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1. Varying growth rates in chickens were induced by different nutritional regimes. The collagen content and architecture of iliotibialis lateralis (ITL) muscle were compared among 21-d-old chick types and broiler types at 80 or 95 d of age. 2. Relative size of ITL muscle was greater in the rapid growing (1.16% of live weight) than the slow growing chicks (1.02% of live weight). The 80-d-old broilers with a compensatory growth phase after an earlier slow growth period produced ITL muscle at 1.65-1.69% of live weight. The ITL muscle in 80- and 95-d-old broilers with restricted later growth after an earlier rapid growth period was 1.29 and 1.49% of live weight, respectively. 3. Collagen content of ITL muscle did not differ between chick types and also among the broiler types. However, collagen concentration decreased from 6.00-6.51 mg/g in the chicks to 3.33-4.00 mg/g in the broilers. 4. Thick and thin perimysia and honeycomb endomysia were viewed by scanning electron microscope (SEM) photography. In the perimysia, a central wide layer of longitudinal collagen fibres and peripheral narrow band of transverse fibres were distinguished. Collagen baskets of adipocytes were observed in the perimysia. 5. Perimysial collagen fibres markedly increased in number and formed a larger fibre cluster during growth from chicks to broilers. Endomysia changed from thin to thicker meshwork with growth. However, the collagen architecture of the muscle in broilers did not change under different nutritional regimes. 6. In conclusion, ITL muscle of chicken develops optimally when body growth is enhanced, but the collagen content and architecture in broilers are not affected by different growth processes.

Varying chicken growth rates were induced with different nutritional regimes, and the collagen content and architecture of M. pectoralis (PT) were compared among 21-day-old chicks and broilers at 80 or 95 days of age. The percentage of muscle weight to live weight was higher in rapid growing chicks (8.4%) than slow growing chicks (6.3%). The 80-day-old broilers engaged in compensatory growth after the early slow growth period producing PT muscle at 11% of live weight. The 80- and 95-day-old chicks with restricted late growth after an early rapid growth period showed PT weight at 8% and 9% of live weight, respectively. Collagen content of the PT muscle markedly decreased from the chicks to the broilers. The collagen concentration was higher in the late-growth restricted broilers (1.67-1.88 mg/g) than the compensatory growth broilers (1.01-1.10 mg/g). Collagen concentration did not differ between the rapid and slow growing chicks (2.72 and 2.94 mg/g). Scanning electron micrographs showed thick and thin perimysia, and honeycomb endomysia. In the perimysia, a stack layer of collagen platelets and a reticular layer of collagen fiber cords were distinguished and collagen baskets of adipocytes were observed. The perimysial collagen fibers became thicker during growth of the chicks to broilers. However, in the late-growth restricted broilers, the perimysial collagen fibers seemed to have retarded development compared with the compensatory growth birds. The PT muscle of chickens develops optimally when body growth is enhanced. The PT muscle of the compensatory growth broilers had improved collagen architecture regardless of the marked decrease in collagen content.

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