In this short review of fish gill morphology we cover some basic gross anatomy as
well as in some more detail the microscopic anatomy of the branchial epithelia from representatives
of the major extant groups of fishes (Agnathans, Elasmobranchs, and Teleosts). The agnathan
hagfishes have primitive gill pouches, while the lampreys have arch-like gills similar to the higher
fishes. In the lampreys and elasmobranchs, the gill filaments are supported by a complete
interbranchial septum and water exits via external branchial slits or pores. In contrast, the teleost
interbranchial septum is much reduced, leaving the ends of the filaments unattached, and the
multiple gill openings are replaced by the single caudal opening of the operculum. The basic
functional unit of the gill is the filament, which supports rows of plate-like lamellae. The lamellae are
designed for gas exchange with a large surface area and a thin epithelium surrounding a wellvascularized
core of pillar cell capillaries. The lamellae are positioned for the blood flow to be
counter-current to the water flow over the gills. Despite marked differences in the gross anatomy of
the gill among the various groups, the cellular constituents of the epithelium are remarkably similar.
The lamellar gas-exchange surface is covered by squamous pavement cells, while large, mitochondriarich,
ionocytes and mucocytes are found in greatest frequency in the filament epithelium. Demands
for ionoregulation can often upset this balance. There has been much study of the structure and
function of the branchial mitochondria-rich cells. These cells are generally characterized by a high
mitochondrial density and an amplification of the basolateral membrane through folding or the
presence of an intracellular tubular system. Morphological subtypes of MRCs as well as some
methods of MRC detection are discussed