Evaluation of Capillary and Myofiber Density in the Pectoralis Major Muscles of Rapidly Growing, High-Yield Broiler Chickens During Increased Heat Stress

Abstract

Skeletal muscle development proceeds from early embryogenesis through marketing age in broiler chickens. While myofiber formation is essentially complete at hatching, myofiber hypertrophy can increase after hatch by assimilation of satellite cell nuclei into myofibers. As the diameter of the myofibers increases, capillary density peripheral to the myofiber is marginalized, limiting oxygen supply and subsequent diffusion into the myofiber, inducing microischemia. The superficial and deep pectoralis muscles constitute 25% of the total body weight in a market-age bird; thus compromise of those muscle groups can have profound economic impact on broiler production. We hypothesized that marginal capillary support relative to the hypertrophic myofibers increases the incidence of microischemia, especially in contemporary high-yield broilers under stressing conditions such as high environmental temperatures. We evaluated the following parameters in four different broiler strains at 39 and 53 days of age when reared under thermoneutral (20 to 25 C) versus hot (30 to 35 C) environmental conditions: capillary density, myofiber density and diameter, and degree of myodegeneration. Our data demonstrate that myofiber diameter significantly increased with age (P > or = 0.0001), while the absolute numbers of capillaries, blood vessels, and myofibers visible in five 400 x microscopic fields decreased (P > or = 0.0001). This is concomitant with marginalization of vascular support in rapidly growing myofibers. The myofiber diameter was significantly lower with hot environmental temperatures (P > or = 0.001); therefore, the absolute number of myofibers visible in five 400X microscopic fields was significantly higher. The incidence and subjective degree of myodegeneration characterized by loss of cross-striations, myocyte hyperrefractility, sarcoplasmic vacuolation, and nuclear pyknosis or loss also increased in hot conditions. Differences among strains were not observed.

Pectoral Vessel Density and Early Ultrastructural Changes in Broiler Chicken Wooden Breast Myopathy

“…The total vessel number decreased with age in unaffected broiler chickens (Fig. 2), which is consistent with previous findings (Joiner et al, 2014;Radaelli et al, 2017).…”

“…In addition to the higher initial myofibre diameter compared with layers, the myofibre diameter increases with age in high-yielding broilers, while the total vessel density decreases (Joiner et al, 2014;Radaelli et al, 2017). Capillaries, rather than other vessels, appear to increase in number in response to the myofibre hypertrophy in order to maintain the necessary blood supply to growing muscle (Joiner et al, 2014;Radaelli et al, 2017).…”

Effects of broiler genetic strain and dietary amino acid reduction on (part I) growth performance and internal organ development

“…However, on day 60, when the birds became very large, they were not able to liberate heat quickly enough, even though the house temperature was only 14.4°C–16.7°C (58.0°F–62.1°F). An earlier study has reported that the relative number of capillaries and blood vessels per unit of body surface area becomes lower with rapid myofiber growth in breast muscle ( Joiner et al., 2014 ), which may compromise the cooling capability of large birds. An insufficient cooling capability in big birds is a common problem in modern broiler production.…”

Blood Gas Disturbances and Disproportionate Body Weight Distribution in Broilers With Wooden Breast

“…Elevated pCO 2 in conjunction with a decline in pH in WB affected birds is possibly indicative of an acid-base disorder called respiratory acidosis. While respiratory acidosis is classically defined in terms of arterial blood gas measurements, the high correlation of arterial and venous pCO 2 and pH is wellestablished in humans, dogs, and chickens, as is the use of venous measurements for investigation of acid-base disturbances (Forster et al, 1972;Ilkiw et al, 1991;Wideman et al, 2003;Yildizdaş et al, 2004). Respiratory acidosis occurs when the body produces more CO 2 than can be removed by the lungs (D'Addesio, 1992).…”

“…Impaired venous return from phlebitis may exacerbate what some characterize as existing vascular insufficiency in the pectoralis major of commercial broilers. At the cellular level, large breast muscles are produced by increasing the number, diameter, and length of muscle fibers (Scheuermann et al, 2004;Roy et al, 2006), causing a reduction of capillary density among other effects (Hoving-Bolink et al, 2000;Joiner et al, 2014).…”