Key Cells

Key cells are mature cells of the superficial layer of the stratified squamous epithelium of the vagina, whose apical surface is densely covered with a layer of adhered bacteria. This gives the cells a characteristic “grainy” or “hairy” appearance, blurs their boundaries and makes the nucleus invisible during microscopy.

Etiology and pathophysiology

The formation of key cells reflects the process of bacterial biofilm formation in vivo. Under conditions of dysbiosis and elevated pH, anaerobic bacteria (primarily coccobacilli Gardnerella vaginalis and Atopobium vaginae) express adhesion factors (pili, fimbriae). This allows them to attach firmly to the cytoplasmic membranes of epithelial cells, displacing the normal lactoflora and forming a protective matrix.

Clinical significance

The detection of key cells during the microscopy of a wet mount or Gram-stained smear is a highly specific marker (specificity >90%). The presence of more than 20% key cells in the visual field is considered a mandatory criterion (according to Amsel and Nugent) for diagnosing bacterial vaginosis. In a physiological norm, vaginal epithelial cells should be “clean,” free from attached coccal flora.