Chromatophore Organs, Reflector Cells, Iridocytes and Leucophores in Cephalopods

Abstract

The chromatophore organs of Lohgo are each composed of five types of cells: a central pigment cell: radially arranged, obliquely striated muscle fibers: neuronal processes; glial cells: and an investment of sheath cells. Sheath cells are absent in Octopus chromatophore organs. The cycle of expansion and retraction of a chromatophore organ may occur within the order of a second. It is clear that the muscle fibers expand the pigment cell and spread out the pigment granules. The pigment is contained within an unusual, filamentous, cytoplasmic compartment called the cytoelastic sacculus. This compartment has elastic properties. Reflector cells and iridocytes produce structural colors even though their components are colorless. Reflector cells in Octopus bear peripheral sets of leaf-like reflecting lamellae called reflectosomes: these contain proteinaceous platelets with a high refractive index (1.42). In each reflectosome the reflecting lamellae are separated by gaps that are about equal to the thickness of the lamellae, but have a lower refractive index (1.33). Reflectosomes are believed to reflect light and to function as thin-film interference devices. Iridocytes in squid and cuttlefish contain iridosomes that are also composed of sets of ribbon-like platelets but these are located centrally within the cell body. The platelets are usually oriented on edge with respect to the surface of the skin. The possibility that dermal iridocytes may act as diffraction gratings is discussed. Leucophores have thousands of processes that contain globules of protein with a high refractive index. These cells scatter light of all wave lengths and appear white in white light.