Intestinal 5'deiodinase activity of developing and adult chickens selected for high and low body weight

Effects of genetic selection on activity of corticotropic and thyrotropic axes in modern broiler chickens

Commercial selection for meat-type (broiler) chickens has produced economically valuable birds with fast growth rates, enhanced muscle mass, and highly efficient feed utilization. The physiological changes that account for this improvement and unintended consequences associated with them remain largely unexplored, despite their potential to guide further advancements in broiler production efficiency. To identify effects of genetic selection on hormonal signaling in the adrenocorticotropic and thyrotropic axes, gene expression in muscle and liver and post-hatch circulating hormone concentrations were measured in legacy [Athens Canadian Random Bred (ACRB)] and modern (Ross 308) male broilers between embryonic days (e) 10 and e18 and post-hatch days (d) 10 and d40. No interactive effects or main effects of line were observed for adrenocorticotropic gene expression during either developmental period, although age effects appeared for corticosteroid-binding globulin in liver during embryogenesis and post-hatch and glucocorticoid receptor in both tissues post-hatch. There was a main line effect for circulating corticosterone, with levels in ACRB greater than those in Ross. Several thyrotropic genes exhibited line-by-age interactions during embryonic or post-hatch development. In liver, embryonic expression of thyroid hormone receptor beta was greater in ACRB on e12, and deiodinase 3 (DIO3) levels were greater in Ross on e14 and e16. In juvenile liver, deiodinase 2 (DIO2) expression was greater in ACRB on d10 but greater in Ross on d20, while DIO3 was higher in ACRB on d30 and d40. Levels of thyroid hormone receptor alpha mRNA exhibited a main line effect, with levels greater in ACRB juvenile breast muscle. Several thyrotropic genes exhibited main age effects, including DIO2 and DIO3 in embryonic breast muscle, thyroid hormone receptor alpha and thyroid hormone receptor beta in post-hatch liver, and DIO2 in post-hatch breast muscle. Circulating triiodothyronine displayed a main line effect, with levels in Ross significantly reduced as compared to ACRB. These findings suggest that in modern broilers, a decrease in levels of hormones that control basal metabolism triiodothyronine and the stress response circulating corticosterone, as well as altered expression of genes regulating thyroid hormone activity, could contribute to lower heat production, reduced stress response, and altered nutrient partitioning, leading to more efficient feed utilization and faster, more productive growth.

The long-term oral administration of thyroxine: effects on blood hematological and biochemical features in broiler breeder hens

Published data on the beneficial effect of short-term administration of thyroxine (T₄) in broiler breeder hens to reduce the ascites incidence in their progeny chicks raises the question as to what extent might the long-term maternal administration of T₄ affect the blood hematological and biochemical attributes in breeder hens. A total of 70 broiler breeder hens (47-wk-old) were randomly allotted to control or thyroxine treated (T₄) groups. Pure T₄ (0.3 mg/bird per day) was orally administered to T₄ birds for 14 successive weeks, whereas the control group received the drinking water only. Blood samples were obtained from the brachial vein prior to the initiation of the trial as well as weeks 50, 53, 55, 57, 59, and 61 of age. Body weight was decreased but egg production was not affected by T₄ treatment. Plasma concentration of T₄, but not triiodothyronine (T₃), was increased in T₄-treated hens (P < 0.05). The total number of leukocytes and erythrocytes were also higher in T₄ birds. A significant effect of time was observed for erythrocyte number and plasma cholesterol concentration (P < 0.05). The long-term administration of T₄ did not affect the concentrations of serum calcium and plasma total protein, albumin, globulin, cholesterol, triglyceride, high density lipoprotein, low density lipoprotein, very low density lipoprotein, alanine amino transferase, and aspartate amino transferase (P > 0.05). However, serum concentrations of phosphorus, glucose, and alkaline phosphatase were higher in T₄ hens as compared to their control counterparts. In spite of differences in circulatory concentrations of a number of traits between the experimental groups, the recorded values were within their reference ranges. Therefore, the administration of T₄ for an extended period of time had no apparent adverse effect on the clinical profile in subjected hens, which may practically support the implementation of this preventative treatment as an approach to decrease the ascites incidence; however, a lower incidence rate in the progeny chicks produced from hens receiving T₄ for long-term periods of time remains to be elucidated.

