Intercellular junctions in the developing arachnoid membrane in the chick

The arachnoid membrane of chick embryos was prepared for electron microscopic study by means of thin sections and freeze‐fracture replicas. Particular attention was given to the relationships among junctional complexes during arachnoid maturation. By 14 days of incubation, the arachnoid had differentiated into morphologically distinct inner and outer zones. Both desmosomes and gap junctions were present among the cells of both layers at this time. Desmosomes were most numerous in the inner arachnoid layer and their structure remained constant. Gap junctions showed a great variation in structure. The large gap junctions contained a particle packing pattern in which rows of intramembranous particles were separated by particle‐free zones. Arched gap junctions were also present. Smaller arrays of gap junctions exhibited a variety of configurations on the membrane P‐face. The first tight‐junctional strands clearly identifiable in freeze‐fractured preparations appeared at 15–17 days. These were closely associated with the particles of gap junctions and consisted of single strands on the P‐face. By hatching age (21 days) a “mature” pattern of tight‐junctional strands was interwoven in several layers. In the interim, more complex arrangements of tight‐junctional strands were in intimate relation with gap junctions.