Histostructure of the tracheal wall of broiler chickens depending on air circulation conditions in closed poultry houses

Currently, optimization of indoor microclimatic conditions in poultry houses is attracting considerable interest due to the intensification of broiler meat production. However, given the increase in flock sizes of broilers, little research has focused on the effect of microclimate parameters in poultry houses on the bird's respiratory system. Insufficient air exchange in the premises can cause functional respiratory disorders in broiler chickens. This paper is the first to report that air circulation in closed poultry houses contributes to maintaining productivity and improves the histostructure and histochemical properties of the tracheal wall in broiler chicks ( Gallus gallus domesticus ). Our work aimed to study the influence of different air circulation regimes in closed poultry houses on histostructure and histochemical characteristic of the trachea in Ross 308 broiler chicks and their productive performance. The study was conducted in 2020-2021 at the LLC Chelny-Broiler poultry farm (Republic of Tatarstan). Ross 308 cross broiler chicks were raised until 39 days of age in five closed premises under different airflow distribution and air circulation (five groups of 35 birds each). For morphometry, 525 preparations of 175 trachea specimens from of all broilers (2500 g bodyweight) were measured. Trachea sections were stained by hematoxylin and eosin procedure. For histochemical studies of acidic and neutral mucins, sections were stained by a combined method for detecting polysaccharides using the Schiff-iodic acid (PAS-reaction) and alcian blue according to the manufacturer's recommended (LLC Labico, Russia). In the control groups 2, 3, and 4, there was no air circulation; in the experimental groups I and V, circulation was provided by forced ventilation, capacity of 8.5 thousand m3/h (SF-550-02, AgroKurs, Russia). Ventilation was run at the 10 day-age of the broiler chicks. Insufficient air circulation in the poultry rearing rooms caused destructive changes in the tracheal mucous membrane, i.e., its own lamina proliferation, edema, a decrease in the height of the epithelium, and destruction of cilia. This led to metaplasia of the epithelium and disruption of mucociliary transport. The thickness of the mucous membrane and its own lamina was minimum in the experimental group 1 (147.2±3.3 µm and 129.1±3.1 µm, respectively) and maximum in the control group 3 (404.7±9.4 µm and 395.7±9.4 µm) (p ≤ 0.01). The thickness of the tracheal epithelial layer significantly increased in the experimental groups 1 and 5 (by 14 % on average) compared to the control groups 1, 2, and 3 (p ≤ 0.01). The lack of indoor air circulation led to a significant decrease in the height of cilia in the control groups 2, 3, and 4 (by 39.5, 58.1, and 67.5 %, respectively) as compared to the experimental groups 1 and 5. The increase in birds' bodyweight at 5 weeks of age in the experimental group 1 increased compared to the control groups 2, 3, and 4 by 6.5, 3.2, and 7.1 %, respectively (p ≤ 0.05). The histochemical characteristics suggests the presence of simple multicellular endoepithelial glands in the tracheal epithelium layer of birds. Thus, with the provision of proper air circulation in an enclosed space, the thickness of the mucous membrane and its own lamina decreases, and the thickness of the epithelial layer and the height of the tracheal cilia increases. These characteristics are indicative of proper airexchange in the poultry houses.