Abstract
Newly hatched poults and chicks have immature thermoregulatory systems and are not able to maintain a constant body temperature when exposed to hot or cold ambient temperatures. During the immediate posthatch period during handling and transport, poults and chicks are commonly exposed to acute thermal conditions. This immediate posthatch period is when satellite cells, adult myoblasts, have their highest period of mitotic activity and are sensitive to extrinsic stimuli including ambient temperature. Variable temperatures can have long-lasting positive or negative effects on skeletal muscle growth. Satellite cells are a self-renewing multipotential stem cell population located between the basement membrane and sarcolemma of muscle fibers. They are responsible for all posthatch growth of muscle through muscle fiber hypertrophy. Results have shown that satellite cells are more sensitive to thermal stress during the period of satellite cell proliferation than during differentiation into multinucleated myotubes, and that pectoralis major muscle satellite cells from growth-selected lines of chickens and turkeys are more sensitive to temperature. Recent findings have shown that the rapamycin, mTOR, signaling pathway which is involved in muscle growth through hypertrophy is differentially affected by thermal stress in a growth-dependent manner. Since satellite cells are a stem cell population, they can also transdifferentiate into other cellular lineages. Anaerobic muscles like the pectoralis major muscle are more prone to converting to an adipogenic lineage compared to aerobic muscles like the biceps femoris. Thus, variable temperatures can impact the intramuscular fat content of the breast muscle. Taken together, changes in ambient temperature alter satellite cell function causing long-term effects on growth, morphological organization, and composition of the pectoralis major, breast, muscle.