Heater cell

Heater cells are specialized modified muscle cells located at the base of the brain in billfish. They act to maintain temperatures in the brain and eye above ambient temperature. Producing heat and maintaining temperature requires high levels of metabolic activity, as such, the heater cells have the highest mitochondrial densities of any animal cell, ranging from 55% to 70% of the total cell volume. <ref>Sherwood, Klandorf, Yancey, Animal Physiology: From Genes to Organisms, 2nd edition)