Transcript Animal Development

Animal
Development
Chapter 47
Ppt courtesy of Tracy Jackson
http://home.att.net/~tljackson/neville.html
Preformation
• 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 grows.
Epigenesis
• 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 embryosthereby validating Aristotle’s theory.
Embryonic Development Stages
Fertilization
• 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
1. Contact & recognition between sperm
and egg
2. Regulation of sperm entry into the egg.
Only one can enter - others inhibited
from entering
3. Fusion of genetic material
4. Activation of egg metabolism to start
development
Sperm Capacitation
• Freshly ejaculated sperm are unable or
poorly able to fertilize.
• Rather, they must first undergo a series of
changes known collectively as
capacitation.
• Capacitation is associated with removal of
adherent seminal plasma proteins,
reorganization of plasma membrane lipids
and proteins.
• Capacitation occurs while sperm
reside in the female reproductive
tract for a period of time, as they
normally do during gamete transport.
• Capacitation appears to destabilize
the sperm's membrane to prepare it
for the acrosome reaction
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
oocyte.
• 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 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.
• 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 activation.
• Prominent effects include a rise in the
intracellular concentration of calcium,
completion of the second meiotic division and
the so-called cortical reaction.
The Zona Reaction
• The cortical reaction refers to a massive
exocytosis of cortical granules seen
shortly after sperm-oocyte fusion.
• Cortical granules contain a mixture of
enzymes, including several proteases, which
diffuse into the zona pellucida following
exocytosis from the egg. These proteases
alter the structure of the zona pellucida,
inducing what is known as the zona reaction.
Components of cortical granules may also
interact with the oocyte plasma membrane.
•
•
The critical importance of the zona
reaction is that it represents the major
block to polyspermy in most
mammals.
This effect is the result of two
measurable changes induced in the
zona pellucida:
1. The zona pellucida hardens.
2. Sperm receptors in the zona pellucida
are destroyed
The Zona Reaction Animation
Stages of Development
• In animals, one can usually
distinguish 4 stages of embryonic
development.
1.Cleavage
2.Patterning
3.Differentiation
4.Growth
Cleavage
• 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.
Patterning
• 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:
1. front and rear (the anterior-posterior
axis)
2. back side and belly side (its dorsalventral axis)
3. left and right sides.
• There is little visible differentiation of the
cells in the various layers, but probes
for cell-specific proteins reveal that
different groups of cells have already
started on specific paths of future
development.
• 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
Differentiation
• In time, the cells of the embryo
differentiate to form the specialized
structures and functions that they will
have in the adult.
• They form neurons, blood cells, skin
cells, muscle cells, etc., etc.
• These are organized into tissues, the
tissues into organs, the organs into
systems.
Growth
• 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 II
• Each of these will have special roles
to play in building the complete
animal.
• Some are listed in the table on the
next slide.
Germ-layer origin of various body tissues.
Ectoderm
Mesoderm
Endoderm
skin
notochord
lining of gut
brain
muscles
lining of lungs
spinal cord
blood
lining of bladder
all other neurons
bone
liver
sense receptors
sex organs
pancreas
Eggs and Zygotes have Animal and
Vegetal Poles
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•
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
material
• 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 pole
• 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
placenta
• Animals which develop into small feeding larvae
(sea urchins, sea stars) also have small, simple
eggs
• 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 mitosis
• 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
blastopore
• Cells at the animal pole grow and spread over
outer surface, forcing other cells inward through
the blastopore
– In bird & mammal embryos there is a long
furrow, the primitive streak instead of a
blastopore