According to the studies a 4-week-long administration of exogenous thyroxine in broiler breeder hens to decline the cold-induced ascites in their offspring suggests that the long-term maternal hyperthyroidism would affect gastrointestinal tract morphology in broiler breeder hens. The birds (70) were fostered in the individual cages and allotted to two treatment groups (five replicates of seven hens each), to either zero (CON) or 0.30 mg thyroxine (hyperthyroid group) for 100 successive days. At the end of the experiment (64 week of age), two birds per replicate (20 hens total) were slaughtered and their gastrointestinal tract were removed for histological evaluation. Thyroxine treatment resulted in an increase in plasma concentration of T4; however, the T3 level was not affected. The results of this study showed that the long-term administration of thyroxine had no adverse effect on gastrointestinal tract morphology in broiler breeder hens; in conclusion it could be recommended the long-term maternal hyperthyroidism treatment to reduce the ascites incidence in their offspring. Although, more studies should be done to make a final conclusion to use of this suppressive treatment to reduce the ascites incidence.
Thyroid hormones have a crucial role in diverse physiological functions such as intestinal development. The gastrointestinal tract needs a large reservoir of O2, which may decline the accessibility of O2 for other organs in hypoxemia condition, as seen in the ascites disorder. The ascites syndrome is pulmonary hypertension which happens in broilers and exacerbated by hypoxemia. Despite the raising the mortality rate, ascites causes the reduced weight gain in broilers. Many authors have demonstrated some methods to diminish ascites incidence such as feed restriction, intermittent lighting, genetic selection, and administrating potassium bicarbonate. The use of thyroid hormones is generally considered as one method to decline ascites syndrome. More recently, it has been revealed that short-term (4 weeks) administration of thyroxine (T4) in broiler breeder hens reduced ascites incidence in their chicks.
The role of thyroid hormones on reducing ascites was ascribed to development of pulmonary system and better efficiency of O2 and CO2 exchange. It is well documented that thyroid hormones have an effect on intestine, a high oxygen-requesting organ; therefore, the objective of the current study was to ascertain whether the ascites-reducing effect of long-term hyperthyroidism is related with histomorphometrical changes in gastrointestinal tract in broiler breeder hens.
All steps in the current work were approved by the Animal Care and Welfare Committee of Shiraz University, Shiraz, Iran. The Cobb 500 breeder hens (47-week-old) were selected into two groups (five replicates of seven hens each), the control (35 birds) and hyperthyroid groups (35 birds). The hyperthyroid group received thyroxine (T4) (0.3 mg/bird/d) for 100 days (Saemi et al., 2018a). The birds were fostered under the same management fed a corn-soybean based diet (2700 kcal metabolizable energy (ME)/kg, and 14%, 2.99% and 0.36% crude protein (CP), calcium and phosphorus, respectively). At the end of the experiment (64 week of age), two birds per replicate (20 hens total) were sacrificed and the gastrointestinal tracts were picked up. The tissues were gathered after slaughter, and washed in normal saline, for histomorphometrical evaluations, the pieces were fixed in 10% buffered formalin and continued for histological methods. After tissue preparation and staining with hemotoxylin and eosin (H & E), histological layers were measured using light microscopy and micrometry method.
This study was carried out in a completely randomized design. The data were analyzed for normality. Data were subjected to the GLM procedure. Differences between means were compared by the least squares procedure and the level of significance was set at P ≤ 0.05.Results and discussionRegarding the role of thyroid hormones in regulating gastrointestinal tract function, we hypothesized thyroid hormones might affect the gastrointestinal tract morphology in broiler breeder hens. The results of the present study showed T4 concentration was enhanced in the hyperthyroid hen as compared with the control group (27.08 versus 10.24 ng/mL; P = 0.003) with no effect on plasma T3 concentration which can be ascribed to situation in which the extra thyroxine completely transformed to inactive T3 (rT3). The comparative histomorphometrical study of different parts of the gastrointestinal tract in hyperthyroid breeder hens revealed the effects of long-term hyperthyroidism on none of the parameters were significant. Some researchers have showed an impeded intestinal growth and devastated intestinal morphology and function in stressed birds.
The stress induced by corticosterone, caused reduced small intestinal epithelial cells growth, which resulted in a decrease in the villus height and crypt depth, the lower villus height caused the reduced absorptive function; however, in this experiment plasma corticosterone concentrations were not measured in hyperthyroidism condition. In this study we did not observe any differences in plasma T3 concentration between the control group and hyperthyroid group which might prevent the systemic path of T3 function for changes in histomorphometrical values for gastrointestinal tract such as duodenal and jejuna villus height and crypt depth in hyperthyroid group. This might mention the result of locally made T3 through intra-intestinal de-iodination.
The long-term hyperthyroidism had no adverse effect on gastrointestinal morphometric values, although the results of the study might be distinct for treatments that using various birds, doses and duration of treatments. Since long-term administration of extra thyroxine in broiler breeder hens had no undesirable effect on gastrointestinal tract morphology, it could be suggested the use of thyroid hormones commercially to reduce the incidence of ascites in breeder hens’ progeny chicks. Although, more studies for example the effects of long-term hyperthyroidism on hatchability and the chicks’ performance and quality should be carried out to make a final conclusion to use of this preventative treatment to reduce the ascites incidence.
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