The relationship between ingested thyroid hormones, thyroid homeostasis and iodine metabolism in humans and teleost fish

Oral l-thyroxine (T4) therapy is used to treat human hypothyroidism but T4 fed to teleost fish does not raise plasma thyroid hormone (TH) levels nor induce growth, even though oral 3,5,3'-triiodo-l-thyronine (T3) is effective. This suggests a major difference in TH metabolism between teleosts and humans, often used as a starting thyroid model for lower vertebrates. To gain further insight on the proximate (mechanistic) and ultimate (survival value) factors underlying this difference, the several steps in TH homeostasis from intestinal TH uptake to hypothalamic-hypophyseal regulation were compared between humans and teleosts, and following dietary TH challenges. A major proximate factor limiting trout T4 uptake is a potent constitutive thiol-inhibited intestinal complete T4 deiodination that is ineffective for T3. At the hepatic level, T4 deiodination, conjugation and extensive biliary excretion with negligible T4 enterohepatic recycling can further block teleost T4 uptake to plasma. Such protection of plasma T4 from dietary T4 may be particularly critical for piscivorous fish consuming thyroid tissue, rich in T4 but not T3. It would prevent disruption by unregulated ingested T4 of the characteristic acute and transient changes in teleost plasma T4 due to diel rhythms, food intake and stress-related factors. These marked natural short-term fluctuations in teleost plasma T4 levels are enabled by the relatively small and rapidly-cleared plasma T4 pool, stemming largely from properties of the plasma T4-binding proteins. Humans, however, due mainly to plasma T4-binding globulin, have a relatively massive circulating pool of T4 and an extremely well-buffered free T4 level, consistent with the major TH role in regulating basal metabolic rate. Furthermore, this large well-buffered and slowly-cleared plasma T4 pool, in conjuction with enterohepatic recycling and relaxation of hypothalamic-hypophyseal negative feedback, allows humans to temporarily 'store' ingested T4 in plasma, thereby sparing endogenous TH secretion and conserving thyroidal iodine reserves. Indeed, iodine conservation is likely the key ultimate factor determining the divergent evolution of the human and teleost systems. For humans, ingested iodine in the form of I^-, or TH and their derivatives, is the sole iodine source and may be limiting in many environments. However, most freshwater teleosts, in addition to their ability to assimilate dietary I^-, can derive sufficient I^- from their copious gill irrigation, with no selective advantage in absorbing dietary T4 which would disrupt their natural acute and transient changes in plasma T4. Thus T4 may act also as a vitamin (vitamone) in humans but not in teleosts; in contrast, T3, naturally ingested at much lower levels, may act as a vitamone in both humans and teleosts.

Growth performance and intestinal morphology in broiler chickens produced from hyperthyroid breeder hens

An association between induced maternal hyperthyroidism and a decreased incidence of cold-induced ascites in broiler chickens has been reported recently. There are also reports suggesting that thyroid hormones have an effect on intestine, a high oxygen-demanding organ. The present study aimed to determine whether the ascites-attenuating effect of maternal hyperthyroidism is associated with morphological changes in intestine and with probable adverse effects on growth performance in progeny chicks. Eighty-eight broiler breeder hens were allotted to control or hyperthyroid [HYPER; thyroxine-treated] groups, and artificially inseminated. Hatching eggs (n = 924) were incubated and day-old male chicks (n = 288) were reared for 42 days under standard or low ambient temperature. Plasma thyroxine was higher in the HYPER hens and their embryos at internal pipping. Maternal hyperthyroidism did not adversely affect the feed : gain ratio and carcass weight, whereas cold stress impaired them. A higher relative weight was found at 42 days of age for all intestinal regions at low ambient temperature. The duodenal and jejunal crypt depths (CD) were lower in HYPER group at 7 days of age. Cold exposure decreased the duodenal and jejunal villus height and CD, and increased the ileal goblet cell number at 21 days. On Day 42, higher values were recorded for CD, goblet cell number, and epithelial thickness of different intestinal regions in cold-exposed birds. In conclusion, growth performance was not adversely influenced by maternal hyperthyroidism and its minimal early effect on intestinal morphology does not appear to be the underlying mechanism for decreased incidence of ascites in broilers.

