Exploring the relationship between tissue morphology andmuscle function across different rearing temperatures in pigs

Fibres in mammalian skeletal muscles are classified into oxidative (red), glycolytic (white), and intermediate (pink), according to the predominant myosin isoform (MyHC I, IIb, and IIa/x, respectively) that defines muscle contractile profile and function. Our study aimed to provide a morpho-functional characterization of Longissimus dorsi (LD:rachis stability) and Semimembranosus (SM:locomotion) by investigating the relationship between muscle morphological features and functions in two nine-month life cycles (December to October, September to July; higher vs lower rearing temperatures). Muscles were sampled from 9-month-old pigs (n = 7/cycle) at the slaughterhouse; morphological and gene expression analyses were carried out to define LD and SM's fibres profile. Both morphological (a) and molecular (b) analyses suggested a different fibre profile between the two muscles. A higher presence of glycolytic fibres was found in LD (a: p < 0.0001; b: p < 0.05), while SM showed more oxidative (a: p < 0.01) and intermediate (a: p < 0.0001; b: p < 0.0001) ones. Moreover, SM showed a different fibre type composition between the two life cycles, with less red (a: p < 0.001) and pink (a: p < 0.001; b: p < 0.05) fibres and a tendency to a higher content of white fibres in animals born in winter and slaughtered in autumn (a: p = 0.07). The higher composition in white fibres of LD may explain its role in stabilizing the rachis through fast not-lasting contractions. SM was found richer in red/pink fibres, necessary to support both fast and slow movements during animals' locomotion. Additionally, an effect of the rearing temperatures on fibre composition was spotted in SM, with fewer oxidative/intermediate fibres in animals that faced extremely hot conditions during their lives, up to slaughter. Literature shows that heat stress can impact muscle metabolism, increasing its glycolytic activity and reducing its oxidative capacity. Hence, we hypothesize that the different fibre compositions may reflect the adaptive behaviour of muscle tissue and metabolism to environmental thermic stress. However, further studies are required to better clarify this aspect.