Skeletal Muscle and Adipose Tissue Cellularity in Runt and Normal Birth Weight Swine2

Two experiments were conducted to examine the effects of prenatal stunting on skeletal muscle and adipose tissue cellularity in swine at 96 and 109 kg live weight. Animals used in Exp. 1 were large (1.56 kg average birth weight) and runt (.97 kg) littermate pigs and pigs with very low birth weight (.79 kg). The pigs with very low birth weight were half sibs to the large and runt littermates. Pigs were slaughtered at 96 kg live weight. Pigs in Exp. 2 were reared in individual cages and pens from birth to slaughter at 109 kg and included large barrows (1.59 kg average birth weight), runt barrows (.83 kg) and runt gilts (.91 kg). Runt pigs in Exp. 1 had fewer Type II white fibers in the semimembranosus muscle than did large pigs, but fiber diameter was similar for pigs in all groups. In Exp. 2, runts had a higher proportion of Type I fibers and fewer Type II fibers than did large birth weight pigs. Since runts that weighed less than .90 kg at birth had less muscle weight and fibers of similar or larger diameter, the data support the hypothesis that lower muscle fiber numbers are associated with prenatal stunting. Frequency distributions of adipocyte diameters were more bimodal in runt than in large birth weight pigs, and average diameter of adipocytes in the perirenal and subcutaneous depots was smaller for runts than for large pigs. There were more adipocytes in the perirenal adipose tissue of runts than in that of large birth weight pigs, and there was a tendency for more adipocytes in the subcutaneous depot of runt pigs. These differences were more pronounced in the more severely runted pigs. The presence of bimodal adipocyte diameter distributions was probably indicative of recruitment of additional cells for lipid filling. The fact that this phenomenon was more prevalent in runt than in large pigs at constant slaughter weight suggests that runt pigs had fewer adipocytes present initially for lipid filling.