ORIGINAL ARTICLE Differentiation and Growth of Myotomal Muscles in a Non-Model Tropical Fish Pterophyllum scalare (Teleostei: Cichlidae) A. Kacperczyk, I. Je ˛ drzejowska and M. Daczewska* Department of Animal Developmental Biology, Zoological Institute, University of Wroclaw, Sienkiewicza 21, 50-335 Wroclaw, Poland Introduction In all vertebrates having a trunk and a tail, skeletal mus- cle fibres derive from somites that arise as a result of paraxial mesoderm segmentation. Among vertebrates, the process of somitogenesis has been extensively studied in birds and mammals. Each somite is an epithelial vesicle whose lumen contains mesenchymal cells called somato- coel cells. During somite maturation, epithelial cells of its medio-ventral part undergo de-epithelialization to form a mesenchymal sclerotome, which is the source of connec- tive tissue of the axial skeleton. The dorsal part of the somite retains its epithelial character and becomes dermomyotome. Subsequently, the dermomyotome dif- ferentiates into a myotome, which gives rise to trunk (myotomal) muscles and the dermatome developing into connective tissue of dermis (Christ and Ordahl, 1995; Stockdale et al., 2000). Muscle progenitor cells that derive from the dermomyotome show a characteristic pattern of gene expression. It is known that genes specifying muscle cell identity and also their proliferation and migration encode ‘paired-box’ transcription factors: Pax-3 and Pax- 7 (Gros et al., 2005; Relaix et al., 2005). In mature mus- culature, the expression of Pax-7 is limited to satellite cells. These cells, first described by Mauro (1961), are mononucleated myogenic cells, located between the sar- colemma and basement membrane of terminally differen- tiated muscle fibres. Satellite cells are normally quiescent in adult muscle, but act as a reserve population of cells capable of proliferation in response to injury, causing regeneration and/or growth of muscle fibres (Cossu et al., 1980; Bischoff, 1994). Growth of muscles occurs through hypertrophy and/or hyperplasia. Hypertrophy consists in growth of pre- existing muscle fibres, whereas hyperplasia refers to the formation of new muscle fibres (Rescan, 2005). In fish, gene expression and morphology of developing skeletal muscles have been well characterized in zebrafish (Brachydanio rerio), a model fish species. In this study, differentiation of myotomal muscle fibres in a non-model *Correspondence: Tel.: +48713754023; fax: +48713222817; e-mail: daczew@biol.uni.wroc.pl With 4 figures Received November 2010; accepted for publication April 2011 doi: 10.1111/j.1439-0264.2011.01086.x Summary Somite differentiation, muscle fibres formation and growth were analysed in a non-model tropical fish Pterophyllum scalare. In this study, it was found that during somite differentiation, a primary myotome appears. The primary myo- tome is filled with multinucleated myotubes that constitute the major part of the somite. Subsequently, Pax-3 (paired-box protein)-positive cells, located externally to the myotomes, appear. In post-hatching stages, mononucleated proliferating cell nuclear antigen-positive cells are observed in the inter-myoto- mal spaces and within the myotomes. The mononucleated cells, situated in the myotomes, first express desmin in their cytoplasm and then Pax-7 (paired-box protein) in their nuclei. Expression of desmin indicates that they will enter myogenic pathway, whereas expression of Pax-7 suggests their role of satellite cells. We assume that mononucleated intramyotomal cells are myogenic pre- cursors involved in muscle growth. In advanced (post-hatching) stages of myo- genesis, myotomes contain both primary and new muscle fibres. Morphometric analyses show that in Pterophyllum scalare, growth of muscle fibres is mainly a result of hypertrophy. Anatomia Histologia Embryologia ª 2011 Blackwell Verlag GmbH • Anat. Histol. Embryol. 40 (2011) 411–418 411