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 How does an egg become an animal?
 Until the end of the 18th century, the prevailing theory
was that the embryo was a miniature infant.
 This idea of preformation also included the thought
that each embryo contained all of the descendents as
smaller embryos.
Much like a Russian Nesting Doll
 Another version of
preformation included the
idea of a homunculus- the
sperm contains a
preformed infant which
 Another theory proposed by Aristotle 2,000 years
earlier was that of epigenesis.
 The form of an animal emerges gradually from a
relatively formless egg.
 Microscopy allowed scientists to witness the
progressive development of embryos- thereby
validating Aristotle’s theory.
Embryonic Development Stages
 Fertilization in vertebrates is, of course, the union of
two haploid gametes to reconstitute a diploid cell - a
cell with the potential to become a new individual.
 Fertilization is a not a single event. Rather, it is a
series of steps that might be said to begin when egg
and sperm first come into contact and end with the
intermingling of haploid genomes.
Events of Fertilization
Contact & recognition between sperm and egg
Regulation of sperm entry into the egg. Only one
can enter - others inhibited from entering
Fusion of genetic material
Activation of egg metabolism to start development
Acrosomal Reaction
 Binding of sperm to the zona pellucida (egg
membrane) is the easy part of fertilization.
 The sperm then faces the daunting task of
penetrating the zona pellucida to get to the
 Evolution's response to this challenge is the
acrosome - a huge modified lysosome that is
packed with zona-digesting enzymes and
located around the anterior part of the sperm's
head - just where it is needed.
 The acrosome reaction provides the sperm with
an enzymatic drill to get throught the zona
 As the acrosome reaction progresses and the
sperm passes through the zona pellucida, more
and more of the plasma membrane and
acrosomal contents are lost.
 By the time the sperm traverses the zona
pellucida, the entire anterior surface of its
head, down to the inner acrosomal membrane,
is denuded.
 The constant propulsive force from the sperm's
flagellating tail, in combination with acrosomal
enzymes, allow the sperm to create a tract through the
zona pellucida.
 Once a sperm penetrates the zona pellucida, it binds
to and fuses with the plasma membrane of the oocyte.
Egg Activation
 Prior to fertilization, the egg is in a quiescent
state, arrested in metaphase of the second
meiotic division.
 Upon binding of a sperm, the egg rapidly
undergoes a number of metabolic and physical
changes that collectively are called egg
 Prominent effects include a rise in the
intracellular concentration of calcium,
completion of the second meiotic division and
the so-called cortical reaction.
Zona Reaction
The critical importance of the zona reaction is that
it represents the major block to polyspermy in most
This effect is the result of two measurable changes
induced in the zona pellucida:
The zona pellucida hardens.
Sperm receptors in the zona pellucida are destroyed
Stages of Development
In animals, one can usually distinguish 4 stages
of embryonic development.
2. Patterning
3. Differentiation
4. Growth
 Mitosis and cytokinesis of the zygote, an unusually
large cell, produces an increasing number of
smaller cells, each with an exact copy of the
genome present in the zygote.
 However, the genes of the zygote are not expressed
at first. The activities of cleavage are controlled by
the mother's genome; that is, by mRNAs and
proteins she deposited in the unfertilized egg.
 Cleavage ends with the formation of a blastula.
During this phase, the cells produced by cleavage
organize themselves in layers and masses, a process
called gastrulation. The pattern of the future
animal appears:
front and rear (the anterior-posterior axis)
2. back side and belly side (its dorsal-ventral axis)
3. left and right sides.
 Gastrulation forms three major "germ layers":
ectoderm, mesoderm, and endoderm.
 By gastrulation, the genes of the zygote genome are
being expressed.
Late Gastrulation in the Frog
 In time, the cells of the embryo differentiate to form
the specialized structures and functions that they will
have in the adult. This is called differentiation.
 They form neurons, blood cells, skin cells, muscle
cells, etc., etc.
