Specialized unique actin cytoskeletal structures are found adjacent to the plasma membrane. Four remarkable examples include red blood cells, human embryonic kidney cells, neurons, and sperm cells. In red blood cells, a spectrin-actin hexagonal lattice is formed by interconnected short actin filaments. In human embryonic kidney cells, the cortical actin forms a scale-free fractal structure. In neuronal axons, actin forms periodic rings that are stabilized by spectrin and adducin. And in mammalian sperm, actin forms a helical structure in the midpiece, i.e., the first segment of the flagellum.