The labelling index is not always reliable. Discrepancies between the labelling index and the mitotic rate in the rat corneal epithelium after intraperitoneal and topical administration of tritiated thymidine and colcemid.

Many cell kinetic studies are based on the assumption that tritiated thymidine injected into an animal is available for incorporation into DNA for only a short time, and that it labels all cells in the S phase. The present study indicates that this is not the fact for the rat corneal epithelium. The labelling index (LI) declines considerably from the limbal area to the central cornea, while the mitotic rate is almost constant all over the corneal epithelium. The LI should therefore not be used as the only criterion in the assessment of proliferation rate.