Since 1982, our group has collaborated with Prof. Gregorio S. Montes in analyzing the composition and function of the extracellular matrix during fin regeneration in teleosts. The structure and ultrastructure of fully formed and regenerating fins of various teleostean species have been studied. The dermal skeleton of fins consists of rays formed by lepidotrichia and which is surrounded by loose connective tissue and a multistratified epidermis. Distally, there are hyperpolymerized macrofibrils of elastoidin, a collagen-like protein, named actinotrichia. Both lepidotrichia and actinotrichia are formed during regeneration. However, whereas the lepidotrichia form distally by the addition of new material, the actinotrichia are synthesized very early at proximal sites and then maintained by a continuous turnover at the distal margin of the regenerating ray blastema, as shown by radioactive pulse-chase experiments. Using the picrosirius-polarization method, as well as immunocytochemistry for various extracellular matrix components and enzymatic digestion, we have established correlations between the various extracellular matrix components; glycosaminoglycans and collagen in the mature adult structure. During regeneration and after wound healing a blastema is formed. Histological analysis of the extracellular matrix has indicated that the blastema consists of the blastema proper, which is rich in hyalunorate and the actinotrichial blastemic region, which is rich in glycosaminoglycan sulphates and collagen. The inhibition of collagen synthesis by several specific drugs during fin regeneration attenuated blastema formation, regenerative outgrowth and extracellular matrix formation. These findings suggest an interesting morphogenetic and regenerative function for collagen which could provide an interesting field for future research.