CELLULAR-LOCALIZATION OF TRANSFORMING GROWTH-FACTOR-BETA-2 AND GROWTH-FACTOR-BETA-3 (TGF-BETA-2 TGF-BETA-3) IN DAMAGED AND REGENERATING SKELETAL-MUSCLES

Regeneration involves a number of cellular processes: revascularisatio n, invasion by haemopoietic cells, removal of necrotic tissue and fina lly reformation of the tissues. These processes have been extensively studied in vitro and are known to be affected by various growth factor s, However, it has proven difficult to extrapolate the in vitro result s to the in vivo situation. This is partially because the response of cells to growth factors is dependent on which other regulatory factors are present. The locations of various growth factors within regenerat ing skeletal muscles have been studied but information is not availabl e for the transforming growth factor-beta2 (TGF-beta 2) or TGF-beta 3, even though the TGF-beta s are putative regulators of revascularisati on, inflammation and the formation of connective tissue and muscle fib res. In this paper, the cellular locations of TGF-beta 2 and TGF-beta 3 in freeze-lesioned skeletal muscle were examined using immunohistoch emistry. The amounts and locations of the TGF-beta s varied depending on the stage of degeneration/regeneration, The first isoform of TGF-be ta to appear within the lesion was TGF-beta 2, which accumulated at th e junctions between the viable and necrotic portions of fibres. The pr oduction of TGF-beta 2 by the damaged fibres occurred immediately prio r to the inflammatory reaction. However, these two events are probably independent of each other as the TGF-beta 2-rich necrotic tissue was not preferentially phagocytosed, The haemopoietic cells contained TGF- beta 3 immunoreactivity and the lesioned area became progressively ric h in TGF-beta 3 as the macrophages accumulated in the lesion and remov ed the TGF-beta 2-rich necrotic tissue. In vitro, the TGF-beta s are p otent inhibitors of myogenic fusion and have been postulated to contro l the onset of myotube formation in vivo, Consistent with this idea, t he formation of myotubes did not occur until the TGF-beta 3-positive h aemopoietic cells had migrated from the ghosts of necrotic fibres, In contrast, fusing satellite cells and newly formed myotubes contained s trong TGF-beta 2 immunoreactivity. This observation, coupled with the recent report that satellite cells require functional TGF-beta recepto rs to fuse in vivo, suggests that TGF-beta 2 may stimulate myotube for mation in vivo. (C) 1997 Wiley-Liss, Inc.