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 (116% of live weight) than the slow growing chicks (102% of live weight). The 80-d-old broilers with a compensatory growth phase after an earlier slow growth period produced ITL muscle at 165-169% of live weight. The ITL muscle in 80- and 95-d-old broilers with restricted later growth after an earlier rapid growth period was 129 and 149% 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 600-651 mg/g in the chicks to 333-400 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.
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