Occasional changes in the genetic makeup of cultured cells allow them to propogate indefinitely, making them effectively immortal. Such lines are said to be transformed, and are often used in research as a standardized cell line. Chinese hamster ovary (CHO) cells were introduced in the early 1960s as a viable epithelial cell line containing twin female X chromosomes. The most common variety of CHO cells has a nutritional requirement for the amino acid proline, which makes this cell line an ideal candidate for genetic studies.
A higher magnification image sequence captured with a 60x water-immersion objective.