Muscle satellite cells (SC) are resident stem-like cells that play an integral role in skeletal muscle growth and repair. Understanding how SC maintain their identities and dynamic properties is critical to animal growth. However, the genetic and environmental factors governing SC behaviors and the underpinning mechanisms remain unknown. To explore whether genetic selection influences SC behaviors, we used 2 lines of chickens selected for over 50 generations with over a 10-fold difference in body weight at 56 d of age-the Virginia high weight selection (HWS) and low weight selection (LWS) lines. To study these 2 lines, we performed both in vivo and in vitro experiments. In vivo, we studied the abundance of SC in normal physiological settings and tested their functional roles in muscle regeneration using a muscle injury model. In vitro, we isolated SC from chicken skeletal muscle and assayed their ability to proliferate and differentiate under cultured conditions. Immunohistochemical staining of breast muscle (pectoralis major) revealed that muscle fibers from HWS chickens possessed more SC than those from LWS. Further analysis showed that the SC pool from HWS muscles contained a higher percentage of activated SC compared to that of LWS. When isolated SC from HWS and LWS muscles were cultured, HWS SC exhibited greater abilities to proliferate and differentiate than those SC from LWS. To test whether the observed in vitro differences in SC properties could be confirmed in vivo, we subjected chicken breast muscle to barium chloride to induce muscle injury and regeneration. Consistent with in vitro data, breast muscle in HWS chicken experienced a faster and more robust recovery than that of LWS, as evidenced by quicker regeneration and larger muscle fiber size. Taken together, these findings suggest divergent selection for body weight not only results in correlated responses in SC number, but also changes SC growth kinetics. Further dissection of the molecular mechanism will aid the identification of the target molecules for growth intervention in chickens.

Aust. J. Bioi. Sci. 1981, 34, 469-78 Ewes which had been lactating for 3-4 weeks and which had been milked by hand from the day of parturition were subjected to food restriction for 4 days. One group of three ewes was fed ad libitum and a second group of four ewes was fed to meet calculated requirements for maintenance and milk production. Over 4 days food intake was reduced by 80 % in both groups of ewes.In response to food restriction, milk yields and body weight decreased. Blood amino acids, plasma glucose, glucose pool size, glucose irreversible loss, insulin, thyroxine and the insulin: glucagon molar ratio decreased. In contrast, plasma glucagon remained relatively unaffected and plasma free fatty acids and growth hormone increased. These changes were similar for both groups of ewes and were reversed when food intake was restored.The results suggest that the hormonal control of gluconeogenesis in the ruminant is similar to that in the non-ruminant.

Glucose biokinetics of six normal pregnant/lactating ewes during the periparturient period were compared with those of five non-pregnant ewes induced to lactate artificially by treatment with oestrogen and progesterone, followed by a series of infusions of oxytocin. Normal ewes produced large amounts of milk (800-900 g/day) on day 1, and yields remained relatively constant until day 8 post partum. Milk production of induced ewes, however, was negligible on day 1 (30 g/day) but increased progressively until day 8 (540 g/day) after the start of milking. The glucose irreversible loss per minute 2-8 days before (6·4 v. 5·6 mg per kilogram body weight o . 7 5), and 2 days after the onset of lactation (9·1 v. 5·5 mg/min per kgo. 75 ) was significantly greater (P < 0·05) in normal pregnant/lactating ewes than in ewes induced to lactate artificially. By the eighth day of lactation rates of glucose irreversible loss per minute (7·4 v. 6·4 mg per kilogram body weighto. 75 ) were not significantly different (P > 0·05). The data were consistent with the hypothesis that glucose supply is rate limiting for milk production for several days after the initiation of lactation in non-pregnant, hormone-treated ewes.In parturient ewes, the rate of glucose irreversible loss was significantly increased 1-2 days post partum.

Lambs suffer from high mortality rates within the first 2-3 days after birth. The importance of sucking behaviour for the ingestion of colostrum and subsequently milk by the newborn animal is therefore seen as crucial for its survival and development. Epidermal growth factor (EGF) is a component of colostrum and milk (Read 1988; Gow et al. 1991) where it is thought to function as a promoter of gut maturation in young animals following ingestion. We recently reported (Gow and Moore 1992) a new physiological function for EGF, that of stimulating fluid intake in adult sheep. Here we present a review of the functions of EGF, particularly as they relate to lactation, body growth, appetite and fluid balance. We hypothesise that another function of EGF in mammary secretions may be to regulate body growth by altering sucking behaviour and activity in the neonate.