DEVELOPMENT OF JUNCTIONAL COMPLEX IN CULTURED CELLS

The development of specialized intercellular junctions in cultured cells was studied ultrastructurally. MDCK cells, derived from dog kidney, were fixed in situ at proper times after replating, and the sections were cut perpendicular to the plane of the monolayer. In three or four days, the apposition of cell membranes and condensation of extracellular flocculent material were observed between the neighboring cells, and such were regarded as the early signs of desmosome formation. In many cases, a desmosome was formed first, and the formation of a tight junction followed on the apical (medium facing) side. Finally, all intercellular spaces were closed by a junctional complex at the apical edge. In the complex, a tight junction, (intermediate junction) and desmosome(s) succeeded each other in a medium-substratum direction in all cases. In glutaraldehyde-O^sO⁴ fixed specimens, the intermediate dense line in the desmosome was ascertained from the infant stage of development, while in O^sO⁴ fixed material, the structure was obscure throughout the observation but side-arm-like projections were more prominent.