The structure and ultrastructure of the rostral cartilage in the spiny eel, Macrognathus siamensis (Teleostei: Mastacembeloidei)

Abstract

An elastic, cell-rich cartilage provides flexible support to the highly mobile, rostral tentacle of the mastacembelid, Macrognathus siamensis. Active movement of the tentacle is effected by skeletal muscles, the muscular bellies of which are located outside the organ. The tentacle returns to its original shape by elastic recoil. The cartilage resembles plant supporting tissue and the cartilages of certain invertebrates. It is surrounded by a thick perichondrium and articulates at a synovial joint with the supraethmoid. The chondrocytes are large and shrunken within lacunae. They contain glycogen and cytoplasmic stores of RNA. The matrix is reduced to thin seams between adjacent cells, and stains strongly with alcian blue and a variety of elastic stains. Parts of the matrix are trilaminar, and such an appearance recalls the distinction between the primary cell walls of adjacent plant cells and the intervening middle lamella. The perichondrium consists of an alternating sequence of cells, circularly arranged collagen fibres and a foamy, amorphous material of unknown composition. Deep to the perichondrium, the chondrocytes are packed with intermediate filaments. Membrane-bound organelles are not prominent, though mitochondria are located at the periphery of the cell. The ultrastructural similarities between these chondrocytes and those of hyaline-cell cartilage (chondroid) support the contention that Schaffer's concept of a rigid distinction between 'true' cartilage and 'chondroid' is no longer tenable. The matrix is devoid of collagen and is most distinctive. In the immediate vicinity of the cells it consists of matrix granules and matrix fibers but, where a trilaminar appearance is easily discernible, there is a central zone that consists of large masses of amorphous material that is presumed to contain elastin. Such amorphous material has not previously been seen in any teleostean elastic tissue. By contrast, elastic system fibres, readily demonstrable elsewhere in teleosts, are conspicuously absent.