Gap junctions and rhombic particle arrays in planaria

Freshwater planaria (Dugesia) were found to have three distinct forms of gap junctions. The three forms differ from one another in morphology and in the kinds of cells that they are found between. Utilizing freeze-fracture and lanthanum tracer preparations, the three types were recognized by differences in the spacing and pattern of their particles or subunits and in the membrane faces to which the particles adhere. One of the three types is associated with gastrodermal, parenchymal, and excretory cells; another is found on muscles and nerves; and the third is localized apparently on secretory cells. The possibility of the three junctional forms being correlated with functional differences is discussed. Muscle and nerve cells also have a nonjunctional type of specialization consisting of intramembranous particles arranged in a rhombic pattern.