MYOEPITHELIAL CELLS ACTIVELY PROLIFERATE DURING ATROPHY OF RAT PAROTID-GLAND

Despite limited supporting evidence, salivary gland myoepithelial cell s are said to be differentiated cells with little or no capacity to re plicate; they presumably develop from stem cells. This study investiga ted the proliferative potential of myoepithelial cells with an antibod y to proliferating cell nuclear antigen and a rat model. This model in volved clamping of the parotid duct causing atrophy of the gland and t hen releasing the duct followed by gland regeneration. Rats were sacri ficed at time points during atrophy and regeneration phases and the nu mber and location of cycling myoepithelial cells assessed. Cycling myo epithelial cells were identified with double immunohistochemical stain ing, cycling cells with proliferating cell nuclear antigen-positive nu clei within muscle-specific actin-positive cytoplasm (the latter ident ified with antibody HHF35). The results show that baseline proliferati ve rates of myoepithelial cells in both the resting and fully regenera ted gland ranged from 0.3% to 2%, similar to rates for other major cel l types in the normal rat gland. A peak myoepithelial cell proliferati ve rate of 23% occurred at day 5 during the atrophy phase. Rates durin g the regenerative phase were not significantly different than the bas eline levels. Similarities of rat and human parotid gland and the defi nite proliferative capacity of myoepithelial cells indicates that thes e specialized cells must be considered one of the potential progenitor cells for human salivary gland tumors.