By Natasha Guenther, Brea Altoya, and Bianca (I can’t
spell her last name so I’m leaving it out)
Key Concepts
 After fertilization, embryonic development proceeds
through cleavage, gastrulation, and organogenesis
 Morphogenesis in animals involves specific changes in
cell shape, position, and adhesion
 The developmental fate of cells depends on their and
on inductive signals
Body-Building Plan for animals
 Organism’s dev. Is determined by the genome of the zygote
and also by the differences that arise b/w early embryonic
cells
 These differences set the stage for the expression of different
genes in different cells
 Cytoplasmic determinates: maternal substances. Affect dev.
Of the cells that inherit them during early mitotic divisions
of the zygote
 Cell division continues
 Cell Differentiation: the specialization of cells in their
structure and function (timely communication is necessary)
 Morphogenesis: the process by which an animal takes shape
and the differentiated cells end up in the appropriate
locations
4 Main Stages: 1. Fertilization
 Main function: To combine haploid sets of
chromosomes from two individuals into a single
diploid cell (the zygote)
 Another important function: Activation of the egg
(contact of the sperm with the egg initiates metabolic
reactions that trigger embryonic dev.)
Acrosomal Reaction and Corticole
Reaction
Definitions
 Acrosomal reaction: Hydrolic enzymes released from
the acrosome make a hole in the jelly coat, while
growing actin filaments. This structure potrudes from
the sperm head and penetrates the jelly coat, binding
to receptors in the egg cell membrane that extend
through the vitelline layer.
 Cortical Reaction: Fusion of the gamete membranes
trigger a release of Ca creating a fertilization envelope.
Internal Fertilization
 Secretions in the female mammalian reproductive tract
alter certain molecules on the surface of sperm cells and
increase sperm motility
 Zona pellucida: extracellular matrix of the egg. Functions as a
sperm receptor , binding to a complimentary molecule on the
surface of the sperm head.
 Key Difference: The haploid nuclei of mammalian sperm
and egg do not fuse immediately (only after the 1st division
do the chromosomes from the two parents come together.)
 Much slower!
2. Cleavage
Important Definitions
 Morula: After further cleavage divisions, the embryo is
in a multicellular ball that is still surrounded by the
fertilization envelope. The blastocoel has begun to
form.
 Blastula: A single layer of cells surrounds a large
blastocoel. The fertilization envelope is still present;
the embryo will soon hatch from it and begin
swimming.
Yolk! Yummyyyyyy stored nutrients
 Eggs and zygotes of sea urchins (and other species)
have definite polarity. During cleavage the planes of
division follow a specific pattern relative to the poles of
the zygote
 The distribution of yolk is a key factor influencing the
pattern of cleavage
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Yolk is more concentrated toward the vegetal pole
Yolk decreases toward the animal pole
Amphibian
Development
1.
2.
3.
The polarity of the egg
determines the anteriorposterior axis before
fertilization.
2. At fertilization the
pigmented cortex slides over
the underlying cytoplasm
toward the point of sperm
entry. This rotation exposes a
region of lighter colored
cytoplasm (which is a mark of
the dorsal side)
3. The first cleavage division
bisects the gray crescent.
More Yolk!
 Has a pronounced effect on cleavage in the eggs of
birds, other reptiles, many fishes, and insects.
 Meroblastic cleavage: Cleavage of the fertilized egg is
restricted to the small disk of yolk-free cytoplasm and
cannot penetrate through the dense yolk. Incomplete
division of a yolk-rich egg. (birds)
 Holoblastic cleavage: The complete division of eggs
having little yolk (as in sea urchins) or a moderate
amount (frogs)
 Blastoderm (birds): a cap of cells formed in early
cleavage divisions in birds, that rests on the undivided
egg yolk.
3. Gastrulation
 A dramatic rearrangement of the cells of the blastula to form a
three-layered embryo with a primitive gut.
 The process is driven by the same mechanisms in all species
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Changes in cell motility
Changes in cell shape
Changes in cellular adhesion to other cells and to molecules of the
extracellular matrix
Gastrula: The three-layered embryo
Germ Layers: The three layers produced (embryonic tissue)
Ectoderm: Forms the outer layer of the gastrula
Endoderm: Lines the embryonic digestive tract
Mesoderm: Partly fills the space between the ectoderm and the
endoderm
 Eventually these 3 cell layers develop into all the tissues and
organs of the adult animal
Gastrulation in Sea
Urchins
Gastrulation in a sea urchin
embryo. Gastrulation begins with
the migration of mesenchyme
(mesoderm) cells from the vegetal
pole into the blastocoel. The
vegetal plate invaginates (buckles
inward). Endoderm cells form the
archenteron. Filopodia (made of
mesenchyme cells) drag the
archenteron across the blastocoel.
Fusion of the archenteron with the
blastocoel wall forms a digestive
tube with a mouth and an anus.
Gastrulation in Frogs
4. Organogenesis
 Various regions of the 3 embryonic germ layers
develop into the rudiments of organs
 Involves more localized morphogenetic changes in
tissue and cell shape

1st evidence of organ building is the appearance of folds, splits,
and dense clustering of cells