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Pole cell

In early Drosophila development, the first 13 passes through mitosis are nuclear divisions (karyokinesis) without cytokinesis, resulting in a multinucleate cell (generally referred to as a syncytium, but strictly a coenocyte ). The first mononucleate cells are created at the posterior pole, where the polar granules are tethered. These cells are called pole cells, and they will form the fly's germ line. Because pole cells develop into the germ line they are an important component for genetic engineering.

Formation
In stage 3 of Drosophila embryo development polar buds are formed. Polar buds are formed at the posterior pole of the embryo and are destined to become pole cells. Pole cells are formed in stage 4 of Drosophila embryo development. In this stage pole buds divide and pinch off forming 12- 14 pole cells.

Differentiation to germline cells
Pole cells are the first to form in the embryo but after formation have the slowest cell cycle. the pole buds have pinched off to form the pole cells they continue to divide. In stage 7 of Drosophila embryogenesis, pole cells move into an invagination and forms a structure called the proctodeal invagination. The pole cells continue to undergo mitosis and become germ line cells.

Genetic engineering
Although the percentage of pole cells that will eventually become germ line cells in Drosophila is disputed, it is known that a significant amount will differentiate to the germ line. This makes pole cells a perfect target for genetic engineering. Genes are introduced or knocked-out of the pole cells genome using genetic techniques; usually by transfection or transformation (using P elements) through microinjection. Some sort of reporter gene is often coupled with the genomic change to determine what cells have been successfully modified. Multiple pole cells are modified to insure that a significant portion of the germline cells will acquire the desired genomic changes. Now the germline cells are likely mosaic; some cells contain the modified genome while other retains the wild type genome. The now adult drosophila are bred with an adult wild type drosophila and offspring with the desired genotype can be identified from the reporter gene (often showing a distinct phenotype).