Download Embryology of Frog

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Embryology of Frog
The frog egg is a huge cell; its volume is over 1.6 million times larger than a normal frog cell. During
embryonic development, the egg will be converted into a tadpole containing millions of cells but containing
the same volume of material.
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The upper hemisphere of the egg — the animal pole — is dark.
The lower hemisphere — the vegetal pole — is light.
When deposited in the water and ready for fertilization, the haploid egg is at metaphase of meiosis II
Fertilization
Entrance of the sperm initiates a sequence of events:
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Meiosis II is completed.
The cytoplasm of the egg rotates about 30 degrees relative to the poles.
In some amphibians (including Xenopus), this is revealed by the appearance of a light-colored band,
the gray crescent.
The gray crescent forms opposite the point where the sperm entered.
It foretells the future pattern of the animal: its dorsal (D) and ventral (V) surfaces; its anterior (A) and
posterior (P); its left and right sides.
The haploid sperm and egg nuclei fuse to form the diploid zygote nucleus.
Fertilized egg of Frog
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Cleavage
The zygote nucleus undergoes a series of mitoses, with the resulting daughter nuclei becoming partitioned
off, by cytokinesis, in separate, and ever-smaller, cells. The first cleavage occurs shortly after the zygote
nucleus forms.
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A furrow appears that runs longitudinally through the poles of the egg, passing through the point at
which the sperm entered and bisecting the gray crescent.
This divides the egg into two halves forming the 2-cell stage
The second cleavage forms the 4-cell stage. The cleavage furrow again runs through the poles but at right
angles to the first furrow.
The furrow in the third cleavage runs horizontally but in a plane closer to the animal than to the vegetal
pole. It produces the 8-cell stage.
Cleavage
The next few cleavages also proceed in synchrony, producing a 16-cell and then a
32-cell embryo.
However, as cleavage continues, the cells in the animal pole begin dividing more
rapidly than those in the vegetal pole and thus become smaller and more numerous.
By the next day, continued cleavage has produced a hollow ball of thousands of
cells called the blastula. A fluid-filled cavity, the blastocoel, forms within it.
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Gastrulation
The start of gastrulation is marked by the pushing inward ("invagination") of cells in the region of the
embryo once occupied by the middle of the gray crescent. This produces:
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an opening (the blastopore) that will be the future anus
a cluster of cells that develops into the Spemann organizer (named after one of the German
embryologists who discovered its remarkable inductive properties).
As gastrulation continues, three distinct "germ layers" are formed:
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ectoderm
mesoderm
endoderm
Each of these will have special roles to play in building the complete animal. Some are listed in the table.
The Spemann organizer (mostly mesoderm) will:
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develop into the notochord, which is the precursor of the backbone;
induce the ectoderm lying above it to begin to form neural tissue instead of skin.
o This ectoderm grows up into two longitudinal folds, forming the neural folds stage.
o In time the lips of the folds fuse to form the neural tube.
o The neural tube eventually develops into the brain and spinal cord.
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Differentiation
Although the various layers of cells in the frog gastrula have definite and different fates in store for them,
these are not readily apparent in their structure. Only by probing for different patterns of gene expression
(e.g., looking for tissue-specific proteins) can their differences be detected.
In due course, however, the cells of the embryo take on the specialized structures and functions that they
have in the tadpole, forming neurons, blood cells, muscle cells, epithelial cells, etc., etc.
Growth
At the time the tadpole hatches, it is a fully-formed organism. However, it has no more organic matter in it
than the original frog egg had. Once able to feed, however, the tadpole can grow. It gains additional
molecules with which it can increase the number of cells that make up its various tissues.