Expression of γ-glutamyl transpeptidase (GGT) in the developing mouse tooth, intervertebral disc, and hair follicle was investigated in terms of its localization during ontogenic stages and its association or lack of association with cell proliferation (labeled nuclei after [3H]thymidine nijection or metaphase-arrested cells after colchicine injection). The data demonstrate that (a) GGT expression followed a program of activity and localization changes that correlated with the progressive emergence of developmental stages and (b) GGT activity in developing tissues derived either from epithelium (enamel-producing cells and hair follicle cells) or from mesenchyme (intervertebral disc cells) was localized only in mitotically quiescent cellular layers or regions associated with the production of specialized tissue products; however, not all postmitotic regions expressed GGT activity.

Although further research is needed to clarify the role of GGT in normal and neoplastic tissues, we conclude that increasing evidence from this and other laboratories implicates GGT as a marker of cell differentiation, cell aging, and/or reduced cell proliferation.

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Dedicated to Van Rensselaer Potter on his retirement, June 30, 1982, from the active faculty of McArdle Laboratory for Cancer Research, the Medical School, University of Wisconsin. Supported in part by Grants CA-22484, CA-07175, and CA-17334 (V. R. Potter) from the National Cancer Institute, NIH, and by the International Cancer Research Data Bank Programme of the National Cancer Institute, NIH, under Contract N01-CO-65341 (International Cancer Research Technology Transfer-ICRETT) with the International Union against Cancer.

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