Effect of breed, slaughter weight and sex on histological properties of ovine muscle.

Thirty-three wether and ewe lambs of two breed groups- Suffolk ram x Suffolk-Rambouillet ewe (Su x R) and Suffolk ram x Finnish Landrace-Southdown ewe (Su x F x So)-and two slaughter weight groups (32 and 41 kg) were studied. Carcass studies, including histological examinations of muscle fiber types, sarcomere lengths and fat cell, were made. The longissimus (LD) muscle at the 13th rib and a sample of the semimembranosus (SM) from the upper proximal location were used. Fibers in both muscles were classified as beta R, alpha R or alpha W. Su x F x So lambs had more alpha W fibers and fewer alphaR fibers in both muscles. Breed type appeared to cause a physiological shift from alpha R to alpha W fiber types. Furthermore, beta R fibers were larger in the SM of the Su x F x So lambs than in that of the Su x R lambs. beta Red fibers in the LD, and all three types fibers in the SM, increased in size with increased weight. Wethers had fewer alpha R fibers in the SM, while ewe lambs had larger alpha R and alpha W fibers in the SM. The more phasic LD muscle had fewer beta R fibers, more alpha W fibers and smaller alpha R and alpha W fibers than the more tonic SM. Su x F x So lambs were fatter, as indicated by larger fat cells in both muscles. In addition, as slaughter weight increased, there was a concurrent increase in fat cell diameters in both muscles. Fat cells were larger in the LD than in the SM. Sarcomere lengths were not affected by breed, slaughter weight or sex. However, sarcomeres were longer in the SM than in the LD. Fat cell size and beta Red cells size for the LD were positively correlated with carcass fat indices, while shear values were negatively correlated with fat cell diameters. There was also a positive relationship between sarcomere length and fat cell size in the SM.