DEVELOPMENTAL BIOLOGY
Fertilization to Gastulation
ANIMAL DEVELOPMENT
Embryology - study of development of the Embryo
5 major stages..
1. Gametogenesis - gamete production
2. Fertilization - gamete --> zygote
3. Cleavage - Zygote --> Blastula
4. Gastrulation - Blastula --> Gastrula
5. Organogenesis - Organ Formation
-i.e. Neurulation- Gastrula --> Neurula
FERTILIZATION
1. Sperm attaches the jelly coat of the egg
Acrosome cap -contains digestive enzymes that
eat away at jelly layer
FERTILIZATION
2. Sperm reach vitelline envelope
Vitelline layer-species-specific boundary involved in
sperm-egg recognition
• ensure other species cannot fertilize the egg

FERTILIZATION

3. Sperm /egg plasma
membrane fuse
Sperm nucleus enters the
egg
Fertilization occurs-sperm
nucleus and egg nucleus
form a 2N zygote
FERTILIZATION
Prevention of Polyspermy – entrance of
multiple sperm
1.Change of electrical potential of the
egg plasma membrane- fast
2.Confusion of sperm- Egg releases all
of their Ca ions
CLEAVAGE
• Cleavage-rapid succession of cell division •
doubling with each division each cell smaller
than zygote
•The produced cells named Blastomeres.

During this stage the size of the embryo does not
change, the blastomeres become smaller with
each division
BLASTULA
 continues
divisions to form a ball of 32 cells called
the morula.
 The morula continues divisions to form the hollow
blastula with up to several hundred cells.
 The cavity of the blastula is the blastocoel– fluid
filled cavity forms at the center of embryo
Vegetal Hemisphere -the lower, yolky portion of the egg;
opposite the animal hemisphere.
CLEAVAGE OF A FROG
Starfish development, unfertilized
egg.
16 blastomeres.
2 blastomeres.
32 blastomeres.
morula
4 blastomeres.
Starfish development,
nonmotile
blastula.
GASTRULATION
GASTRULATION (literal meaning - to form a
stomach) is a complex series of cell movements
Blastula (hollow ball of cells) transformed into the
Gastrula (three layered stage)
GASTULATION
1.
rearranges cells, giving them new neighbors
(and thus potentially new signals from other
cells and the environment)
2.
results in the formation of 3 GERM LAYERS
that will form the subsequent embryo:
ECTODERM, ENDODERM, and MESODERM
GASTRULATION
1. Gastrulation begins- Blastopore formed
Blastopore - midway opening on one side
of the blastula
• Site of cell migration from the surface
into the interior
• Future site of anus (Deuterostome) or
mouth (Protostome)
GASTRULATION
2. Cell migrating to
form layers
• Archenteron –
primitive gut
formed (endoderm)
• The open end of the
archenteron is
called the
blastopore
An echinoderm gastrula.
A - ectoderm;
B - blastocoel;
C - archenteron;
D - endoderm;
E - blastopore.
GASTRULATION
3.Gastrulation complete - Gastrula formed:
• Endoderm and archenteron -replace the
blastocoel
• Mesoderm - forms a layer between the ectoderm
and endoderm
• Ectoderm- forms the outer layer except for a
cluster of endodermal cells (yolk plug)
• Yolk plug- (endoderm) marks the site of the
blastopore and of the future anus
DEVELOPMENT OF EGG CELLS
4 stages of embryonic
development
1.Cleavage (Mitosis and
cytokinesis of thezygote, )
2.Patterning (organize
themselves in layers and
masses)
3.Differentiation
4.Grow
PROTOSTOME
Protostome: animals whose
development is characterized by:
the mouth is derived from the
blastopore
 Determinate is the form of
cleavage in most protostomes. It
results in the developmental fate
of the cells being set early in the
embryo development.
Molluscs


spiral determinate cleavage
Arthropod
Rotifers
DEUTEROSTOME


Deuterostome: animals
whose development is
characterized by:
Echinoderm
the mouth is not derived from
the blastopore
Chordates

Indeterminate - when the
original cell in a deuterostome
embryo divides, the two
resulting cells can be
separated, and each one can
individually develop into a
whole organism
Hemichordata
HUMAN
DnaTube.com - Human Development and Stem Cells_WMV V9.wmv
DEVELOPMENT AND STEM CELLS
Early in
development,
a group of
cells called
the inner cell
mass (ICM)
forms. These
cells are able
to produce all
the tissues of
the body.
Later in
development,
during
gastrulation,
the three
germ layers
form, and
most cells
become more
restricted in
the types of
cells that they
can produce
ORGANOGENESIS
Organogenesis is the formation of the organs
 The layers are germ layers; they have specific
fates in the developing embryo:

Endoderm: The innermost lining of digestive
tract, liver, pancreas, lungs



Mesoderm:The middle layer. Goes on to form
the blood and muscles, skeleton, gonads,
excretory system, circulatory system.
Ectoderm :The outermost. Goes on to form the skin
and nervous system
Human reproduction is an inefficient process:

~ 50% of concepti do not implant
(implantation 8-10 dpf, Heart beat at 21 dpf).

a further ~30% die and abort after
implantation.