Epithelial distribution in the human fetal larynx.

Scanning and transmission electron microscopy and light microscopy were used to study the distribution of squamous epithelium and ciliated respiratory type epithelium in ten midtrimester human fetal larynges. Particular attention was paid to the subglottic region, where previous studies have reported areas of squamous metaplasia in neonates, infants, and nonsmoking adults. Both types of epithelium were found in different proportions in the supraglottic, glottic, and subglottic regions. There was considerable downward extension of squamous epithelium from the glottis into the subglottic region, with a wide transition zone where both types of epithelium were found in close association. These observations would suggest that such areas of squamous epithelium in the nonglottic larynx are normal, rather than always indicating a response to extrinsic trauma or irritation. Therefore, the term squamous metaplasia may be inappropriate when applied to such areas in the adult larynx.