Light microscopy on semithin sections demonstrates numerous tubules (T) generated between fibers of the polyester fleece (PF). Development of tubules is observed in series with (a,c,e) CO2 Independent Medium and (b,d,f) Leibovitz’s L-15 Medium. Generated tubules exhibit a lumen (arrow) and a basal lamina (asterisk). a,b) Fixation of specimens in conventional glutaraldehyde followed staining by Richardson solution shows tubules with an inconspicuously looking homogeneous cell population. In contrast, specimens generated in (c,e) CO2 Independent Medium or (d,f) Leibovitz’s L-15 Medium but fixed by glutaraldehyde containing tannic acid show tubules with a dark and bright cell population (TD/B) or tubules containing only bright cells (TB).