A light and electron microscope study of the gills of larval lampreys (Geotria australis) with particular reference to the water‐blood pathway.

Abstract

The gills of ammocoetes of the Southern Hemisphere lamprey Geotria australis have been studied using light and electron microscopy. Emphasis has been placed on describing the structures and vessels involved in gaseous exchange, and on providing quantitative data for the water‐blood barrier, including diffusion distance, diffusing capacity and the relative volumes of the component tissues. Although lamprey gills lie inside rather than outside the branchial skeleton as in gnathostomatous fishes. the morphology and ultrastructure of the gill filaments and secondary lamellae of G. australis larvae are very similar to those of teleost fishes. The extensive blood spaces within the secondary lamellae are enclosed by pillar cell bodies and pillar cell flanges which support two layers of epithelial cells. The outer surfaces of the epithelial cells are ridged and covered in a flocculent material which probably represents mucus. Differences were observed in the components of the water‐blood barrier at the distal edges and at the surface of the secondary lamellae. At the distal edge, the lining of the marginal channel consisted of an endothelial cell rather than the pillar cell flanges which line the blood spaces of other regions. Based on light micrograph measurements, these differences result in a reduction in the arithmetic mean thickness of the water‐blood barrier from 3.62 μm over the pillar cells to 2.22 μm over the marginal channel. Using values for the water‐blood barrier obtained from light micrographs, the arithmetic and harmonic mean diffusing capacities were calculated as 1.1046 and 1.7589 ml O2min/mm Hg/Kg.