Cellular localisation of transforming growth factor‐beta 2 and ‐beta 3 (TGF‐β2, TGF‐β3) in damaged and regenerating skeletal muscles

Abstract

Regeneration involves a number of cellular processes: revascularisation, invasion by haemopoietic cells, removal of necrotic tissue and finally reformation of the tissues. These processes have been extensively studied in vitro and are known to be affected by various growth factors. However, it has proven difficult to extrapolate the in vitro results 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 regenerating skeletal muscles have been studied but information is not available for the transforming growth factor-beta2 (TGF-beta2) or TGF-beta3, even though the TGF-betas are putative regulators of revascularisation, inflammation and the formation of connective tissue and muscle fibres. In this paper, the cellular locations of TGF-beta2 and TGF-beta3 in freeze-lesioned skeletal muscle were examined using immunohistochemistry. The amounts and locations of the TGF-betas varied depending on the stage of degeneration/regeneration. The first isoform of TGF-beta to appear within the lesion was TGF-beta2, which accumulated at the junctions between the viable and necrotic portions of fibres. The production of TGF-beta2 by the damaged fibres occurred immediately prior to the inflammatory reaction. However, these two events are probably independent of each other as the TGF-beta2-rich necrotic tissue was not preferentially phagocytosed. The haemopoietic cells contained TGF-beta3 immunoreactivity and the lesioned area became progressively rich in TGF-beta3 as the macrophages accumulated in the lesion and removed the TGF-beta2-rich necrotic tissue. In vitro, the TGF-betas are potent inhibitors of myogenic fusion and have been postulated to control the onset of myotube formation in vivo. Consistent with this idea, the formation of myotubes did not occur until the TGF-beta3-positive haemopoietic cells had migrated from the ghosts of necrotic fibres. In contrast, fusing satellite cells and newly formed myotubes contained strong TGF-beta2 immunoreactivity. This observation, coupled with the recent report that satellite cells require functional TGF-beta receptors to fuse in vivo, suggests that TGF-beta2 may stimulate myotube formation in vivo.