The Hypothalamic-Pituitary-Thyroid (HPT) Axis in Birds and Its Role in Bird Development and Reproduction

This article reviews thyroid function and its hypothalamic-pituitary-thyroid (HPT) axis control in birds with emphasis on the similarities and differences in thyroid function compared to mammals and other vertebrate classes. Thyroid hormones are important in metabolism and the thermogenesis required for homeothermy in birds, as in mammals, the other homeothermic class of vertebrates. Thyroid hormones play important roles in development and growth in birds, as is the case for all vertebrate classes. The developmental effects of thyroid hormones in birds are presented in the context of differences in precocial and altricial patterns of development and growth with emphasis on oviparous development. The sections on thyroid hormone actions include discussion of effects on the development of a number of tissue types as well as on seasonal organismal processes and interactions of the thyroid axis with reproduction. The current picture of how environmental chemicals may disrupt avian thyroid function is relatively limited and is presented in the context of the assessment endpoints that have been used to date. These endpoints are categorized as thyroid and HPT axis endpoints versus target organ endpoints. The final section discusses two recommended assay protocols, the avian two-generation toxicity assay and the avian one-generation assay, and whether these protocols can evaluate thyroid disruption in birds.

Reprint of "Avian thyroid development and adaptive plasticity" [Gen. Comp. Endocrinol. 147, 93-101]

Precocial and altricial modes of avian development are characterized by different degrees of maturation and physiological capabilities at hatching. In precocial birds, thyroid function and its control are well developed during the latter part of incubation and hatchlings exhibit metabolic responses to cooling and relatively mature sensory and locomotor capabilities. In altricial birds, thyroid function shows little maturation until after hatch as also is the case for thermoregulatory, sensory, and motor functions. This review describes the patterns of precocial and altricial thyroid development, their hypothalamic-pituitary control, extrathyroidal control of hormone activation and deactivation, and target tissue effects during development. Our knowledge is greatest for precocial galliform birds although the organismal picture of thyroid development has been investigated in several altricial avian species.

Avian thyroid development and adaptive plasticity

Precocial and altricial modes of avian development are characterized by different degrees of maturation and physiological capabilities at hatching. In precocial birds, thyroid function and its control are well developed during the latter part of incubation and hatchlings exhibit metabolic responses to cooling and relatively mature sensory and locomotor capabilities. In altricial birds, thyroid function shows little maturation until after hatch as also is the case for thermoregulatory, sensory, and motor functions. This review describes the patterns of precocial and altricial thyroid development, their hypothalamic-pituitary control, extrathyroidal control of hormone activation and deactivation, and target tissue effects during development. Our knowledge is greatest for precocial galliform birds although the organismal picture of thyroid development has been investigated in several altricial avian species.

