REGULAR ARTICLE Cellular morphology and markers of cartilage and bone in the marine teleost Sparus auratus M. Dulce Estêvão & Nadia Silva & Begona Redruello & Rita Costa & Silvia Gregório & Adelino V. M. Canário & Deborah M. Power Received: 27 June 2010 / Accepted: 24 November 2010 / Published online: 15 January 2011 # Springer-Verlag 2011 Abstract Modifications have been characterised in terms of cellular organisation and the extracellular matrix (ECM) during bone ontogeny in the sea bream (Sparus auratus). During endochondral development, the agglomeration of matrix-secreting cells gives rise to chondrones; these chondrones frequently contain proliferating-cell-nuclear- antigen-positive cells, which subsequently become large collagen-II-positive cells with the characteristics of chon- drocytes. Moreover, the matrix:cell ratio within the peri- chondrium increases, accompanied by a modification in ECM composition. Mineralisation of cartilage ECM is marked by a rapid fall in cell number, the switching off of collagen II transcription and the switching on of collagen X transcription, followed by collagen I transcription and bone mineralisation. The formation of dermal structures initiated upon the condensation of mesenchyme cells defines the future location of the dermal bone. Subsequent cellular differentiation gives rise to cells on the bone surface; these cells are positive for collagen I and osteonectin transcripts. The fish skeleton, with the exception of vertebrae, tends to comprise flattened bones that are covered by a monolayer of cells, the periosteum. A third type of tissue, present in gills, consists of chondrocyte-like cells embedded in a mineralised matrix resembling chondroid bone in mam- mals. The results suggest that the cellular organisation and ontogeny of endochondral and dermal bone in the sea bream are similar to those described in other vertebrates. Keywords Skeletogenesis . Molecular markers . Ontogeny . Extracellular matrix . Sea bream, Sparus auratus (Teleostei) Introduction As in all other vertebrates, the skeleton of fish is important for the protection of internal organs and for body shape. However, it is not a haematopoietic tissue in fish as it does not have bone marrow and is less important for support or as a reservoir of calcium, as this ion is readily available in the surrounding environment (Kardong 1998). In mammals, the skeleton is reasonably well characterised and is composed of cortical (i.e. compact) bone and cancellous (otherwise known as trabecular) bone (Compston 2001; Hillier and Bell 2007). The reports that exist about fish bone indicate that its structure differs from that of terrestrial vertebrates, although its formation might share key features M.D.E. received a fellowship from the Programme for Educational Development of Portugal. This work was funded by the Portuguese National Science Foundation, grant no. POCI 2010 (POCI/CVT/ 61052/2004). M. D. Estêvão (*) : R. Costa : S. Gregório : A. V. M. Canário : D. M. Power Centro de Ciências do Mar, CIMAR-Laboratório Associado, Universidade do Algarve, Campus de Gambelas, 8005-139 Faro, Portugal e-mail: mestevao@ualg.pt M. D. Estêvão Escola Superior de Saúde, Universidade do Algarve, Av. Dr. Adelino da Palma Carlos, 8000-510 Faro, Portugal Present Address: N. Silva Medical Physics, The Queen’ s Medical Research Institute, University of Edinburgh, Edinburgh, UK Present Address: B. Redruello Instituto de Productos Lácteos de Asturias, IPLA-CSIC, Carretera de Infiesto s/n, 33300 Villaviciosa, Principado de Asturias, Spain Cell Tissue Res (2011) 343:619–635 DOI 10.1007/s00441-010-1109-y