HISTOLOGICAL IDENTIFICATION OF OSTEOCYTES IN THE ALLEGEDLY ACELLULAR BONE OF THE SEA BREAMS ACANTHOPAGRUS-AUSTRALIS, PAGRUS-AURATUS AND RHABDOSARGUS-SARBA (SPARIDAE, PERCIFORMES, TELEOSTEI)

The bone of advanced teleost fishes such as those of the family Sparid ae is said to lack osteocytes or to be acellular. Acellularity has bee n determined by apparent lack of osteocyte lacunae. This study questio ns the validity of this criterion. Scanning electron and light microsc opy of paraffin and resin sections were used to show that the sides of sea bream mandibles consist of laminar parallel-fibred bone that we c all tubular bone, because it contains tubules, and localised regions o f Sharpey fibre bone. Osteocytes lie along the walls of tubules that a lso contain collagen fibril bundles (T-fibres), or in the lumens of tu bules that do not contain T-fibres. We show that the osteocytes are de rived from osteoblasts. The T-fibre system is different from other fib re systems that have been described. The tubules enclose wide T-fibres (lenticular in cross-section, maximum width about 8 mu m) that taper at their ends and continue as thin T-fibres (round in cross-section, a bout 2 mu m wide). The T-fibres originate in the periosteum. In mature tubular bone, spaces of increasing size develop around the osteocytes . Osteocytes are released from the bone matrix arid become postosteocy tes or bone-lining cells. Secondary bone lines the largest spaces. In Sharpey fibre bone, small osteocytes in small lacunae (about 2 mu m wi de) are found in columns parallel to the Sharpey fibres. Large osteocy tes are found in large round spaces and are much larger than comparabl e osteocytes in lacunae in the bone of the salmon Salmo salar. We conc lude that an absence of visible or conventional osteocyte lacunae does not mean that the cells themselves are absent. There are cells and tw o types of collagen fibre bundle in the tubules. The cells are osteocy tes derived from osteoblasts, and these osteocytes apparently resorb b one with the result that large amounts of bone are destroyed. ''Acellu lar'' tubular and Sharpey fibre bone are types of cellular bone that d iffer from each other and from conventional cellular bone.