INTERCELLULAR-JUNCTIONS IN THE WATER-BLOOD BARRIER OF THE GILL LAMELLA IN THE ADULT LAMPREY (GEOTRIA-AUSTRALIS, LAMPETRA-FLUVIATILIS)

Thin sections and freeze-fracture replicas of the water-blood barrier in the gill lamellae of adult lampreys (Geotria australis, Lampetra fl uviatilis) demonstrate that the occluding junctions between epithelial pavement cells differ markedly from those between ''endothelial'' pil lar cells in the structure and arrangement of their strands. The zonul ae occludentes between pavement cells typically consist of complex net works of 4-6 strands, the mean number of which undergoes a small but s ignificant decline when the animal is acclimated to seawater. In compa rison, the occluding junctions between pillar cells are less elaborate and may represent maculae or fasciae, rather than zonulae occludentes . They do not apparently undergo a change when the animal enters saltw ater. The results indicate that the main part of the paracellular diff usion barrier to proteins and ions is located in the epithelium rather than the endothelium. Communicating (gap) junctions are present betwe en adjacent pavement cells, between pavement and basal cells and betwe en pillar cells. These findings suggest that the epithelial cells and the pillar cells in the water-blood barrier of lampreys both form `fun ctional syncytia'. The results are discussed in the context of ion-tra nsporting epithelia in other aquatic vertebrates.