Biophysical Basis of Breast Muscle Myopathies in Broilers

Artificial selection for fast growth and high breast yield has considerably marked up the pressure on breast muscle development rate and mass in modern broiler chicken hybrids, leading to the onset and progressive increase in the prevalence of growth-related muscle abnormalities and meat quality alterations (namely White Striping - WS, Wooden Breast - WB, and Spaghetti Meat - SM). Each defect manifests with distinctive phenotypes: the appearance of white striations running parallel to the muscle fibers direction (WS), out bulging and pale areas of hardened consistency (WB) and an overall detachment of the fiber bundles (SM). Besides the microscopic traits, which may help to explain their phenotypes, myopathic muscles share similar histological features including an overall re-organization of the muscle architecture that involves both the fibers (i.e., giant fibers with rounded profile and internal nuclei, degeneration up to lysis, regeneration) and the connective tissue composing the perimysial and endomysial septa (i.e., lipidosis, fibrosis, remodeling of the extracellular matrix). In the past few years, several studies have been carried out to investigate the biological mechanisms and cellular processes potentially involved in the phenotypical expression of these abnormalities. However, the exact mechanism underpinning their occurrence is not fully understood. Investigations carried out so far strengthened the hypothesis of a polygenic inheritance of these defects and allowed to identify different factors and time series of events which may be responsible for their development. Among them, vascular defects, altered lipid and glucose metabolisms, and endoplasmic reticulum stress are worth to be mentioned. It has also been recently suggested that particular attention should be addressed to embryonic development to select for hyperplasia and satellite cells activity. This will represent an alternative to post-hatch selection aimed at increasing muscle mass accretion through the hypertrophic growth of the existing myofibers and concurrently provide satellite cell pools able to support muscle growth. Indeed, it is commonly recognized that the occurrence of growth-related breast meat abnormalities boosts with increasing growth rate, slaughter age as well as breast yield and weight. Within this context, it seems that the artificial selection for broilers growth is close to biological limits and further improvements might be restrained by muscle biological potential and related animal welfare concerns. This review is therefore intended to make a summary of the historical background, main features, and possible causative mechanisms of breast muscle myopathies in broilers.