Animated coloured scanning electron micrograph (SEM) Fresh water convict cichlid (Cichlasoma nigrofasciatum) fish gills. Shown here are the fish gill arches with many branchial filaments and respiratory gill lamellae. Gill lamellae consist of mostly 'pavement' cells (epithelial ridged cells), which are the major site of gas exchange. Mitochondria rich cells (MRC) are produced in the gill lamellae and increase the sonoregulatory capacity in response to osmotic challenges. MRCs are believed to be the site of ion uptake by fish in fresh water. Gills of vertebrates are developed in the walls of the pharynx along a series of gill slits opening to the exterior. In fish, the gills are located on both sides of the pharynx. Gills are made of 'branchial' filaments and respiratory gill lamellae that help increase surface area for oxygen exchange. In bony fish, a bony cover called an operculum covers the gills.