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An organism’s development is planned by a genetic
program involving the genome of the zygote and the
molecules placed in the egg by the mother
› These molecules are called cytoplasmic determinants
› As the zygote divides, differences occur between early
embryonic cells due to the uneven distribution of
cytoplasmic determinants and to signals from neighboring
cells
› The process of cell specialization in structure and function
is called cell differentiation
› Development involves morphogenesis , the process by
which an organism takes shape and the differentiated
cells occupy their appropriate locations
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The main function of fertilization is the combining of
haploid sets of chromosomes from two individuals
into a single diploid cell
› Contact of the sperm with the egg’s surface starts
metabolic reactions within the egg and “activating” it
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The acrosomol reaction begins when a specialized
vesicle at the tip of the sperm, called the acrosome,
discharges hydrolytic enzymes
Egg activation is the outcome of the sharp rise of
Ca2+ concentration in the egg’s cytosol, as well as
the binding and fusion of sperm
Unfertilized eggs of many species can be artificially
activated by the injection of Ca2+
Fertilization in mammals is mostly internal, which
causes an increase in sperm motility
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During cleavage, cells carry out the S and M phases of the cell
cycle. Cleavage divides the cytoplasm of a zygote into smaller
cells called blastomeres
Eggs and zygotes have a definite polarity which have a specific pattern
that needs to be followed
› The polarity is defined by the uneven distribution of substances in the
cytoplasm, including cytoplasmic determinants and yolk (stored
nutrients)
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The distribution of yolk is a key factor influencing the pattern of
cleavage. It’s often concentrated toward one pole of the egg,
called the vegetal pole; the yolk concentration decreases
toward the opposite pole, the animal pole
 Establishment of the three body axes occurs early in
development. The animal and vegetal hemispheres of the
zygote can be distinguished by color (animal is gray and vegetal
is yellow)
 Cortical rotation is when the plasma membrane and associated
cortex rotate with respect to the inner cytoplasm
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Is the morphongenetic process in which
cells take up new locations that will allow
the formation of tissues and organs. The
result is that some cells at or near the
surface of the blastula move to a new
interior location, and three cell layers are
established
 http://www.youtube.com/watch?v=f0tZ
UkTaGYw
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During organogenesis, various regions of
the three embryonic germ layers
develop into the rudiments of organs
› Involves localized shape changes in both
tissues and individual cells
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Is a dynamic structure that maintains cell
shape, protects the cell, enables cellular
motion (using structures such as flagella,
cilia, and lamellipodia), and plays
important roles in both intracellular
transport and cellular division
› Cell adhesion molecules (CAMs) are a key
group of proteins that contribute to cell
migration and stable migration and stable
tissue structure
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Fate maps of embryos have shown that
specific regions of the zygote or blastula
develop into specific parts of older
embryos
In nonamniotes, unevenly distributed
cytoplasmic determinants in the egg are
important in establishing the body axes
and in setting up differences between
the blastomeres resulting from cleavage
of the zygote
 Cells that receive different cytoplasmic
determinants result in different outcomes
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Cells in a developing embryo receive
and respond to positional information
that differs with location
› This information is in the form of signaling
molecules hidden by cells in special
“organizer” regions of the embryo
 The signaling molecules influence gene
expression in the cells that receive them