Origin of the brushborder in the differentiating midgut of Melasoma saliceti (Chrysomelidae, Coleoptera) embryos.

The embryonic development of Melasoma saliceti takes eight days at room temperature. At the beginning of the 5th day the endoderm cells have already formed a unilayered epithelium of the midgut primordium. The midgut epithelium is formed by flat cells that are not connected by specialized intercellular junctions. Large vesicles can be seen in dilated intercellular spaces of the epithelium. Cytoplasmic projections, similar to microvilli, appear in the vesicles. During the 5th day ofdevelopment, the vesicles grow and become enclosed by the intercellular junctions of a zonula adherens type. During the 6th day of development the cell junctions surrounding the vesicles become transformed into a septate type. On the 8th day of development the vesicles come close to the apical sides of the midgut cells and open towards the yolk. At the same time the microvilli spread over the apical surface of the midgut primordium to form the regular brushborder of the larval midgut. In the species studied the vesicles appear to "prefabricate" the apical surfaces of the future midgut epithelium.