Relationships between thyroid status, tissue oxidative metabolism, and muscle differentiation in bovine fetuses
Introduction
Thyroid hormones (TH) play a crucial role in growth, development, and function of most vertebrate tissues, such as brain, bone, adipose tissue, and skeletal muscles [1]. They affect both tissue accretion and differentiation in the fetus by a combination of metabolic and non-metabolic mechanisms [2]. Thyroid hormones also exert critical effects on a variety of metabolic pathways, especially energy metabolism, and are key regulators of postnatal growth. Heart and skeletal muscles are important TH targets [3] especially the developing skeletal muscle [4]. In rat, TH control myosin heavy chain (MyHC) isoform transitions [5], [6]. A role for TH is also suspected in coordinating the expression of contractile and metabolic muscular proteins [7].
Most studies relate to postnatal TH effects on experimental animals, mainly rodents. However, TH regulation of muscle development occurs as early as in the fetus for species which are mature at birth based on muscle biology, such as ruminants [8]. In fetal sheep, an intact thyroid gland is required for normal development and for biochemical and contractile differentiation of muscle masses [9].
Developmental and metabolic effects of TH are mediated by triiodothyronine (T3), which is mainly produced by peripheral 5′-deiodination of thyroxine (T4) [10]. However, the mechanisms of a TH action during fetal development of muscles are poorly documented although local interactions with the activity of the somatotrophic axis have been shown [11].
In this study, we investigated the influence of TH on tissue differentiation (heart, liver and skeletal muscles). We have examined temporal relationships between thyroid status, tissue oxidative capacity, and muscle differentiation in bovine fetuses during the two last trimesters of gestation. For this purpose, we have applied a multivariate analysis procedure [12].
Section snippets
Animals and tissue samples
This study was carried out as part of a research program approved by the “Institut National de la Recherche Agronomique” (INRA, France) Ethical Committee. The study included 33 fetuses of comparable chronological age collected at day 110 (111.5 ± 6.5; n = 6), 180 (181.7 ± 5.9; n = 7), 210 (207.8 ± 4.8; n = 10), and 260 (259.6 ± 2.5; n = 10) post-conception (p.c.). Fetuses were obtained by artificial insemination of Charolais heifers with Charolais sperm. The cows were bred and slaughtered, and fetuses were
Thyroid hormone status in fetuses
Mean plasma concentrations of iodothyronines in fetuses ranging from day 110 to day 260 p.c. are shown in Fig. 1. Plasma T4 was detected from day 110 p.c. onwards (Fig. 1A). Plasma T4 concentrations increased from a mean concentration of 8.7 ng/mL at day 110 p.c to 63 ng/mL (P < 0.001) at day180 p.c. An increase was recorded between day 180 and day 210 p.c. (56%; P < 0.01). Plasma T4 concentrations increased also in the last third trimester and reached 130.5 ng/mL at the end of gestation (P < 0.01
Thyroid status ontogeny
A histological study by Koneff et al. [21] has shown that the thyroid gland in bovine fetuses differentiates by day 90 p.c. It undergoes thereafter a gradual development during the third trimester of gestation, and displays a marked increase in functional activity near term coinciding with biochemical maturation [22]. In the present study, the pattern of TH concentrations fits well with the data describing ontogeny of the bovine fetal thyroid. Plasma TH concentrations increased with gestational
Acknowledgements
The authors thank Rt Jailler and Rd Jailler for animal management and slaughtering, C. Barboiron, N. Guivier, and J.L. Montel for excellent technical assistance. They are particularly grateful to Dr C. Jurie and to Y. Anglaret for statistical analysis.
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