Growth-related changes in the histochemical properties and collagen architecture of the Musculus pectoralis were compared among Silky, layer-type and meat-type cockerels. Histochemical and immunohistochemical methods were used and collagen architecture was studied using scanning electron microscopy. The total amount of collagen present was also measured. The diameter of type IIB myofibers was similar or rather larger in the layer-type birds compared with the meat-type. The collagen content was generally low for 5-10 weeks across the breeds and then increased in the other breeds except for Silky. In the perimysium, the collagen bundles gradually increased in size and the density of the fibrils also increased during growth. At 30 weeks of age, the layer-type birds showed compact collagen bundles while the meat-type had loose bundles. The endomysial collagen network appeared relatively denser in the meat-type chicks compared to the others at week 1. At 30 weeks of age, compact and felt-like structure of endomysium was shown by Silky and layer-type chickens, while the meat-type showed a relatively loose arrangement of tissue in the endomysial collagen. From these results, it appears that the meat-type chicken can produce a large M. pectoralis with many, relatively thinner myofibers and a relatively undeveloped form of intramuscular collagen structure.
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