Proliferative compartments in the normal nail unit.

The main distinction between the germinal matrix and the nail bed is that the former is the origin of all or most of the nail plate and the latter provides an epithelial surface to which the emerging nail can adhere. It has been argued that the nail bed may contribute substance to the nail plate and it is likely that if this was the case, epithelial proliferation in the nail bed would match that in the germinal matrix by a proportion appropriate to its contribution. We have measured the labelling index (LI) in the three major anatomical sites of the nail unit using two antibodies to antigens expressed in cycling cells. Using Ki-67 (MIB-1) and proliferating cell nuclear antigen (PCNA) as markers of cell proliferation we have defined the LI in the germinal matrix (MIB-1: 22.1%, PCNA: 33%), nail bed (MIB-1: 0.75%, PCNA: 1%) and digit pulp (MIB-1: 16.8%, PCNA: 17.4%). This suggests a low degree of proliferation in normal nail bed consistent with a minimal or non-existent contribution to the nail plate. This may change in hyperkeratotic nail dystrophies, where the nail bed LI is raised (MIB-1: 31%, PCNA: 29%), illustrating how nail bed behaviour can alter with disease.