Sir,

We are grateful to the comments from both centres which enable us to expand on the case of limbal stem cell deficiency (LSCD) that we originally described.1 Firstly, donor epithelial defects can be intimately related to limbal stem cell function as early as the first day postoperatively—Ahmed and Ahmed have shown that corneal epithelial cell migration rate is 61 μm/h during the active healing phase of the epithelium, which means that the defect may close in 18–48 h, or even faster.2 This rapid rate of recovery is a response to wounding, which has been conclusively shown by Lehrer et al3 using double-labelling techniques. The latter experiments demonstrate three mechanisms to account for the persistent epithelial defect on day 1 following corneal grafting. First, replication of limbal stem cells. Second, additional cycles of cell proliferation that normally remain in reserve. Third, enhancement of transient amplifying cell (TAC) proliferation via a shortening of the cycling time. These processes may also be under circadian control.4, 5 The observed clinical epithelial defect day 1 postop in our patient is a clinical correlate of this upregulation of cell turnover in the limbus. This cellular upregulation has a further possible significance, as we shall discuss later specifically in response to Dr Zaher and Dr Ramesh's correspondence.

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