Declining muscle hyperplasia in juvenile trout is associated with a significant impairment of the supportive function of the myogenic progenitor niche

Unlike mammals and birds, where new muscle fiber formation (hyperplasia) ceases around birth, large and fast-growing fish such as trout undergo a spectacular post-hatching surge of hyperplasia, followed by a considerably delayed hyperplasia decline. This study investigated the role of muscle stem cells (MuSCs) and their niche in this process by assessing changes in their abundance, myogenic potential and niche functionality.

Hyperplasia kinetics were investigated by measuring the total number of fibers and their cross-sectional area (CSA) in white muscle across juvenile stages (10 g to 2 kg). Quantification of MuSCs during growth was performed by pax7 in situ hybridization. To assess the supportive capacity of the MuSCs niche, muscle-derived cells (MDCs) extracted from the Tg(mlc2:gfp) trout line were transplanted into muscle of wild-type trout at different juvenile stages. Expression of GFP in transplanted muscle was measured as an indicator of myogenic progenitor differentiation.

Histological analysis revealed a significant decrease in hyperplasia and MuSCs density (defined here as pax7⁺ cells) between 10 and 500 g trout. Transplantation experiments using MDCs from Tg(mlc2:gfp) trout (10 g donors into 10 g to 2 kg recipients) showed alterations in niche functionality as the trout grew from 10 to 500 g. The transplantation of Tg(mlc2:gfp) MDCs from early to late juvenile donor trout (100 g to 2 kg) into 10 g WT recipients showed a decrease in the GFP signal as the donor weight increased. Detailed analyses of GFP⁺ fibers produced after transplantation showed an enrichment of small-CSA GFP⁺ fibers in 10 g but not 100 g trout recipient muscles, indicating a rapid impairment in niche ability to support hyperplasia. In addition, by comparing trout of the same age but different weights, we demonstrated that weight gain, rather than chronological aging, is a key factor driving this decline.

Overall, these results indicate that the decline in muscle hyperplasia in trout is associated with an early impairment of the MuSC niche, along with a reduced MuSC density. Also, weight gain was found to play a more critical role than aging. These original findings provide new insights into the mechanisms underlying muscle growth as hyperplasia declines in vertebrates.