Asymmetry in growth of the broiler chicken: histochemical and anatomical studies on the musculo-skeletal system

Abstract

The aim of this work was to study the apparent asymmetry in the breast muscles in some individual broiler chickens strain "Cobb 500", with a view to revealing the cause(s) of this asymmetrical growth. Birds showing apparent asymmetry in the breast were selected to study their breast muscles anatomically and histochemically in comparison with unselected birds as controls. The skeleton was studied in these birds too. Selected chickens had heavier body and muscle weight, and higher growth rate than the controls. However, no significant differences were obtained between the two sides of the pectoralis or supracoracoideus muscles in either group of chickens, although the degree of asymmetry of the pectoralis muscle in selected chickens was higher than in the controls. The distribution of pectoralis muscle weight (degree of asymmetry) was normally distributed in both groups of chickens. The histochemical study on the pectoralis muscle revealed that there were significant differences in fibre number and diameter between the anterior (region A) and mid part (region B) of the pectoralis muscle in both groups of chickens, in that there were more FG fibres in region B, whereas region A has more FOG and SO fibre number. The diameter of fibre type in region A was significantly larger than in region B. Differences in fibre diameters were obtained between the right and left side of the pectoralis muscle. FG and SO fibres in the left anterior side of pectoralis muscle were growing significantly faster than in the right side in selected chickens, and control (against body weight or muscle weight). However, no significant differences were obtained for fibre numbers per square millimeter either between the two sides in control or in selected chickens. Many measurements on the skeleton were taken to study the shape of the sternum and the rib-cage in both groups of chickens. The essential differences were the depth of the keel, shape of the rib-cage, and the shape of the ribs. In selected chickens, depth of the keel at the right side was significantly deeper than the left, consequently the width and height of the keel in the left side were significantly greater than in the right side. As a result, the breast angle at the right side was significantly larger than the left one. Statistical analysis revealed significant differences in the shape of the ribs between the two sides. Ribs at the left side had significantly greater: arc and chord length, enclosed area and height; than the right side. In addition the orientation dorsal angle of the left ribs was significantly greater than the right. As a result, selected birds had faster bone growth, shorter, and less bone weight than the controls, in addition to the deformities in the shape of the keel, rib-cage, and ribs. From the results, it would appear that the asymmetry in the shape of the keel and rib-cage could be the consequence of the high growth rate in body weight and increased breast muscle weight, without increase of the growth of skeletal mass. Such disproportionate change in body parts could be the result of direct selection for increased amount of breast muscles. This problem could be reduced by restriction of early growth and include the concept of the skeletal growth in the selection-programme indices.