| Abstract: | At 110 d of gestation, fetuses were removed from Ossabaw, Yorkshire and crossbred sows and from sows selected for high (obese) or for low (lean) backfat thickness. Ossabaw and obese fetuses were smaller than lean, Yorkshire and crossbred fetuses (595 +/- 32 and 863 +/- 44 g vs 1,030 +/- 77, 1,380 +/- 1,144 +/- 80 g; means +/- SE), respectively. Minimum fiber diameters in the semitendinosus muscle were larger in obese, lean and Ossabaw fetuses than in Yorkshire and crossbred fetuses (12.9 +/- .3, 12.5 +/- .2 and 11.8 +/- .2 micron vs 10.2 +/- .2 and 11.1 +/- .8 micron), respectively. Histochemical analysis for NADH-tetrazoleum reductase (NADH-TR) and esterase activities indicated no fiber type differentiation and no strain differences. Fiber type differentiation was obvious with acid ATPase histochemistry in muscles from all fetuses. The white portion of the semitendinosus from Ossabaw, obese and lean fetuses had many fibers that contained no histochemically detectable lipid (oil red O staining). The unstained fibers (oil red O) were always the most peripherally located fibers in a fasciculi. In some instances, 50% of the fibers in a fasciculi were not stained for lipid. All the fibers in the red portion of muscle from Ossabaw, obese and lean fetuses contained lipid. All the fibers in the red and white portions of muscles from crossbred and Yorkshire fetuses contained lipid. Muscles from young (1 to 2 d old) Ossabaw, Yorkshire and crossbred pigs were also histochemically analyzed. Analysis for NADH-TR, esterase and alkaline phosphatase (capillary staining) activities indicated no fiber type differentiation and no strain differences. As in the fetuses, the white portion of muscle from Ossabaw pigs had many fibers with no lipid (oil red O). Lipid was present in all fibers in the deep portions of muscle from Ossabaws and in all fibers in both portions of muscle from crossbred and Yorkshire pigs. These results indicate that when lipid staining is used as the criterion, fiber type differentiation is evident in muscle from fetuses and young pigs from strains not genetically selected for muscling (Ossabaw, obese and lean strains). Furthermore, fiber type differentiation is not evident in muscle from strains of pigs genetically selected for greater muscling (crossbred and Yorkshires). |
