Frog Embryology – RCN Corporation

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 amount of organic matter. 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 Entrance of the sperm initiates a sequence of events: 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. 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.

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.

The Spemann organizer (mostly mesoderm) will:

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.

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Frog Embryology - RCN Corporation