Ultrastructure of the epistellar body of the octopus

Summary

The epistellar body of Octopus appears to be a vestigial photoreceptive organ. Electron-microscope examination reveals that the wall of the epistellar organ is composed of four layers: 1. a layer of neuron-like cells subjacent to the capsule, which send processes into the lumen of the organ; 2. the capsule, which includes flat pigment-laden cells; 3. a layer of neuron-like cells, which also send processes into the lumen; and 4. a layer of smaller epithelioid cells bordering on the lumen, apparently supportive to the neuronal processes. The neurite-like processes are covered with tubular projections, similar to those found on the processes of the retinula cells of the cephalopod retina. When several epistellar processes are in contact they may become organized in a manner roughly equivalent to the rhabdome of the octopus eye. The lumen-directed processes and the perikarya of the epistellar neuron-like cells also contain lamellated membrane systems similar to the “myeloid” bodies found in squid retinula cell bodies by Zonana. The general structure of the epistellar body suggests a non-functional photoreceptive organ. No ultrastructural evidence could be adduced to support a neurosecretory (or secretory) status for this structure.