Fertilization is a complex chain of events that must occur successfully for conception. It begins with changes to the sperm to prepare for fusion with an egg. A single sperm must fuse with the egg to prevent polyspermy, which leads to embryonic death. Following fusion, the genetic material from the sperm and egg combine and the zygote undergoes rapid cell division through cleavage to form a blastula or blastocyst. This initiates differentiation of cells and early embryonic development.

2. Fertilization- is more a chain of events than a single, isolated
Fertilization
phenomenon. Indeed, interruption of any step in the chain will
almost certainly cause fertilization failure. The chain begins with a
group of changes affecting the sperm, which prepares them for the
task ahead.
Successful fertilization requires not only that a sperm and
egg fuse, but that not more than one sperm fuses with the egg.
Fertilization by more than one sperm - polyspermy - almost
inevitably leads to early embryonic death. At the end of the chain
are links that have evolved to efficiently prevent polyspermy.

3. How Does Fertilization Occur?
The number of eggs produced by females of different
species depends on how fertilization occurs. In some species,
such as the frog you see here, fertilization is external. In the
mating season, the male mounts the female and squeezes her
abdomen, forcing the eggs out. The male deposits sperm on the
mass of eggs and fertilization occurs. This is a bit haphazard and
the developing tadpoles are subject to many predators, so
thousands of eggs are deposited to ensure that some will survive.
In other species, fertilization is internal...the male deposits
sperm within the reproductive tract of the female and fertilization
occurs there. This type of fertilization should sound at least
vaguely familiar to you, since it applies to the human species.
Chances of an egg being fertilized are greater with internal
fertilization, so fewer eggs are produced by the female.

4. During the spawning season, a sea urchin male releases
billions of sperm into seawater. The presence of the sperm triggers
nearby females to release millions of eggs and fertilization is
external.
We can induce urchins to release their gametes by injecting
dilute potassium chloride into the animal's body through the
membrane surrounding its mouth, which is on the lower side of the
body. Since you can't tell the sex of a sea urchin by looking at it,
you have to look at the gametes the animal produces.

5. The male releases a whiter viscous mass of sperm from tiny
pores on the upper surface of the body. A female urchin produces a
yellowish-orange mass of eggs when injected, and we can collect the
the eggs in a beaker of sea water.
An electron micrograph shows urchin sperm swarming
around an egg. As soon as one sperm penetrates the egg, a
protective membrane lifts off the surface of the egg. It is called the
fertilization membrane.

6. The Acrosome Reaction
* Binding of sperm to the zona pellucida is the easy
part of fertilization.
* The acrosome reaction provides the sperm with
an enzymatic drill to get through the zona pellucida.
* The same zona pellucida protein that serves as a
sperm receptor also stimulates a series of events
that lead to many areas of fusion between the
plasma membrane and outer acrosomal membrane.
* Membrane fusion (actually an exocytosis) and
vesiculation expose the acrosomal contents, leading
to leakage of acrosomal enzymes from the sperm's
head.
* Sperm that lose their acrosomes before
encountering the oocyte are unable to bind to the
zona pellucida and thereby unable to fertilize.

7. Stages of Fertilization
• * Contact and recognition between sperm and egg
• * Regulation of sperm entry into the egg
• * Fusion of the genetic material of sperm and egg
* Activation of egg metabolism to start development

8. First stage of fertilization
*Fertilization occurs
*Zygote implants itself in the lining of the uterus
*Rapid cell division occurs
*Embryonic stage lasts from 2 weeks to 8 weeks
*Cells differentiate into three distinct layers:
the ectoderm, the mesoderm, and the endoderm
*Nervous system begins to develop
*Embryo is 1/2 inch long

10. Sperm-Oocyte Binding
Once a sperm penetrates the zona pellucida, it binds to
and fuses with the plasma membrane of the oocyte. Binding
occurs at the posterior (post-acrosomal) region of the sperm
head.
The molecular nature of sperm-oocyte binding is not
completely resolved. A leading candidate in some species is a
dimeric sperm glycoprotein called fertilin, which binds to a
protein in the oocyte plasma membrane and may also induce
fusion. Interestingly, humans and apes have inactivating
mutations in the gene encoding one of the subunits of fertilin,
suggesting that they use a different molecule to bind oocytes.

11. Early Development
After fertilization occurs, what happens next? The
diploid zygote is just the beginning of a new organism. It will
have to develop into a multicelled embryo by undergoing cell
divisions called cleavage. Turn to Roman Numeral III of your
lab guide and read the definition of cleavage.
Soon after fertilization, the zygote will undergo cleavage
to form two cells. Since this is a mitotic division, each cell will
have a nucleus with genetic information identical to that of the
zygote, but will be half the size of the zygote. These cells don't
have time to grow.

12. They cleave again by mitosis to form four cells and again the
cells are reduced in size. The divisions continue.
The four-celled stage is followed by an eight-celled stage, then
16, 32, and so on. As the number of cells increases, the cells migrate
outwards to form a hollow ball of cells.
This hollow ball is called a blastula. In the sea urchin embryo,
the blastula cells develop cilia and the blastula revolves in the
seawater. The cells on one side of the blastula begin to push in or
invaginate.
You can see this beginning to happen here. This indentation
marks the gastrula stage of development. In a late gastrula, the
indentation will deepen. Since cells on the inside of the gastrula
encounter different conditions than those on the outside, they begin to
differentiate into different types. The formation of tissues has begun.

