Loss of totipotency
§  Influenced by the extracellular environment and the cell
itself acting on the its own genome
v Differentiation
§  Follows determination
§  Results from differential gene expression
§  Cells become structurally and functionally specialized
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Cell Determination: Cytoplasmic Segregation & Induction
v  Chemical signals control cell differentiation
v  Two signal mechanisms are known
1.  Cytoplasmic segregation
§  A factor (signal) is unequally distributed in the
zygote and ends up in some daughter cells but not
others
§  These cytoplasmic determinants are often
transcription factors (proteins that bind DNA to
control gene expression)
2.  Induction
Cell Determination: Cytoplasmic Segregation
v Polarity occurs early in development
§  Establishes major body axes
§  Anterior–posterior axis (head – tail)
§  Dorsal–ventral axis (back – belly)
§  The earliest step in differentiation
v Materials are distributed unequally in egg
and zygote
§  Dividing cells receive unequal amounts
§  Generates polarity in early embryo
§  A factor (signal) is produced and secreted by some
cells to induce other cells to differentiate
Cytoplasmic Segregation: Asymmetry in the Early Embryo (Part 1)
Cytoplasmic Segregation: Asymmetry in the Early Embryo (Part 2)
Cell Determination: Induction
The Stages of Animal Development
Example: development of the vertebrate eye
1.  Developing forebrain bulges out at both sides & forms optic vesicles
2.  Optic vesicles signal surface tissue à induce it to form lens placodes
3.  Lens placodes differentiate into lenses
4.  Developing lens induces surface tissue to develop into the cornea
1.  Cytoplasmic determinants: Eggs and Sperm are not
created equal
2.  Fertilization: the onset of development
3.  Cleavage: Repackaging the Cytoplasm
4.  Gastrulation: Producing the Body Plan
5.  Neurulation: Initiating the Nervous System
6.  Extraembryonic Membranes
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1. Sperm & Egg differ in size
2. Fertilization
v Eggs contain cytoplasmic determinants
§  Regulate expression of genes affecting developmental fate
of cells
§  Strong influence on early development
§  Cytoplasmic factors set anterior/posterior, and dorsal/
ventral axes
v  Union of gametes to form zygote (usually diploid)
The acrosomal reaction (Fig 47.3)
2. Fertilization
v  Contact of (only 1!) sperm with egg’s surface
§  Initiates metabolic reactions within egg that trigger onset
of embryonic development
1.  Acrosomal reaction: 1-3 seconds after contact
§  Sperm releases hydrolytic enzymes that digest material
surrounding the egg
§  Depolarization of egg cell membrane = fast block to
polyspermy
2.  Cortical reaction: rise in Ca2+ stimulates cortical granules
1 Contact. The
sperm cell
contacts the
egg’s jelly coat,
triggering
exocytosis from the
sperm’s acrosome.
2 Acrosomal reaction. Hydrolytic
enzymes released from the
acrosome make a hole in the
jelly coat, while growing actin
filaments form the acrosomal
process. This structure protrudes
from the sperm head and
penetrates the jelly coat, binding
to receptors in the egg cell
membrane that extend through
the vitelline layer.
3 Contact and fusion of sperm 4 Entry of
and egg membranes. A hole
sperm nucleus.
is made in the vitelline layer,
allowing contact and fusion of
the gamete plasma membranes.
The membrane becomes
depolarized, resulting in the
fast block to polyspermy.
Sperm plasma
membrane
Sperm
nucleus
5 Cortical reaction. Fusion of the
gamete membranes triggers an
increase of Ca2+ in the egg’s
cytosol, causing cortical granules
in the egg to fuse with the plasma
membrane and discharge their
contents. This leads to swelling of the
perivitelline space, hardening of the
vitelline layer, and clipping of
sperm-binding receptors. The resulting
fertilization envelope is the slow block
to polyspermy.
to release contents into perivitelline space
Acrosomal
process
Basal body
(centriole)
Fertilization
envelope
Sperm
head
Actin
Acrosome
Jelly coat
(Cumulus)
Sperm-binding
receptors
Fused plasma
Cortical membranes
granule
Perivitelline
Hydrolytic enzymes
space
Vitelline layer (zona
pelucida)
Egg plasma
membrane
Cortical granule
membrane
EGG CYTOPLASM
2. Fertilization
Types of Fertilization
v These changes cause the formation of a fertilization envelope
§  Functions as a slow block to polyspermy
1.  Internal fertilization
§  Male transfers sperm via a penis or other organ into
female’s body
2.  External fertilization
§  “Spawning”- fishes, amphibians, reptiles
Salmon spawning video: http://www.youtube.com/watch?v=SOllUTBYZtE
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3. Cleavage
The Mammalian Zygote Becomes a Blastocyst (Part 1)
v Rapid DNA replication and mitosis
§  Little actual growth
§  Little gene expression
v Formation of blastula
§  Large zygote à morula à blastula with blastocoel
§  Blastomeres = Individual cells of blastula
v Influenced by:
§  Yolk (impedes cleavage furrow- cleavage often incomplete/superficial)
§  Maternal cytoplasmic determinants
§  Mitotic spindle orientation determines cleavage plane
§  Radial pattern à frogs, sea urchins
§  Spiral pattern à molluscs (snail shells)
§  Rotational pattern à (mammals = us)
Cleavage in the Mammalian Embryo
Cleavage in Xenopus (frog): Fate Maps
v Specific blastomeres generate specific tissues & organs
§  Determined by unequal complements of nutrients and
cytoplasmic determinants in the egg
Day 6-7
Day 1-2
Day 5
Day 2-3
Day 3-4
4. Gastrulation: Producing the Body Plan (Review)
Gastrulation can look very different between groups
v Gastrulation
§  Blastula transforms into embryo with three germ layers,
and defined body axes
v Three germ layers form:
§  Endoderm à inner germ layer
§  Gives rise to digestive, circulatory, and respiratory tracts
§  Mesoderm à middle germ layer
§  Bone, muscle, liver, heart, and blood vessels
§  Ectoderm à outer germ layer
§  Epidermis, nervous system
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Gastrulation in Sea Urchins
Gastrulation in the Frog Embryo
*Primary embryonic organizer
The dorsal lip is very important to development
Gastrulation in the Chick Embryo
5. Organogenesis and Neurulation
Neurulation in the Frog Embryo (Part 1)
v Gastrulation produces an embryo with three germ layers
v Organogenesis occurs next- formation of organs and organ
systems
v Neurulation occurs early in organogenesis- begins the
formation of the nervous system in vertebrates
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Neurulation in the Frog Embryo (Part 2)
6. Extraembryonic Membranes
v Extraembryonic membranes
§  Originate from germ layers of embryo
§  Function in nutrition, gas exchange, waste removal
v In the chicken,
§  Yolk sac forms first
§  Extension of the endodermal tissue of the hypoblast
§  Constricts at the top to create a tube continuous with
embryo gut
§  Yolk is digested by the endodermal cells of the yolk sac
§  Nutrients transported through blood vessels lining outer
surface of yolk sac
Chick Embryo Extraembryonic Membranes
Human Embryo Development
The Mammalian Placenta
Summary of Development/Reproduction
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Useful video resources from Khan Academy
Stages of development (video, ~10 min):
https://www.khanacademy.org/partner-content/crash-course1/crashcourse-biology/v/crash-course-biology-115
Genes (such as Hox) that regulate development (video, ~10 min):
https://www.khanacademy.org/partner-content/crash-course1/crashcourse-biology/v/crash-course-biology-116
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