The Gills of Hagfishes

Abstract

The gills of hagfishes form lens-shaped pouches, rather than holobranches that characterize those of lampreys and gnathostome fishes. Their internal surface is enlarged by radial folds, which extend between the medial and lateral walls of the pouch and contain the branchial microcirculation. Blood and water flow through the gills are arranged in a countercurrent system, thereby facilitating gas exchange. The circulation of the hagfish gills consists of an arterio-arterial and an arteriovenous component. The arterio-arterial circulation connects the afferent and efferent branchial arteries. The arteriovenous circulation comprises an intrabranchial sinusoid system, which is supplied by the arterio-arterial circulation through arteriovenous anastomoses and connected to the peribranchial sinus through gaps in the muscle layer of the gill pouch. The majority of the epithelial surface is formed by mucus-secreting pavement cells. In the lateral half of the gill pouch, mitochondria-rich (MR) cells are intercalated between the pavement cells, which are characterized by an extensive amplification of the basolateral cell membrane and, in freeze-fracture replicas, by assemblies of linear arrays of particles and fibrils in the apical cell membrane. Since the MR cells show strong histochemical reactions for both Na+/K+-ATPase and carbonic anhydrase, they may be engaged in ion transport and/or acid-base regulation.