13. As development and differentiation continue, a free swimming
larval stage appears...it is called a Pluteus larva. The larva feeds,
grows, and undergoes structural changes to form a juvenile sea urchin
which resembles the adult.
Sperm cells and egg cells both are specialized to perform their
specific roles in the fertilization process. Regardless of whether
fertilization is external or internal, the fusion of a sperm and egg
results in a zygote that contains a mixture of parental genetic
information. All zygotes, whether they are destined to become baby
rhinos, sea urchins, or humans, are programmed to go through very
similar early stages of development.

14. New cells are formed by cleavages, they differentiate and
eventually the embryo takes on a recognizable form.
Result of meiosis, fertilization and development is an
offspring that has the characteristics of its species, but has a
combination of characteristics that make it unique.

15. Progression of development
1. 1 cell --> many cells
2. uniform cells --> specialized cells, differentiation
3. simple multicelled shape --> complex multicelled
shape
4. Not just morphology, also physiology and behavior
Development is fundamentally a process of control of
gene expression.
DNA --> mRNA--> proteins -----> phenotype

16. Recognition of Egg and Sperm
• * The chemoatraction of the sperm to the egg
• * Exocytosis of the acrosomal vesicle
• * Binding of the sperm to the extracellular envelope
• * Passing of the sperm through the extracellular
envelope
• * Fusion of egg and sperm plasma membranes

17. Post-fertilization Events
Following fusion of the fertilizing sperm with the
oocyte, the sperm head is incorporated into the egg cytoplasm.
The nuclear envelope of the sperm disperses, and the
chromatin rapidly loosens from its tightly packed state in a
process called decondensation. In vertebrates, other sperm
components, including mitochondria, are degraded rather than
incorporated into the embryo.
Chromatin from both the
sperm and egg are soon encapsulated
in a nuclear membrane, forming
pronuclei.

18. Cell Cleavage
1. Sea Urchin- Cleavage begins. Mitosis occurs. Cytokinesis is
uneven, partitioning different amounts and different portions of
cytoplasm to progeny cells. Smaller cells mark the animal pole
and larger cells (containing more yolk, among other things) mark
the vegetal pole.
2. Morula forms early as a solid ball of cells. (< or = 32
s).
3. As cell division progresses (500-2,000 cells), a blastula is
formed that resembles a hollow ball. The space in the center
of this ball is called a blastocoel. The blastocoel simply
vides an internal space for the movement of cells during
trulation.

19. 4. Frog
Steps 1, 2 and 3 as in sea urchin except that the increased
amount of yolk causes the cells at the vegetal pole to divide
much slower leading to more and smaller cells at animal pole.
Blastocoel forms, not in the center, but toward the animal pole.
5. Chick
Cleavage occurs in cells lying atop the large yolk mass so that a
disk (blastocyst or blastodisc) forms. Top and bottom layers
of cells separate forming the blastocoel cavity.

20. The presence of the blastocyst indicates that
two cell types are forming: the embryoblast (inner
cell mass on the inside of the blastocele), and the
trophoblast (the cells on the outside of the
blastocele).

21. Cell Cleavage and Blastomere
The zygote now begins to cleave,
with each division occurring into two
cells called blastomeres. The zygote's first
cell division begins a series of divisions,
with each division occurring
approximately every twenty hours. Each
blastomere within the zona pellucida
becomes smaller and smaller with each
subsequent division.
When cell division ungenerated
about sixteen cells, the zygote becomes a
morula (mulberry shaped). It leaves the
fallopian tube and enters the uterine
cavity three to four days after

22. Early Blastocytes
About four days after fertilization, the morula
enters the uterine cavity. Cell division continues, and a
cavity known as a blastocele forms in the center of the
morula. Cells flatten and compact on the inside of the
cavity while the zona pellucida remains the same size.
With the appearance of the cavity in the center, the entire
structure is now called a blastocyst.

23. Properties of Embryo During Cleavage
and Blastulation
•During cleavage and blastulation the embryo is simple in structure
and alteration in its cell arrangement are straightforward.
•Many properties of the embryo and its constituents cells change
during this period.
•In sea urchin, isolated blastomeres at 2- or 4- cell stage give or
develope to complete individual.

24. •Chick blastoderms (30,000 cells) was cut into two halves, each
half was able to form complete chick embryo.
•Once mammalian blastocyst has become subdivided into the
trophoblast and inner cell mass, these two regions are not
equivalent to constitute complete individual.
•Partial or complete separation of the inner cell mass will give
twinning phenomenon.
•Tertraparental mice.
•In others such as the mollusk, 2- or 4-cell embryos give rise to
incomplete embryo.
•Blastomeres isolated from mammalian embryo show steadily
decreasing ability to form complete individuals from 2- to 8-cell
stage.