Fig. 1. Basic structure of the human oesophageal epithelium. H&E staining (top image) shows the normal stratified squamous epithelium of the human oesophagus. Invaginations of the basement membrane produce characteristic tall papillary structures (P) at regular intervals. The schematic representation (lower image) demonstrates the complexity of the epithelium. The epithelium is divisible into two zones, a differentiated zone (yellow) consisting of progressively flattened terminally differentiated keratinocytes and a generative basal zone. The latter zone is complex, and three cellular compartments can be identified. The single layer of cells directly adherent to the basement membrane (the basal layer) is divisible into two components: the flat interpapillary basal layer (IBL, red) and the papillary basal layer (PBL, green). Above the basal layer there are multiple layers of basophilic cells that have initiated the squamous differentiation program but are still capable of dividing (the epibasal layers, gray). The schematic is not drawn to scale. In vivo, the differentiated layer (yellow) is approximately 20 to 24 cell layers deep. In normal subjects the length of the papillae and basal zone thickness are said to be less than 66% and 10% of the total thickness of the epithelium, respectively (Geboes and Desmet, 1978; Ismail-Beigi et al., 1970). The relative thickness of the differentiated zone and basal zone varies along the oesophagus and between individuals (see text) (Ismail-Beigi et al., 1970; Behar and Sheahan, 1975).