Freeze-fracture study of the turtle lung. 1. Intercellular junctions in the air-blood barrier of Pseudemys scripta

The air-blood barrier in the lung of the turtle Pseudemys scripta was studied by means of freeze-fracture replicas in an attempt to give a detailed account on the structural organization of the intercellular junctions. Between the pneumocytes, zonulae occludentes containing 4-19 strands in the apico-basal direction are present; they are considered to be physiologically very tight. In the endothelium, fasciae occludentes, i.e., discontinuous occluding junctions can be found, composed of up to 4 strands. These junctions are regarded to be very leaky. The findings are discussed in relation to recent physiological results, suggesting that in comparison with 'dry' mammalian lungs, the turtle lung is a rather wet lung based on its much larger transcapillary fluid filtration into the interstitium. In addition, small maculae communicantes are demonstrated between the pneumocytes; they possibly serve for metabolic coupling.