Embryology
Cellular and Molecular
Mechanisms Involved in
Fertilization
and Development
Sea Urchin
Fertilization
Contains
digestive
enzymes
unfertilized
sea urchin egg
Animation
43-01
fertilized
sea urchin egg
Egg plasma membrane
Vitelline envelope
Sea Urchin Fertilization
Acrosomal
enzymes digest
jelly coat
Acrosomal
process forms
by actin
polymerization
Bindin on acrosomal
process contacts
receptors on vitelline
envelope
Egg plasma
membrane forms
fertilization cone to
engulf sperm head
Preventing Polyspermy
• Polyspermy = fertilization of egg by more
than one sperm
• Blocked in two ways
– Fast block to polyspermy: Change in
charge across the egg plasma membrane,
mediated by influx of sodium ions
– Slow block to polyspermy: cortical
granules release enzymes and waterabsorbing materials to form fertilization
envelope around egg, mediated by
release of stored calcium from
endoplasmic reticulum
Sea Urchin Fertilization
Cortical granules
fuse with plasma
membrane and
release
– Enzymes to catalyze
reactions that
• break bonds between
vitelline envelope and
plasma membrane
• remove bindin
receptors
• harden vitelline
envelope
fertilization envelope
– Substances that
absorb water and
raise vitelline
envelope
Applying Your Knowledge
1.
2.
3.
4.
5.
Bindin
Cortical Granule
Acrosome
Vitelline envelope
Fertilization cone
A. Which structure of the egg has a receptor for
sperm?
B. Which structure is directly responsible for
bringing the sperm head into the egg ?
C. Which part of the sperm contains enzymes
for digesting through the jelly coat
surrounding the egg?
Development
• A series of progressive changes that
generates the structures of the
organism throughout its life cycle
• Includes
– growth: change in size due to increase in
cell number and/or cell size
– differentiation: cellular specialization
– morphogenesis: establishing body and
organ shapes
Determination Precedes Differentiation
• Determination: commitment
of a cell to a particular fate,
occurs by the activation of
specific genes
• Affected by
– Cytoplasmic
segregation
– Induction
• Differentiation is
the result of
determination
Induction: process in which a substance or tissue
influences the fate of a group of adjacent cells
Optic vesicle
induces lens
placode
Optic cup
induces lens
Lens induces cornea
Transplant of tissue from dorsal lip of
blastopore induces second invagination
and formation of secondary embryo.
Embryonic Stages
• Cleavage: series of rapid cell
divisions following fertilization,
leading to
– morula: ball of cells
– blastula: hollow sphere of cells
Blastocoel =
fluid-filled
space
Blastomere =
individual cell
of blastula
Morula
Blastula
Yolk Content Affects Cleavage Patterns
Blastodisc=
layer of cells
on top of yolk
Orientation of mitotic spindles in early
cleavage is dependent on the organization
of cytoplasmic determinants in the egg
Mosaic
Development
Regulative
Development
Cytoplasmic Determinants Influence Cleavage Patterns
Cell Adhesion Molecules
Influence Blastula Organization
• Cell Adhesion Molecules (CAMs) are transmembrane cell surface proteins that bind to CAMs
on other cells.
• CAMs are involved in cell migration and producing
stable tissue structure.
• Experiments have shown that cadherin (one type of
CAM) stabilizes the frog blastula.
Embryonic Stages
Invagination
Gastrulation: cellular movements
forming three tissue layers and distinct
body axes
Sea Urchin
blood vessels, muscle,
bone, liver and heart
mesoderm
Migration
nervous system, skin
linings of digestive and
respiratory tracts
primitive gut
Sea Urchin Gastrulation
Cytoskeletal Components Directing Invagination
Embryonic Stages
Gastrulation in the Frog
Involution
Embryonic Stages
Gastrulation in the Chick
Migration
Neurulation in the Frog
Notochord, derived from
mesoderm, induces the
overlying ectoderm to form
the neural tube
Cytoskeletal
Components
Involved
with
Neurulation
Tissues of the
Frog Neurula
Neurulation in the Chick Embryo
Applying Your Knowledge
1.
2.
3.
4.
5.
Gastrula
Neurula
Blastula
Ectoderm
Mesoderm
A. Which one represents a hollow ball of cells?
B. Which tissue induces formation of neural
structures?
C. Which one involves invagination or involution
of cells?
Morphogenesis
• Pattern Formation: organization of
differentiated tissues into specific
structures
– Body segmentation in fruit flies is
controlled by
• gradients of materials established in the
egg
• actions of a series of embryonic genes
Animation
19-03
Genes Affecting Body Segmentation
Products of Maternal
Effect Genes establish
gradients in the egg
Gap genes define
broad areas and
regulate Pair-rule
genes
Pair-rule genes refine
segment locations and
regulate Segment Polarity
genes
Segment Polarity
genes determine
segment boundaries
and orientations
Homeotic genes
define the role of
each segment
Mutation in a Homeotic Gene Causes
Legs to Grow Where Antenna Should Be
Homology between
Genes Controlling
Segmentation in Fruit
Flies and Mice
Morphogenesis May Require Apoptosis
Apoptosis = Programmed Cell Death
Cellular and Molecular Mechanisms
Essential to Development
• Specificity of receptors and enzymes
• Cytoplasmic organization
• Cell movement through the action of
cytoskeletal elements and cell adhesion
molecules
• Orientation of tissue layers leading to
Induction
• Differential Gene Expression
Applying Your Knowledge
1. Differentiation
2. Growth
3. Morphogenesis
A. Which process leads to a morula?
B. Which process leads to the formation of
somites, the precursors to vertebrae?
C. Which process is directly preceded by
determination?