Chapter 47: Animal
Development
AP
BIOLOGY
SHANNON
BRADY
2010
A Body Building Plan for Animals
 Cytoplasmic determinants: maternal substances that cause early
embryonic cells to become different because of their uneven
distribution within the unfertilized egg.
 Cell Differentiation: The Specialization of cells in their structure and
function.
Fertilization; Uniting the sperm & egg
 The main function of fertilization is to combine haploid sets
of chromosomes from two individuals into a single diploid
cell, the zygote.
 There are two main processes that occur during
fertilization; the Acrosomal Reaction and the Cortical
Reaction.
The Acrosomal Reaction
The Acrosomal Reaction Cont.
1.
2.
3.
4.
Contact. The sperm cell contacts the egg’s jelly coat, triggering
exocytosis from the sperm’s acrosome.
Acrosomal Reaction. Hydrolytic enzymes released from the
acrosome make a hole in the jelly coat, while growing actin filaments
from the acrosomal process. This structure protrudes from the sperm
head and penetrates the jelly coat, binding receptors in the egg cell
membrane that extend through the vitelline layer.
Contact & Fusion of sperm & egg membranes. A hole is made
in the vitelline layer, allowing contact and fusion of the gamete
plasma membranes. The fused membrane becomes depolarized,
resulting in the fast block to polyspermy ( prevents additional sperm
cells from fertilizing the egg)
Entry of Sperm Nucleus.
The Cortical Reaction
The Cortical Reaction Cont.
Fusion of the gamete membranes triggers the
release of Ca2+ from the ER into the egg’s cytosol,
causing cortical granules in the egg to fuse with
the plasma membrane and discharge their
contents. This leads to swelling of the
perivitelline space, hardening of the vitelline
layer, and clipping of sperm- binding receptors.
The resulting fertilization envelope is the slow
block to ployspermy.
1.
Activation of the Egg
 Another result of the increase in Ca2+ is substantial
increase in the rates of cellular respiration and
protein synthesis by the egg cell.

 This artificial activation switches on the metabolic
responses of the egg and causes it to begin developing by
parthenogenesis.
Fertilization in Mammals
1.
2.
3.
4.
5.
The sperm migrates through the coat of follicle cells and binds
to receptor molecules in the zona pellucida (extracellular
matrix) of the egg.
This binding induces the acrosomal reaction, in which sperm
releases hydrolytic enzymes into the zona pellucida.
Breakdown of the zona pellucida by these enzymes allows the
sperm to reach the plasma membrane of the egg. Membrane
proteins of the sperm bind receptors on the egg membrane,
and the two membranes fuse.
The nucleus and other components of the sperm cell enter the
egg.
Enzymes released during the cortical reaction harden the zona
pellucida, which now functions as a block to polyspermy.
Cleavage
 During Cleavage a cell undergoes DNA synthesis and
Mitosis but not protein synthesis.
 Cleavage partitions the cytoplasm of one large cell,
the zygote, into many smaller cells called
blastomeres, each with it’s own nucleus.


 The first 5-7 divisions form a cluster known as the morula
(lobed surface embryo).
A fluid filled cavity called the blastocoel begins to form
within the morula and is fully formed into the blastula ( a
hollow ball of cells.)
Body Axes Establishment in Amphibians
The eggs and zygotes of sea urchins and other animals have a
definite polarity.
 yolk is most concentrated toward the vegetal pole; but decreases
significantly toward the opposite animal pole.
Following fusion of the egg and sperm, rearrangement of the
amphibian egg cytoplasm establishes One of the body axes.
 1) The polarity of the egg determines the anterior posterior axis in
fertilization.
 2) At fertilization the pigmented cortex slides over the underlying
cytoplasm toward the point of sperm entry. This rotation exposes a
region of light colored cytoplasm, the gray crescent, which marks the
dorsal side.
 3) The first cleave division bisects the gray crescent. Once the anterior
posterior, and dorsal-ventral axes are defined, so if the left-right axis.
Cleavage in a Chick Embryo
Most of the zygote is yolk with a small disk of cytoplasm located at the
animal pole. The cell is surrounded by egg whites, but cleavage is
restricted to the disk of cytoplasm.


Meroblastic Cleavage: The incomplete division of yolk-rich egg (chick embryo).
Holoblastic Cleavage: The complete division of eggs having little yolk (sea
urchins) or some yolk (frogs).
Early cleavage division in a bird embryo produce a cape of cells
called a blastoderm, which rests undivided on the yolk.
 The blastoderm sorts into upper and lower layers; the epiblast and hypoblast.
The cavity between the two is the avian version of the blastocoel.
Gastrulation
What is it?
-A dramatic rearrangement of the cells of the blastula to form
a three layered embryo with a primitive gut.
-Changes in motility, cell shape, and cellular adhesion to
other cells and to molecules of extra cellular matrix.
The result. A three layered embryo called the gastrula.


The three layers form the embryonic germ layers; the ectoderm forms
the outer layer; the endoderm lines the embryonic digestive tract; and the
mesoderm partly fills the space between.
These germ layers eventually turn into adult tissue and organs.
Gastrulation Diagram
Organogenesis
Organogenesis involves more
localized morphogenic
changes in tissue and cell shape.
The first evidence
of this process is the appearance
of folds, splits, and
dense clustering of cells.
Developmental Adaptations of Amniotes
 All vertebrate embryos require an aqueous
environment for development.
 Only two such structures exist today that allow
vertebrates to reproduce in dry environments.
1) the shelled egg of birds and other reptiles.
2) the uterus of placental mammals. (embryos are surrounded
by fluid within a sac by a membrane called the amnion)


Reptiles including birds are therefore called
amniotes.
In some shelled eggs “extra life support” is needed in
the form of embryonic membranes.
Mammalian Development
FOUR STAGES
1) Fertilization
 2) Cleavage.
 3) Gastrulation.
 4) Organogenesis.
Morphogenesis
 The movement of cells through:
 - reorganization of the skeleton
 - cell migration
 - convergent extension.
 ( depicted to the right )
Roles of the Extracellular Matrix & Adhesion
Molecules
 Several methods of guiding cells in the right
direction during morphogenesis include:
 ECM, (extracellular matrix) which is a mixture of secreted
glycoproteins.
  CAM’s ( Cell Adhesion Molecules )
  Cadherins, an important type of cell to cell adhesion
molecules.

The Development Fate of Cells
 Developmental fate depends on history and inductive
signals, two general principles describe where
differentiation occurs in cells:
 1) During Cleavage divisions, embryonic cells must somehow become
different from one another.
2) Once initial cells asymmetries are set up, subsequent interactions
among the embryonic cells influence their fate, usually by causing
changes in gene. (Induction)
Establishing Cellular Asymmetries
The Axis of the body plan must be established.
Bilaterally symmetrical animals have an anterior-posterior axis, ventraldorsal axis, and left and right sides.
The “Organizer” of Spemann & Mangold
Cell fate by Inductive Signals.
In a 1920’s experiment Hans Spemann & Hilde
Mangold concluded that the dorsal lip of the
balstopore in early gastrula functions as an organizer
of the embryo by initiating a chain of induction that
result in the formation of the notochord, neural tube,
etc. A growth factor BMP-4 is suspected to cause
induction.
Formation of the vertebrate Limb
 Positional information controls pattern formation that
forms limbs in the body.
 The two major limb bud organizers are the:
 1) apical ectodermal ridge (AER)
 2) zone of polarizing activity (ZPA)
 These two organizers send signals to the cells, which
determine whether a limb will be a fore limb or a hind limb
based on developmental histories.