Abstract
The structure of the capillary plexus in the gas exchange zone in the lungs of five neonates was investigated by electron microscopy and cytochemical techniques. The results show that the capillaries associated with the terminal respiratory saccules are narrow and that they generally possess two to three endothelial cells per cross section. "Seamless" and protoplasmic capillaries were not observed. A capillary plexus was located on either side of each intersaccular septum and the mesh size of this plexus was relatively large. The gas diffusion pathway in the neonatal lung was found to be very short (mean 1.84 micron). It is suggested that the layer of periodate reactive material lining the luminal surfaces of the capillary endothelial cells may have important functions in vivo.