The changes below were made to the article on the date noted. These changes have been incorporated in the article and the details are documented here.

30 December 1997:

In the caption of Figure 4c, the sentence:

TUNEL positive cells were present to a lesser degree in serumless/defined medium (arrows) than in serum-containing medium.

was corrected to:

TUNEL positive cells were present to a lesser degree in serumless/defined medium (arrows) than in death-induced cell conditioned medium.