THE CLONAL ORGANIZATION OF THE SQUAMOUS EPITHELIUM OF THE TONGUE

Knowledge of the kinetics and stem cell localization of the mouse lingual epithelium is largely based on studies using DNA labelling techniques. We have adopted a different approach, using histochemistry for the X-linked enzyme glucose-6-phosphate dehydrogenase (G6PD). We have deduced clone size and morphology from studies of patch size and distribution in mice heterozygous for G6PD deficiency and from the identification of clonal enzyme loss induced in normal mice by application of a mutagen. Lingual epithelium of female mice (CBA x GPDX) heterozygous for G6PD deficiency showed multiple clearly defined patches of strong or weak enzyme activity, corresponding in intensity to the strong staining uniformly present in the normal parental strain (CBA) or to the weak staining uniformly present in the G6PD deficient parental strain (GPDX). This pattern results from the random suppression of either the paternal or the maternal X chromosome in each cell early in embryonic development, and the subsequent inheritance of X inactivation in daughter cells, giving rise to phenotypic patches each composed of one or more clones. The patch borders intersected the base of the lingual epithelium at small indentations or at the apices of connective tissue papillae; the surface intersection in some cases bisected filiform papillae. Patch width measured in tissue sections at the mid rete ridge level, showed a clear mode close to 40-mu-m, corresponding very closely to the mode for rete ridge width (i.e. distance between connective tissue papillae). Further evidence for clonal organization was obtained by inducing mutations in the lingual epithelium of CBA mice by topical mutagen application. A few clearly defined patches of enzyme loss were found with a mean diameter of 36-mu-m. Their morphology was very similar to that of patches in the heterozygous animals. We interpret these patches as clones derived from stem cells with induced somatic G6PD mutations. We conclude that the mouse lingual epithelium is a stem cell epithelium composed of clonal units of about 40-mu-m diameter, based on the rete ridge structure and that both connective tissue papillae and filiform papillae occur at the junction of two or more epithelial clones.