Fig. 3. Colony morphology following in vitro culture of keratinocytes from the oesophageal basal layer. (A) After 20 days in culture, basal oesophageal keratinocytes have formed multiple large (stem cell) colonies (stained with rhodanile blue). Small transit colonies (<32 cells) are rare. (B) Upon microscopic examination after one week of in vitro culture, the colonies were observed to be composed of both small `basal-type' cells and larger differentiated squames (arrow). Oesophageal keratinocytes are cultured in the presence of a feeder layer of mitomycin-treated fibroblasts according to the method of Rheinwald and Green (Rheinwald and Green, 1975).