Cell replication and terminal differentiation in the embryonic chicken lens: normal and forced initiation of lens fibre formation.

The relation between cell replication and lens fibre formation was studied in normal 5-day chicken embryos and in embryos in which the lens was reversed. After this microsurgery the epithelial cells were facing the retina and were forced to start fibre differentiation. Embryos were treated with [3H]thymidine followed by autoradiography or with Colcemid to induce metaphase arrest. About half of the control epithelial cells were actively replicating (in contrast, in the earliest stages of lens formation all cells are multiplying). Half of the non-dividing population was accounted for by the annual pad cells. The other half, probably in a resting phase (GO) of the cell cycle, was perhaps slightly more common in the central epithelium. This suggests that a germinative zone may be forming in the lens epithelium as early as 5 days of embryonic life. Normal epithelium had a cell cycle time of about 11 h, with S-phase, M, G1 and G2 lasting 5.5, 0.5, 2, and 3 h. After reversal the labelling index remained at control levels for about 4 h and then dropped to zero at 9 h. Similarly, the mitotic index gradually decreased to zero in 15. It appears that reversal forces the epithelial cells to cease replication when they reach G1. Cells already in GO may form fibres without first going through DNA synthesis. The very sharp boundary between dividing and non-dividing cells at the border of the optic cup suggests that the retina may regulate the irreversible cessation of cell replication associated with lens fibre differentiation.