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Early Development in
Selected Invertebrates
Gastrulation in Molluscs
Chapter 5
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Early Development in Selected
Invertebrates
• Fertilization
– initiating stage in development
• Zygote
– genetically fused single cell (2n)
• Cleavage
– stage of rapid cell division, dividing the
cytoplasm of a fertilized egg into numerous
cells
• Gastrulation
– process whereby cells move to different parts
of the embryo and acquire new neighbor cells 2
Cleavage
• Cleavage
– Process of developing multicellular
organisms
– Series of mitotic division
– Blastomeres – cleavage stage cells
– Most species (except mammals) control this
division with oocytes storing up proteins and
mRNAs (maternal factors)
• Later controlled by nuclear genome
• Cytoplasmic volume does not increase
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Cleavage
• Cleavage
– 1st division into half …. 2nd division into
quarters … 3rd eighths…..4th so on…..
– Division without cytoplasm increasing
volume
• Accomplished by abolishing gap periods (G1 &
G2)
– Nuclear division occurs rapidly
• Frog egg – 37,000 cells/43hrs
• Drosophila – 50,000 cells/12hrs
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Early Development Overview
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Early Development Overview
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Cleavage
• From fertilization to cleavage
– MPF (mitosis-promoting factor)
• Causes transition from fertilization to cleavage
• Responsible for resuming mitotic division
– Biphasic cell cycle (2 steps: M-to-S)
• M (mitosis)
– MPF factors 
• S (DNA synthesis)
– MPF factors 
• Shift between M to S
– MPF factors degraded
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Fertilization to Cleavage
• From fertilization to cleavage
– What causes MPF levels to cycle?
• MPF is made up of 2 subunits
– Cyclin B – large subunit
• Shows cyclical behavior (key to mitotic
regulation)
• Accumulation in S phase
• Degraded after cells have reached the M phase
– Cyclin dependent kinase (cdc2) – small
• Activates kinase by phosphorylating target
proteins
• i.e. histones, nuclear envelope lamin proteins, and
cytoplasmic myosin subunits
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Fertilization to Cleavage
5.2 Cell cycles of somatic cells and early blastomeres
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Fertilization to Cleavage
•
Mid-blastula transition
–
Point where cytoplasmic components are used
up and the nucleus begins to synthesize new
proteins
1. G1 – G2 (gap stages) are now added back to cell
cycle
•
•
2.
Synchronicity of cell division is lost
•
3.
Frog – at 12th cleavage cycle
Drosophila – G2 after 14th, G1 after 17th
Different cells synthesize different regulators of MPF
New mRNAs are transcribed
•
Many are necessary for gastrulation even though the
embryo is not able to initiate gastrulation
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Cytoskeleton Mechanisms of Mitosis
•
Cytoskeleton mechanisms of mitosis
– 2 coordinated processes
1. Karyokinesis
– Mitotic division of the cell’s nucleus
– Uses Mitotic spindles made up of microtubules
(tubulin)
2. Cytokinesis
–
–
Contractile rings made up of microfilaments
(actin)
Cleavage furrow uses perpendicular tightening
microfilament ring
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Cytoskeleton Mechanisms of Mitosis
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Cytoskeleton Mechanisms of Mitosis
5.2 Role of microtubules and microfilaments in cell division
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Patterns of Embryonic Cleavage
•
Patterns of Embryonic Cleavage
– Patterns are species specific
– Determined by 2 major parameters
1. Amount and distribution of yolk protein
within the cytoplasm
2. Factors in the egg cytoplasm that
influence the angle of the mitotic spindle
and the timing of its formation
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Patterns of Embryonic Cleavage
• Amount and distribution of yolk
– Determines where cleavage can occur
– Determines relative size of blastomeres
– One pole – yolk free
• Called Animal Pole
• Cell division fast
– Other pole – yolk rich
• Called Vegetal Pole
• Inhibits cleavage
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Patterns of Embryonic Cleavage
• Classification
of cleavage
symmetry and
patterns
5.3 Summary of
the main patterns
of cleavage
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Patterns of Embryonic Cleavage
•
Holoblastic Cleavage – cleavage furrow
through the entire egg
–
Isolecithal – equally spaced yolk
•
•
Cleavage furrows through the entire egg
4 major patterns of cleavage:
–
–
–
–
–
Radial
Spiral
Bilateral
Rotational
Mesolecithal
•
•
moderate vegetal yolk distribution
Displaced radial
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Patterns of Cleavage
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Patterns of Embryonic Cleavage
• Meroblastic cleavage
– Portion of cytoplasm cleaves over large yolk
accumulation
– Cleavage does not penetrate the yolky portion
• Impedes membrane formation
– Telolecithal – free of yolk
• Bilateral
• Discoidal
• Birds, fishes, reptiles
– Centrolecithal – yolk in center
• insects
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Gastrulation
• Remember blastula consists of numerous cells,
their position was established in cleavage
• Cells are now given new positions/neighbors
• Will form:
– Endoderm
– Mesoderm
– Ectoderm
• 3 germ layers
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Gastrulation
• Involves combinations of cell movement
– Organisms and patterns vary
– Basic type of movement common:
• Invagination – infolding
• Involution – inturning/inward movement of
expanding outer layer becomes mesenchymal
• Ingression – migration of individual cells from the
surface to the interior
5.4 Types of cell
movements during
gastrulation
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Gastrulation
– (cont) Basic type of movement common:
• Delamination – splitting of one cellular sheet
into 2 parallel sheets
• Epiboly – movement of epithelial sheet (usually
ectoderm) as a unit to enclose a deeper layer
5.4 Types of cell
movements during
gastrulation
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Cell Specification and Axis Formation
• Cell fate specified by cell-to-cell
interaction
• Asymmetric distribution of transcribed
molecules:
– Molecules bound to cytoplasm – given to
cell that bonds to that part
– Molecules are actively transported along the
cytoskeleton to a particular cell
– Molecule becomes associated with specific
centrosome
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Cell Specification and Axis Formation
• Once asymmetry occurs – cells
communicate to neighboring cells
(Paracrine, Juxtacrine interactions)
• Must develop 3 crucial axes
– Anterior-posterior axis
– Dorsal-ventral axis
– Right-left axis
• Some organs not symmetrical
– heart, liver….
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Cell Specification and Axis Formation
5.5 Axes of a bilaterally
symmetrical animal
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