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Fertilization, Part 3
Early Development - Overview
(Cleavage and Gastrulation)
Gilbert - Chapter 7 pp. 193-195
Chapter 8 pp. 211-216
Today’s Goals
• Finish Fertilization (see last lesson goals)
• Define the processes of Cleavage and
Gastrulation and compare the unique ways in
which these processes happen in various
model organisms.
• Discuss how the amount of yolk in the egg
affects the type of cleavage and gastrulation
that occur and that the amount of yolk in the
egg relates to the evolution of the organism.
• Describe specific types of cellular movements
that occur during Gastrulation including:
epiboly, convergent extension, involution
Fertilization: 4 major events
• Sperm and other as the same species
• ONE (and only one) sperm enters egg
make contact and must recognize each
• Fusion of the genetic material
• Activation of egg to begin development
Fusion of genetic material
(Sea Urchin model)
• Sperm nucleus and centriole enter egg,
but mitochondria and flagella
• Thus, mitochondrial DNA is solely from
maternal contribution
• The sperm centriole will provide part of
the mitotic spindle (all species but
Fusion of the Genetic Material
(Sea Urchin Model)
• Haploid female pronucleus* in egg
• Sperm nucleus decondenses to the
male pronucleus
– Includes breakdown of nuclear envelope
and decondensation of chromatin
• Sperm pronucleus then rotates 180º
and positions the centriole between
sperm and egg pronuclei
*Mammals - must undergo 2nd meiotic division
• Sperm centriole then acts as a
microtubule organizing center (MTOC)
which extends microtubules to form an
• Microtubules extend to female
pronucleus and pull the two pronuclei
• Fusion of these pronuclei forms the
diploid zygote nucleus
Pronuclear movements during human fertilization
Sperm Enters on
B - Egg
Meiosis 2
C - 15 hours
later, nuclei fuse
Fertilization: 4 major events
• Sperm and egg make contact and must
recognize each other as the same
• ONE (and only one) sperm enters egg
• Fusion of the genetic material
• Activation of egg to begin development
Activation of egg metabolism
• Occurs as a result of sperm entry
– “Early” responses: within seconds of
cortical granule reaction
– “Late” responses: several minutes
after fertilization occurs
Early Responses
• Ca2+ is also required for the egg to reenter the cell cycle, and activate protein
• This triggers several metabolic events
– Increase in lipid biosynthesis
– Increase in oxygen use (respiration)
Late Responses
• Activation of DNA synthesis
– Increase in pH after sperm entry (sea urchin)
and Calcium activation of MAP kinase allow
DNA synthesis to resume
• Activation of protein synthesis
– Occurs several minutes after sperm entry
– Not dependent on transcription of new mRNA’s
(uses maternal mRNA stored in egg)
– Maternal mRNA’s include histones, tubulins,
actins, morphogenetic factors
Rearrangement of the Egg
• Fertilization can cause distinct changes in the
arrangement of the egg cytoplasm
• This is especially important and easily viewed
in amphibian eggs
• Amphibian eggs have an animal pole and a
vegetal pole
– Animal pole has dark pigment
– Egg is radially symmetrical around the A-V axis
• Sperm can enter anywhere on animal half
Formation of the Grey Crescent Amphibians
• Once sperm enters, the darkly
pigmented cortical (outer) cytoplasm
rotates relative to the clear inner
cytoplasm (about 30°)
• This exposes some of the more diffuse
pigment granules in the animal half,
which appear grey - “grey crescent”
– 180° from the point of sperm entry
• As the frog develops, this area will mark
the place where gastrulation begins
Cleavage and Gastrulation
• Cleavage
– Rapid cell division
– Little or no cell growth
• Gastrulation
– Rearrangement of cells
– Formation of 3 germ layers
• Axis formation
– Dorsoventral, anterior-posterior, left-right
– Depending on species, determined during either fertilization,
cleavage or gastrulation
• Zygote becomes a multicellular organism:
Blastula (often becomes a hollow ball of cells)
• Egg cytoplasm is divided into smaller nucleated
cells called blastomeres
• Cell divisions are synchronous
• Cells do not grow - volume of the embryo stays
the same
• Rate and pattern of cell division is controlled by
maternal mRNAs (except in mammals)
• Rapid!!
– Frog can divide into 37,000 cells in 43 hours (see
next slide – Rana pipiens
Synchronous divisions
Blastula does not increase in size
Cell Cycle During Cleavage
• Rapid
– Due to the lack of G1 and G2 phases of cell cycle
• Stores of Cyclin B from maternal mRNAs
allow cells to proceed from M to S phase
without G phases
• When the stores of Cyclin B become used up,
the transcription of zygotic cyclins begins
• Mid-blastula transition
– Embryonic genes begin to become expressed
– Gap phases return to the cell cycle
– Cells no longer divide synchronously
Cells in cleavage:
NO Interphase
Normal Somatic
Patterns of Embryonic
• Determined by
– The amount and distribution of yolk in the
NEXT – Factors in the egg cytoplasm that influence
WEEK the location of the mitotic spindle
Yolk and Embryonic Cleavage
• Yolk inhibits cleavage
– If one pole of egg has less yolk than the
other pole, cleavage will occur unequally
– Amphibians
• Animal pole: little yolk
• Vegetal pole: yolk-rich
• Yolk determines where blastocoel
(cavity) can form
Holoblastic (complete)
• Ex. Sea urchin, snail, mammal
• Isolecithal yolk distribution
– Evenly distributed
– Very Little Yolk (why??)
• Results in holoblastic cleavage
– Cleavage furrow extends throughout entire egg
– Many patterns can occur depending on cleavage planes
• Radial, Spiral, Bilateral, Rotational cleavage
Holoblastic Cleavage
• Mesolecithal eggs can also undergo
holoblastic cleavage
– Eggs have uneven yolk disposition
– Animal and vegetal pole (like amphibians)
– Result is cleavage occurs through whole
egg, but after first few divisions, animal
pole divides faster than vegetal pole
Meroblastic (Incomplete)
• Occurs in Telolecithal eggs
– Dense yolk throughout most of the egg (why?)
– Ex. Birds, fish, reptiles, molluscs
• Occurs in Centrolecithal eggs
– Yolk in the center of egg
– Ex. Insects
• Only a portion of the cytoplasm is cleaved
• Cleavage furrow does not penetrate through the
whole egg
– Discoidal cleavage, bilateral cleavage, superficial
• Rearrangement of the cells of the blastula
• Establishes 3 “germ layers” of the embryo
– Ectoderm, mesoderm and endoderm
• Involves the whole embryo and must be coordinated
– If cells migrate in one location, other cells may need to move
• Occurs very differently in various species
– Depends on yolk, cleavage, gene expression
• Uses several kinds of cell movements
Sea Urchin
Cell Movements in
• Invagination:
Infolding of cells into
Cell Movements in
• Involution: Inturning
of outer cell sheet
– Outer cell sheet
spreads over internal
Cell Movements in
• Ingression:
Migration of
individual cells into
the interior
– Cells that were
epithelial become
– Migrate
Cell Movements in
• Delamination:
Splitting of one
cellular sheet into 2
Cell Movements in
• Epiboly: Movement
of epithelial sheets
of cells
– Cells spread as a
– Can occur by cells
dividing or changing
Axis Formation
• 3 crucial axes must be developed
– Anterior-posterior (head to tail)
– Dorso-ventral (back to belly)
– Left-right (lateral symmetry)
• HOW? Sample Mechanisms:`
– Site of gastrulation
– Maternal mRNA distribution
– Cleavage planes