Thyroid hormone deiodination in birds

Because the avian thyroid gland secretes almost exclusively thyroxine (T4), the availability of receptor-active 3,3',5-triiodothyronine (T3) has to be regulated in the extrathyroidal tissues, essentially by deiodination. Like mammals and most other vertebrates, birds possess three types of iodothyronine deiodinases (D1, D2, and D3) that closely resemble their mammalian counterparts, as shown by biochemical characterization studies in several avian species and by cDNA cloning of the three enzymes in chicken. The tissue distribution of these deiodinases has been studied in detail in chicken at the level of activity and mRNA expression. More recently specific antibodies were used to study cellular localization at the protein level. The abundance and distribution of the different deiodinases shows substantial variation during embryonic development and postnatal life. Deiodination in birds is subject to regulation by hormones from several endocrine axes, including thyroid hormones, growth hormone and glucocorticoids. In addition, deiodination is also influenced by external parameters, such as nutrition, temperature, light and also a number of environmental pollutants. The balance between the outer and inner ring deiodination resulting from the impact of all these factors ultimately controls T3 availability.

Halometabolites and cellular dehalogenase systems: an evolutionary perspective

We review the role of iodothyronine deiodinases (IDs) in the evolution of vertebrate thyroidal systems within the larger context of biological metabolism of halogens. Since the beginning of life, the ubiquity of organohalogens in the biosphere has provided a major selective pressure for the evolution and conservation of cellular mechanisms specialized in halogen metabolism. Among naturally available halogens, iodine emerged as a critical component of unique developmental and metabolic messengers. Metabolism of iodinated compounds occurs in the three major domains of life, and invertebrate deuterostomes possess several biochemical traits and molecular homologs of vertebrate thyroidal systems, including ancestral homologs of IDs identified in urochordates. The finely tuned cellular regulation of iodometabolite uptake and disposal is a remarkable event in evolution and might have been decisive for the explosive diversification of ontogenetic strategies in vertebrates.

Differential expression of iodothyronine deiodinases in chicken tissues during the last week of embryonic development

In the current study, the authors examined the type 1 (D1), type 2 (D2), and type 3 deiodinase (D3) activity and mRNA expression patterns in thyroid, lung, brain, pituitary, heart, liver, spleen, gonads, skin, muscle, intestine, Fabricius' bursa, and kidney during the last week of chicken embryonic development and the first 2 days posthatch. The D3 was the most widely expressed, occurring in all examined tissues. Also, the D1 knows a widespread distribution, although no D1 activity or mRNA expression could be detected in the brain, the thyroid, the muscle, and the skin. In contrast, the D2 has a much more restricted expression pattern, since the brain is the only organ where, prior to hatching, both in vitro D2 activity and D2 mRNA expression can be detected. Taken together, these results demonstrate that during the last week of chicken embryonic development, the majority of tissues express D3, together with either D1 or D2, indicating that each tissue possesses the necessary tools to regulate local thyroid hormone levels at least partly independent from T(3) and T(4) levels in plasma. In addition, the deiodinase expression data could be correlated to certain thyroid hormone dependent tissue-specific developmental events. This strongly suggests that in birds, as in mammals and amphibians, the correct spatial and temporal expression of iodothyronine deiodinases are essential for normal embryonic development.

Maternal thyroid hormones in Japanese quail eggs and their influence on embryonic development

We addressed the relationship between the thyroid status of hens and the thyroid hormone content of their eggs, as well as the influences of egg hormones on embryonic development. Methods for measuring thyroid hormones in egg yolk were verified by demonstrating consistency in the recovery of yolk thyroid hormones following a methanol/chloroform extraction and in the measurement of thyroid hormones by RIA for a range of hormone concentrations in yolk extracts. Untreated hens produced eggs with yolk thyroxine (T4) concentrations that were low relative to plasma T4, but yolk triiodothyronine (T3) concentrations comparable to those of plasma. Hens dosed twice daily with T4 (1 or 3x the daily thyroid secretion rate, TSR, of T4 per dose) had significantly higher plasma and egg yolk T4 concentrations than did control hens dosed with saline. In general, the T4 concentration of egg yolk varied with the thyroid status of the hen. When the relationship between each hen's plasma T4 and the yolk T4 concentration of her eggs was examined, hens appeared to regulate T4 deposition into yolk at "levels" characteristic of the "levels" of thyroid status produced by the different doses of T4. Embryonic pelvic cartilage, a thyroid hormone-responsive tissue, showed enhanced growth and differentiation in embryos from eggs of hens given the highest dose of T4. Specifically, alkaline phosphatase activity (a marker of differentiation) and pelvic cartilage wet and dry weights were significantly greater in embryos from high T4 eggs (hens on the 3x TSR dose) than those in controls. However, embryos from high T4 eggs did not differ in general body growth (body weight, length, and general morphology) or hatchability compared to controls. In a single T3 experiment, hens were dosed twice daily with 1 microg T3. The embryos from eggs of these hens had accelerated differentiation/maturation of pelvic cartilages (sampled at Day 12) compared to those from control eggs; body growth did not differ from that of controls.

