Examining the Roles of Macrophages and Vascular Inflammation in Broiler White Striping

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Institution: University of California, Davis

Principal Investigator: Michael J. Mienaltowski, DVM, PhD
Annie J. King, PhD
University of California, Davis
2251 Meyer Hall
One Shields Ave.
Davis, CA 95616

Broiler chickens have been genetically selected to grow quickly and efficiently. Although efficient growth is desirable for poultry producers, larger, fast-growing broiler chickens may be more likely to display certain myopathies. Some myopathies can lead to carcass condemnations, while others may affect the aesthetic appearance, texture and nutritional quality of the meat. In this study, researchers were concerned with white striping, which affects several muscles of the carcass – breast muscle in particular. Estimates in the United States indicate that as much as 32.3% of broiler breasts have white striping, which represents an annual loss of $200 million due to discarding meat, downgrading product, or decreased yields from trimming, cook loss, or drip loss. Due to these economic losses, this research project was designed to gain knowledge into white striping by following broilers fed a commercial diet from hatching to market weight.

An integrative analysis was performed over four levels to better define the causes of white striping (whole bird, pectoralis muscle tissue, cells within the tissue, and molecular biological processes) occurring during the growth process. The objectives of the study included: (1) To determine when microvascular damage first appears in broiler pectoralis major muscle (2) To determine when foam cells begin to contribute to the pathology (3) To determine which gene expression pathways are associated with foam cell formation and if they coincide with white striping formation.

In Objective 1 of the study, white striping and associated pathology was first detectable as early as week 2 in a small number of birds. However, by the third week, the severity of the pathology had escalated upon gross inspection of the tissue, and thereafter became more and more severe. Changes in the muscle were also examined microscopically for Objective 2; by the third week, there was significant infiltration of large mobile white blood cell macrophages into the muscle with significant muscle degradation occurring in the weeks 4-6. All of these detrimental changes were highly correlated with the age and cull weight of the broilers.

Within Objective 3, analyses of gene expression in the pectoralis major muscle demonstrated significant activity of white blood cells within the muscle. There was an indication of the release of signaling molecules within the tissue that beckon white blood cells to the muscle. Moreover, active pathways were detected that would allow for white blood cells to migrate from blood vessels into the muscle. Furthermore, the analyses indicated the presence of natural killer cell-like white blood cells that could promote fibrosis – fibrous muscle tissue. Because university and county COVID-19 precautions restricted access to the laboratory, specific follow-up work will confirm the presence of foam cells and other white blood cells with immunohistochemistry.

In the long-term, these findings help to advance our understanding of the progression of white striping. From our findings thus far, we have determined that future projects should focus on interventions at weeks 2-4 of growth in the broiler to reverse white blood cell infiltration in the muscle.

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