Human amnion cells in vitro did not increase in number over a period of 2–3 months but maintained a stable population, morphologically similar to the primary cultures.

Some of these cells were capable of transforming into rapidly growing, altered cell lines. Seven cell strains were derived from cultures of four of the eight amniotic membranes used in these studies. Transformation occurred with various media and at various time intervals. In cultures prepared without embryonic extract, newly formed cells appeared spontaneously after 3–3½ months, whereas in cultures containing embryonic extract the cells passed through two clearly distinguishable intermediate stages, yielding altered cells after 6 months.

It was not determined whether the transformed cells were of epithelial or fibroblastic origin.

It is suggested: (a) The culture medium may influence the time at which transformation occurs and the selection of a specific cell type within a culture. (b) The amnion cell may provide a means to study the transition phase in the establishment of a cell strain.

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Aided by the American Cancer Society Instituional Grant 43H.

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