Supplemental thyroid hormones and molting in turkey breeder hens

The objective of the current study was to determine whether thyroid physiology may affect molting time in turkeys. Two trials using approximately 144 hens were conducted to elucidate thyroidal factors that limit the molting process. Thyroid hormones or a thyroid blocker (thiouracil) were given to the hens during a molt by supplementing the diet with thyroxine (T4), triiodothyronine (T3), or thiouracil. Supplementing with T4 reduced the number of days to return to egg production, whereas supplementing with thiouracil or T3 prolonged days to first egg. The observations support previous suggestions of separate functions for T3 and T4 during molting. As had been observed many times previously, the feeding of thiouracil delayed the molt but did not completely stop the molting process. The hens fed thiouracil returned to 50% egg production nearly 10 d after the control group, whereas T3 prolonged the return to 50% egg production nearly 1 wk later. The data indicate the endogenous low levels of T4, but not T3 in modern strains of turkeys may contribute to a relatively longer molting period of turkey breeder hens induced to molt out of season.

Long-term divergent selection for eight-week body weight in white Plymouth rock chickens

This paper provides an overview of results from a long-term (38 generations) selection experiment. Lines were developed from individual phenotypic selection for high or low body weight at 8 wk of age. Included are data for the selected lines, sublines in which selection was relaxed, crosses of the selected lines, and sublines in which the sex-linked dw gene was introduced. Periodically (and in some cases every generation) data were obtained for unselected traits. These unselected traits included feed consumption and intake behavior, reproduction, allomorphic relationships, and metabolic, immunological, endocrine, and molecular factors. These responses have been integrated into a resource allocation paradigm.

Physiological differences in late embryos from turkey breeders at different ages

Physiological mechanisms were measured in embryos from turkey hens of different ages to determine associations with declines in hatchability as breeder hens age. As the hens aged from 32 to 54 wk of age, embryonic viability declined (P < 0.05). The greatest proportional increase (P < 0.01) in embryonic mortality of aging hens occurred at the plateau stage in oxygen consumption or immediately thereafter at pipping. Eggshell conductance constants increased (P < 0.01) as hens aged but did not change after mid-lay, suggesting an alteration in respiration for the embryos in eggs produced by older hens compared to eggs produced by the same hens at younger ages. The alteration may cause embryos in eggs from older hens to reach the plateau stage in oxygen consumption (approximately 25 to 26 d of incubation) earlier in development than embryos from young hens. Hepatic and cardiac glycogen concentrations were greatest (P < 0.001) in embryos from hens at the youngest age and then declined (P < 0.05) as the hens aged. Embryonic blood plasma glucose concentrations declined (P <0.05) similarly. Plasma thyroxine (T4) and triiodothyronine (T3) concentrations were measured in embryos from the hens at different ages as well. Increased (P < 0.05) T4 was evidenced in embryos from the youngest hens, whereas increased (P < 0.05) T3 activity was evident in embryos from hens of older ages. It was concluded that the decline in hatchability seen as turkey breeder hens age may have a basis in the differences seen in the physiology of hatching in embryos. Specifically, thyroid influences on growth and carbohydrate metabolism may be involved in decreased embryonic viability.