Animal
Development
Fertilization,
Cleavage,
Gastrulation
Mrs. Oldendorf
AP Biology
Frog development
Fertilization:
Haploid + Haploid  Diploid
The sperm:
Note the acrosome and the
nucleus and location of
mitochondria
The Acrosomal Reaction:
Sea Urchins (external development)

upon contact with egg’s jelly coat, sperm’s
acrosome is released

enzymes make hole in the jelly coat

fusion of egg and sperm membranes triggers
depolarization of the membrane (fast block to
polyspermy)

cortical granules fuse with plasma membrane,
clip off sperm binding receptors (slow block
to polyspermy)
In Mammals
(internal fertilization)

sperm need the environment of the
female tract to be able to fertilize
(capacitation – 6 hrs)

follicle cells surround the ova (egg) –
called the zona pellucia – have
receptors for sperm that induces an
acrosomal reaction

no fast block system in mammals, only
slow block
Development
(cleavage and gastrulation)
Terms:

Cleavage – stages of early development (cell
divisions)

Blastula – 1st 5 – 7 cleavages  hollow ball of cells
surrounded by blastocoel

Blastocoel – fluid filled cavity that is the future body
cavity (animal hemisphere)

Vegetal pole – where yolk (stored nutrients) is
concentrated

Animal pole – the opposite pole of where the yolk is
More terms:

Archenteron – future gut (digestive
system)

Blastopore – future anus

Germ layers (3) – ectoderm, endoderm,
mesoderm

Gastrula – a 2 or 3 layered embryo
The EGG

The upper hemisphere of the egg —
the animal pole — is dark.

The lower hemisphere — the vegetal
pole — is light.
Holoblastic Cleavage

In eggs that contain no or only
moderate amounts of yolk,
cytokinesis divides the cells
completely.
- (like frogs and mammals)
Meroblastic Cleavage

In eggs that contain a large amount of
yolk, cytokinesis does not divide the
egg completely.

- also found in the eggs of fish,
reptiles, and 4 species of mammals —
the monotremes
Cleavage

Series of mitoses in zygotic nucleus
First cleavage

A furrow appears that runs longitudinally
through the poles of the egg

This divides the egg into two halves
forming the 2-cell stage
As cleavage continues…

the cells in the animal pole begin dividing more
rapidly than those in the vegetal pole (and thus
become smaller and more numerous.)

By the next day, a hollow ball of thousands of
cells called the blastula has been produced.

A fluid-filled cavity, the blastocoel, forms
within it.
During this entire process…

There has been no growth of the embryo

Not until the blastula > 4000 cells is there
any transcription of zygote genes
– (up until now the activities have been run by
gene products – mRNA and proteins from the
mother)
Gastrulation

The invagination (pushing inward) of cells
in the region of the embryo once occupied
by they grey crescent

This produces:
– An opening (blastopore) that will be the anus
– A cluster of cells that becomes Spemann
Organizer
3 Germ Layers are formed:
Ectoderm
Mesoderm
Endoderm
Spemann Organizer will.
Develop into the notochord (precursor to
backbone)
 Induce ectoderm to form neural tissue
 Neural fold stage  then forms the neural
tube  develops into brain and spinal
cord


FOLIC ACID!!!
Differentiation

Cells of the embryo take on specific
functions

Neurons, blood cells, muscle cells,
epithelial cells, etc etc etc
Genetic Regulation of Cell
Differentiation

** Control Factors in cytoplasm of
egg
– Evidence: remove nucleus from fertilized
egg and cleavage still occurs
 Egg has maternal mRNA in unfertilized egg
 proteins direct change

** Events of Fertilization
– Point where sperm enters determines
location of grey crescent (across from)
determines dorsal axis (nervous system)

** Role of Nucleus
– All cells of embryo have same set of
chromosomes
Morphagenesis
– Formation of overall body pattern
– Driven by chemical concentration gradients
set up by morphogens (signaling molecules)
that diffuse through the tissues of an embryo
during early development, concentration
gradients
– involves homeobox genes or homeotic
gene
 Arranged in a definite order and determine
the anterior and posterior ends of body