Experimental study on pathogenesis of polydactyly of the thumb.

The pathogenesis of polydactyly of the thumb was investigated by administration of single dose of cytosine arabinoside (ara-C, 100 mg/kg) to pregnant SD rats on day 11. Marked growth discrepancy between ectoderm and mesoderm of the limb bud was observed both morphologically and biochemically 24 hours after treatment. The growth discrepancy was induced by interference with proliferation process of mesodermal cells. The different susceptibility of ectodermal cells and mesodermal cells to the teratogenic agent was found to depend on the stage difference of cellular differentiation of each tissue. Cephalocaudal asymmetry of the limb bud, as well as relative overgrowth of ectodermal tissue, was postulated to be the cause of protrusion of the preaxial border of the limb bud, which was an initial sign of extra first digit formation.