Introduction Callus tissue of plant origin has been examined widely for its cytological and morphological characteristics but very little are known about the differences between cells of intact tissue and the corresponding cells grown on an artifi cial medium. The present study compares anatomical and cytological structure of apple fruit explants and apple callus. Material and methods Fruits of apples Malus domestica Borkh were used in the experiments. Apple callus was obtained from the fruit by aseptic culture on modifi ed MS medium containing the supplements. Fresh apple callus was analyzed in stereoscopic or phase-contrast microscope. Clusters of the cell line callus were examined in modular confocal laser scanning microscope Leica DM IRB/E with relative fl uorochrome treat ments and pseudo-coloured of cytoskeletal constituents. Results and discussions The initiation of callusing in Malus explants occurred within 35-45 days. After 3-4 subcultivation carpoexplants formed three types of callus: green and white, comparatively homogenous, friable, and rose-colored, more compact and dense. Comparative microscopical analysis of green callus cells showed that this tissue type consists of small thin walled isodiametric cells with large intercellular spaces. The cells of green callus were characterized by spherical cells which the containing of a conside rable quantity of starch. White callus consists of prosenchymatous and worm-like cells and protein intraplastidial crystal-like inclusions (Matienco et al., 1997). The rose-colored callus was characterized by a predominance of spherical and slightly oval vacuolated cells, which contained anthocyanins. Intensively stained cell groups were placed on the callus surface. In all types of callus binucleate and polynucleate cells were observed. Binucleate fruit cells were widely described in vivo and in vitro in the callus of leaf, stem, root and fruit origine (Marinescu, Matienco, 1994; Rumyantseva et al., 1998). Interestingly, those binucleate cells were previously found in vivo in parenchyma cells of succ ulent apple pericarp (Colesnicova, Matienco, 1994) of different species and cultivars, i.e. in natural conditions. We have presumably connected this process with the evolution phenomenon of hypergenesis or hyperthely, gigas excessive growth (Matienco, 1969). This biological instrument might be conditioned by endogenous hormones synthesized by giant fruits and transmitted (shifted) through carpoexplants to callus proliferative potential in vitro. Conclusions Thus, tissue callus polyploidization represents one of the pushing forces in parenchyma proliferation in vivo and was all present in vitro with its multiplicity, variability of the expression. Consistent with this idea, the proliferation in vitro, under nutritional stress, the presence of exogenous hormones (auxins and cytokinins) and other factors exploit the same multiply mechanism that was established in vivo, during the evolution of hypertrophy and hyperthely.