Developmental patterns of muscle, bone, and organ growth in foetal lambs of five genotypes

Patterns of change in the weights of organs (brain, liver, kidney, heart, lungs, thymus, and thyroid) and some muscles and bones of the hind leg were assessed using 106 lamb foetuses distributed evenly over the foetal age range from 69 to 146 days. Differences between five genotypes (Merino, Romney, Merino x Romney, Drysdale, and Wiltshire), two sexes (male and female), and two pregnancy rank categories (single and multiple) for foetal growth indicated that Merino foetuses grew more slowly than all other genotypes (P < 0.01) and that females were lighter than males (P < 0.001). Crown-rump lengths at a foetal weight of 3 kg were lower for the Romney and Drysdale genotypes (P < 0.05) and for males relative to females. There were several significant genotype effects on organ weights but these did not appear to form a consistent pattern. Thus the Romney had the lightest brain, the Merino the lightest liver but the heaviest brain and lungs, and the Drysdale had the lightest thyroid and thymus. Femur and tibia weights at a foetal weight of 3 kg were greatest for the Merino and for females, but the weights of muscles around the femur differed little between genotypes or sexes. Allometric growth ratios for the weights of organs (except the heart), bones, and muscles decreased with increasing foetal weight. When adjusted to a constant bone weight, both the femur and tibia were longest for the Merino group and females. Leg muscle to bone ratio was lower for females, but did not differ between genotypes. Leg muscularity in contrast, was lower for the Merino group than for any of the others, and was also lower for females. It is concluded that appreciable genotype differences exist in patterns of foetal growth.