SOURCES OF VARIATION IN MYOFIBER NUMBER WITHIN AND BETWEEN LITTERS OF PIGS

A study of the determinants of inter- and intra-litter variation in muscle fibre number was carried out on five litters of Large White piglets. Fresh frozen, whole mid-belly sections of m. semitendinosus were stained to demonstrate acid-stable myosin adenosine triphosphatase activity. From these sections it was possible to identify which fibres had developed as primary and which as secondary fibres. Estimations of total muscle fibre number, total primary fibre number and ratio of secondary fibres to primary fibres were made for each animal. Results demonstrated that primary fibre number varied between litters (P < 0.01) and was responsible for the variation in total muscle fibre number (P < 0.05) between litters since there was no significant variation in secondary : primary ratio. Within-litter differences in total fibre number could be attributed to both the secondary : primary ratio and primary fibre number, in almost equal contributions. However, when only the largest and smallest extremes of the litters were compared, variation in fibre number was due to the significant difference in the secondary : primary ratio (P < 0.01). Taken as a whole, the results appear to show that primary fibre number is responsible for all the variation in muscle fibre number between litters, and also makes a significant contribution, with secondary : primary fibre ratio, to the variation present within a litter. The factors responsible for variations in primary and secondary fibre numbers are discussed.