The head cartilage of cephalopods. I. Architecture and ultrastructure of the extracellular matrix

Abstract

The morphology of head cartilage of the cephalopods Sepia officinalis and Octopus vulgaris has been studied on samples fixed and embedded for light- and electron microscopy and on fresh frozen sections viewed by polarizing microscopy. The organization of extracellular matrix (ECM) varies in different regions of the head cartilage. Superficial zones are made up of densely packed collagenous laminae parallel to the cartilage surface, while radially arranged laminae form a deeper zone where territorial and interterritorial areas are present. A compact arrangement of banded collagen fibrils (10–25 nm in diameter) forms the laminae of the superficial zones and of the interterritorial areas; a loose three-dimensional network of fibrils (10–20 nm) with many proteoglycan aggregates forms the territorial areas. A pericellular matrix surrounds the bodies of extremely branched territorial chondrocytes. Peculiar anchoring devices (ADs) are dispersed with variable orientation within the ECM. A perichondrium is present, but often connectival and muscular bundles are fused with the superficial layers of cartilage. Some vessels were also observed within the superficial inner zone and clusters of hemocyanin molecules were demonstrated both in the ECM and in some cells. The cephalopod head cartilage seems to share some morphological characteristics with both hyaline cartilage and bone tissue of vertebrates.