Throughout the differentiation of eye lens epithelium into fibers, an extensive system of intercellular junctions develops. The junctional assembly is initially characterized by the accumulation of 9.0-nm intramombranous particles, forming linear rows in the matching plasma membranes of adjoining fibers. At the final stage of the fiber differentiation, the junctional particles are assembled in geometrically packed arrays.The formation of linear rows and bidiniensional lattices of intramembranous particles probably favors reciprocal recognition of cell surfaces and specific cell-to-cell inter. locking. Moreover, the existence of a rather rigid lipid core of the plasma membrane of eye lens fiber may promote the clustered distribution of intramembranous particles and facilitate the junctional assembly.In. almost all animal tissues, low-resistance passageways and metabolic cooperation correlate with the presence of "gap" junctions (1-3). Freeze-fracturing has revealed that this type of junction is characterized by arrays of repeating particles associated with the inner hydrophobic matrix of the junctional membranes. Moreover, the arrays of particles are interlocked by a hexagonal lattice of pitted facets, likely originating from the merging of both outer membrane leaflets (4, 5). Although many membrane-mediated functions, such as fusion (6) and interaction of cell surface receptors with specific ligands (7), have been tentatively correlated with the spatial rearrangement and perturbation of the inner membrane particles made visible by freeze-fracturing, circumstantial evidence illustrating the differentiation of plasma membranes during cell contact and assembly of low-resistance junctions is still incomplete.Some data have been already presented illustrating the junctional features in embryonic and developing tissues (8,9). From these interesting studies it is, however, not easily apparent whether some plasma membrane aspects identified in freeze-fracturing experiments represent the assembly of "gap" junctions or stages of its breakdown and dilution during organogenesis and tissue differentiation. In the present study, the development of intercellular junctions in the eye lens is investigated. The vertebrate lens has several unique features that make this avascular tissue particularly suitable for the analysis of differentiation processes. In contrast to many other developing tissues, the lens is composed of one type of epithelium which during the entire life of the animal will continuously, and in one direction, differentiate into lens fibers (10). Furthermore, the latter cells are extensively connected by a rather unique type of junction (11,12).We present some evidence that in the region of cellular elongation, where the epithelium loses the ability to divide and differentiates into cortical fibers, the assembly of newly formed junctions is characterized by the progressive accumulation of particles. Initially these inner membrane particles form linear rows. Later they assemble in geometrically packed arra...