Morphological evaluation of Semimembranosus muscle quality in the Nero di Lomellina pig: a contribution to porcine biodiversity

Background: This study provides a morphological characterization of the thigh and Semimembranosus muscle (SM) of the Nero di Lomellina (NL) pig, considering two different rearing periods, December-October (rearing period 1) and September-July (rearing period 2), and using a commercial crossbreed (CH) as a reference. Methods: Fat content in the thigh was evaluated using computed tomography, while SM characteristics were assessed by comparing muscle fiber area, density, and total number. Fiber type composition was determined by immunofluorescence and molecular analyses through the localization and expression of myosin isoforms, including MyHC I, MyHC IIb, and MyHC IIa/x (red slow oxidative, white fast glycolytic, and intermediate fast oxidative glycolytic fibers, respectively). Additionally, physicochemical analyses were conducted to assess muscle quality. Results: The results showed that both breeds had higher intermuscular fat deposition in the rearing period 2. In period 1, NL pigs exhibited larger muscle fibers than CH pigs, whereas no differences were observed in period 2. Immunofluorescence revealed that CH pigs in period 2 had a higher proportion of red and intermediate fibers, while NL pigs showed no significant changes in fiber composition related to the rearing period. Conclusions: In conclusion, lower temperatures in period 2 promoted an oxidative phenotype in CH, whereas NL pigs showed greater stability across rearing periods and better adaptability to temperature fluctuations. These findings suggest that NL pigs exhibit greater resilience to environmental changes, supporting previous studies on breedspecific responses to temperature variation.