Ultrastructure and function of cephalopod chromatophores.

Abstract

synopsis. Each chromatophore organ consists of a pigment cell and of several radial muscle fibers that represent separate cells. The pigment granules are contained within an elastic sacculus within the pigment cell. The sacculus is attached around the equator of the chromatophore to the cell membrane by zonal haptosomes. In turn, the cell membrane is attached to the radial muscle fibers by a dense basal lamina. The cell membrane of the retracted chromatophore is highly folded. Contraction of the radial muscle fibers is initiated by (a) excitatory junction potentials, (b) miniature potentials, or (c) spike potentials. The latter arise spontaneously in the muscle fibers when these have undergone some internal (metabolic?) change. The contraction of the muscle fibers causes expansion of the pigment-containing sacculus. Relaxation of the muscle fibers permits the sacculus to assume its original lenticular or near-spherical shape; the energy for this is stored within the expanded elastic components of the sacculus. In normal skin the chromatophore organs are entirely under the control of the central nervous system, the muscle fibers being activated only by local, excitatory postsynaptic potentials initiated by motor nerve impulses. That postsynaptic potentials are non-propagating insures that individual motor fibers can be activated individually, thus permitting a delicate control of skin color by recruitment as well as by frequency. Tonic contractions and pulsations, involving spontaneous release of transmitter from nerve terminals and spike generation within the muscle fibers, respectively, are the result of altered, abnormal conditions within the skin.