The Mitochondria-Rich Cells in the Gills of Lampreys and Their Presence in Various Freshwater Species

Three types of mitochondria-rich (MR) cells occur in the gill epithelium of anadromous lampreys, although at different phases of the life cycle. 1) Ammocoete MR cells are confined to the larval phase and not involved in osmoregulation, 2) intercalated MR cells are present during all fresh water phases but absent in sea water and responsible for ion uptake from fresh water, and 3) chloride cells develop during metamorphosis and disappear during the spawning migration and are engaged in osmoregulation only in sea water. In freshwater species, ammocoete MR cells are also restricted to the larval phase, whereas intercalated MR cells occur throughout all phases of the life cycle. Chloride cells are retained in metamorphosed nonparasitic Lampetra planeri and Lethenteron appendix and in landlocked Petromyzon marinus, which have been recently derived from their parasitic and anadromous ancestors. These cells are absent in the nonparasitic relict species Lampetra aepyptera and in the freshwater parasitic species Ichthyomyzon unicuspis and Ichthyomyzon castaneus, which are close to the base of the phylogeny tree of contemporary northern hemisphere lampreys. We propose that the length of time a lamprey species has been restricted to fresh water is the overriding factor determining whether that species possesses chloride cells.