 These are organized into tissues, the tissues into
organs, the organs into systems – This is called
 After all the systems are formed, most
animals go through a period of growth.
Growth occurs by the formation of new
cells and more extracellular matrix.
Germ Layers
 Each of these will have special roles to
play in building the complete animal.
 Some are listed in the table on the next
Germ-layer origin of various body tissues.
lining of gut
lining of lungs
spinal cord
lining of bladder
all other neurons
sense receptors
sex organs
Eggs and Zygotes have Animal and Vegetal
 Egg cells are very large:
 Sea urchin: ~70 to 150 microns
 Human ~100 microns
 Frogs & fishes, some insect eggs: 1000 to 2000
microns(1-2 mm)
 Birds & reptiles: millions of microns (many cm)
 Eggs store materials needed for development of
the embryo
 Yolk: lipids, carbohydrates and proteins organized
into granules
 Yolk settles to bottom of egg,
producing a gradient of stored
 Top of egg, with little yolk, is called
the animal pole
 Bottom of egg, rich in yolk, is called
the vegetal pole
 Polar axis goes from animal to vegetal
 Eggs have different amounts of yolk
 Large animals developing outside mothers body
(birds, reptiles) have large eggs with lots of yolk
 Large animals developing within mother's body
(mammals) have small eggs with very little yolk;
they get their food from the mother through the
 Animals which develop into small feeding larvae
(sea urchins, sea stars) also have small, simple
 Frogs and fish are intermediate in egg size and
yolk content
 Almost all of the zygote volume comes from the
egg, giving the zygote an animal & vegetal pole
Embryological Development
 To go from a single-cell to an organism
the embryo must repeatedly divide by
 The early set of rapid cell divisions is
called cleavage
In a Series of Mitotic Divisions the Zygote
Becomes a Hollow Ball (Blastula)
 The first set of cleavage divisions are
synchronized and there is no cell growth
between divisions
 The size of the embryo does not change, but
the egg material is partitioned into more and
more cells
 DNA synthesis does occur between divisions
since each new cell needs a nucleus.
 The cells arrange themselves into a ball
(blastula; called blastocyst in mammals) with
the cell layer surrounding the fluid-filled
interior (blastocoel)
The Archenteron
 After the blastula is finished the wall folds inward
at one point
 Forms a tube, the archenteron or primitive gut
 The opening to the archenteron is called the
 Cells at the animal pole grow and spread over outer
surface, forcing other cells inward through the
 In bird & mammal embryos there is a long furrow,
the primitive streak instead of a blastopore
 In chordates the first organ to form is the notochord
and the neural tube (later becomes the central
nervous system – brain and spinal cord)
 The neural tube forms from ectoderm
 The notochord forms from the mesoderm
 Embryonic Induction –The ability of a group of
embryonic cells to influence the development of
another group of embryonic cells.
Cytoplasmic Determinants
 Certain materials are distributed unequally within the
egg. These substances are not the same in each of the
cells that are formed and determine the fate of the
cells and the cell’s pattern of gene expression.
 Hans Spemann did an experiment with the gray
crescent of an embryo to show the importance of the
cytoplasm in development.
 Dissected an embryo in the two ball stage in different
ways and only the one with the gray crescent
developed normally.
The striped area is the Grey Crescent shows that if it is
cut in a way where it does not get some of the grey
crescent the embryo does not develop properly.
 Discovered by Spemann
 Proved that the dorsal lip of the bastopore normally
initiates inductions that forms the neural tube.
 Grafted a piece of dorsal lip from one embryo onto a
second embryo and cause the development of two
neural tubes.
 The grafted tissue caused the abdomen tissue to
become neural tissue.
 Spemann called the dorsal lip the primary organizer.
Two Headed Frog!!!!
Homeotic, Homeobox, or Hox
 Master genes that control the expression for
anatomical features. It is like a paint by number –
Place legs here, arms here, head here, etc. They direct
the placement of structures in the correct place.