Satellite cells and their regulation in livestock.

Satellite cells are the myogenic stem and progenitor population found in skeletal muscle. These cells typically reside in a quiescent state until called upon to support repair, regeneration, or muscle growth. The activities of satellite cells are orchestrated by systemic hormones, autocrine and paracrine growth factors, and the composition of the basal lamina of the muscle fiber. Several key intracellular signaling events are initiated in response to changes in the local environment causing exit from quiescence, proliferation, and differentiation. Signals emanating from Notch, wingless-type mouse mammary tumor virus integration site family members, and transforming growth factor-β proteins mediate the reversible exit from growth 0 phase while those initiated by members of the fibroblast growth factor and insulin-like growth factor families direct proliferation and differentiation. Many of these pathways impinge upon the myogenic regulatory factors (MRF), myogenic factor 5, myogenic differentiation factor D, myogenin and MRF4, and the lineage determinate, Paired box 7, to alter transcription and subsequent satellite cell decisions. In the recent past, insight into mouse transgenic models has led to a firm understanding of regulatory events that control satellite cell metabolism and myogenesis. Many of these niche-regulated functions offer subtle differences from their counterparts in livestock pointing to the existence of species-specific controls. The purpose of this review is to examine the mechanisms that mediate large animal satellite cell activity and their relationship to those present in rodents.