Growth-related changes in the histochemical properties and collagen architecture of the Musculus puboischiofemoralis pars medialis were compared among Silky, layer-type, and meat-type cockerels. Histochemical and immunohistochemical methods were employed and collagen architecture was studied using scanning electron microscopy. Total intramuscular collagen was also determined. The myofibers were categorized as type I, type IIA and a transitional form, type I-tr. The proportion of type I-tr myofibers diminished and these myofibers were transformed entirely into type I myofibers in meat-type but incompletely in the others. The largest diameter of type I myofiber was found in layer-type at 30 weeks of age. At 30 weeks of age, layer-type birds had attained well-developed perimysial collagen bundles while meat-type birds had less developed bundles. The endomysial collagen network had a large mesh size at 1 week and then accumulated many collagen fibrils to form a felt-like fabric of fibrils by 30 weeks of age. Silky birds developed the thickest endomysial collagen plates of all the breeds. From these results it appears that growth-related changes in the histological structure of M. puboischiofemoralis pars medialis are not necessarily causally affected by the different growth rates of chicken breeds.
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