Early physiologic squamous metaplasia of the cervix: light and electron microscopic observations.

A light and electron microscopic study was undertaken of the early phases of cervical squamous metaplasia. A study of areas of columnar epithelium with one row of subcolumnar cells revealed a spectrum of squamous differentiation ranging from cytologically undifferentiation to more characteristically basal type squamous cells. With increasing differentiation, squamous characteristics, such as tonofibrils and desmosomes, became qualitatively and quantitatively more evident in the cells. Ultrastructural examination also revealed cells in the subepithelial stroma with prominent basal lamina and hemidesmosome-like structures along the periphery. From a light and electron microscopic viewpoint, the most likely origin for the subcolumnar progenitor cells appears to be the subepithelial stromal cells. The resemblance of these subepithelial stromal cells to the subcolumnar cells in very early stages of metaplasia is often striking.