DEVELOPMENTAL PATTERNS OF MUSCLE, BONE, AND ORGAN GROWTH IN FETAL LAMBS OF 5 GENOTYPES

Patterns of change in the weights of organs (brain, liver, kidney, hea rt, 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 genoty pes (Merino, Romney, Merino x Romney, Drysdale, and Wiltshire), two se xes (male and female), and two pregnancy rank categories (single and m ultiple) for foetal growth indicated that Merino foetuses grew more sl owly 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 m ales relative to females. There were several significant genotype effe cts on organ weights but these did not appear to form a consistent pat tern. Thus the Romney had the lightest brain, the Merino the lightest liver but the heaviest brain and lungs, and the Drysdale had the light est 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. A llometric growth ratios for the weights of organs (except the heart), bones, and muscles decreased with increasing foetal weight. When adjus ted to a constant bone weight, both the femur and tibia were longest f or the Merino group and females. Leg muscle to bone ratio was lower fo r females, but did not differ between genotypes. Leg muscularity in co ntrast, 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.