Depending on the criteria you choose, several types of cells could be nominated as the most specialized cell in the mammalian body. The top candidate for this honor, of course, would be the cells that produce gametes, the mammalian sperm and eggs responsible for the continuation of the specific species of mammal!

The second choice is the red blood cells that carry oxygen and carbon dioxide in the blood—perhaps the most highly specialized cells in mammals. They live for roughly 120 days, but during that time they may travel more than 500 miles (800 kilometers) through various organs and blood vessels. Red blood cells lack a nucleus, so they are unable to reproduce; thus, new red blood cells form in the bone marrow.