Angle-dependent light scattering in tissue phantoms for the case of thin bone layers with predominant forward scattering.

The cochlea forms a key element of the human auditory system in the temporal bone. Damage to the cochlea continues to produce significant impairment for sensory reception of environmental stimuli. To improve this impairment, the optical cochlear implant forms a new research approach.

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.

Role of corticotropin-releasing hormone as a thyrotropin-releasing factor in non-mammalian vertebrates.

The finding that thyrotropin-releasing hormone does not always act as a thyrotropin (TSH)-releasing factor in non-mammalian vertebrates has led researchers to believe that another hypothalamic factor may exhibit this function. In representatives of all non-mammalian vertebrate classes, corticotropin-releasing hormone (CRH) appears to be a potent stimulator of hypophyseal TSH secretion, and might therefore function as a common regulator of both the thyroidal and adrenal/interrenal axes. CRH exerts its dual hypophysiotropic action through two different types of CRH receptors.

Cloning and hypothalamic distribution of the chicken thyrotropin-releasing hormone precursor cDNA.

In this paper we report the cloning of the chicken preprothyrotropin-releasing hormone (TRH) cDNA and the study of its hypothalamic distribution. Chicken pre-proTRH contains five exact copies of the TRH progenitor sequence (Glu-His-Pro-Gly) of which only four are flanked by pairs of basic amino acids. In addition, the amino acid sequence contains three sequences that resemble the TRH progenitor sequence but seem to have lost their TRH-coding function during vertebrate evolution.

Corticotropin-releasing hormone-mediated metamorphosis in the neotenic axolotl Ambystoma mexicanum: synergistic involvement of thyroxine and corticoids on brain type II deiodinase.

In the present study, morphological changes leading to complete metamorphosis have been induced in the neotenic axolotl Ambystoma mexicanum using a submetamorphic dose of T(4) together with an injection of corticotropin-releasing hormone (CRH). An injection of CRH alone is ineffective in this regard presumably due to a lack of thyrotropic stimulation. Using this low hormone profile for induction of metamorphosis, the deiodinating enzymes D2 and D3 known to be present in amphibians were measured in liver and brain 24h following an intraperitoneal injection.

Corticosterone-induced negative feedback mechanisms within the hypothalamo-pituitary-adrenal axis of the chicken.

This paper reports the results of in vivo and in vitro experiments on the feedback effects of corticosterone on the hypothalamo-pituitary-adrenal axis in embryos at day 18 of incubation and in 9-day-old chickens. In vivo, a significant negative feedback was detected on the levels of corticotropin-releasing factor (CRF) precursor (proCRF) mRNA and on the plasma concentration of corticosterone, two hours after a single intravenous injection with 40 microg corticosterone. In contrast, the levels of CRF peptide in the hypothalamic area, the CRF receptor type 1 (CRF-R1) mRNA and pro-opiomelanocortin (POMC) mRNA levels in the pituitary were not affected by the in vivo administration of corticosterone.

Hypothalamic control of the thyroidal axis in the chicken: over the boundaries of the classical hormonal axes.

The pituitary gland, occupying a central position in the hypothalamo-pituitary thyroidal axis, produces thyrotropin (TSH), which is known to stimulate the thyroid gland to synthetize and release its products, thyroid hormones. TSH is produced by a specific cell population in the pituitary, the so-called thyrotropes. Their secretory activity is controlled by the hypothalamus, releasing both stimulatory and inhibitory factors that reach the pituitary through a portal system of blood vessels.

The use of real-time PCR to study the expression of thyroid hormone receptor beta 2 in the developing chicken.

Thyroid hormones and their receptors (TRs) have critical functions in development and metabolism. In chicken, three TRs are known: TRalpha, TRbeta0, and TRbeta2. The latter was isolated from chicken eye, but its presence in other tissues has not yet been extensively investigated.

Molecular cloning and developmental expression of corticotropin-releasing factor in the chicken.

We have characterized the structure of the chicken corticotropin-releasing factor (CRF) gene through cDNA cloning and genomic sequence analysis, and we analyzed the expression of CRF mRNA and peptide in the diencephalon of the chick throughout embryonic development. The structure of the chicken CRF gene is similar to other vertebrate CRF genes and contains two exons and a single intron. The primary structure of the mature chicken CRF peptide is identical to human and rat CRF.

Cloning and tissue distribution of the chicken type 2 corticotropin-releasing hormone receptor.

We report the cloning of the complete coding sequence of the putative chicken type 2 corticotropin-releasing hormone receptor (CRH-R2) by rapid amplification of cDNA ends (RACE). The chicken CRH-R2 is a 412-amino acid 7-transmembrane G protein-coupled receptor, showing 87% identity to the Xenopus laevis and Oncorhynchus keta CRH-R2s, and 78-80% to mammalian CRH-R2s. The distribution of CRH-R2 mRNA was studied by RT-PCR analysis and compared to CRH-R1 distribution.

Low submetamorphic doses of dexamethasone and thyroxine induce complete metamorphosis in the axolotl (Ambystoma mexicanum) when injected together.

Entanglement of functions between the adrenal (or interrenal) and thyroid axis has been well described for all vertebrates and can be tracked down up to the level of gene expression. Both thyroid hormones and corticosteroids may induce morphological changes leading to metamorphosis climax in the neotenic Mexican axolotl (Ambystoma mexicanum). In a first series of experiments, metamorphosis was induced with an injection of 25 microg T(4) on three alternate days as judged by a decrease in body weight and tail height together with complete gill resorption.

Type I iodothyronine deiodinase in euthyroid and hypothyroid chicken cerebellum.

Immunocytochemistry using polyclonal anti-type I deiodinase (D1) led to the localization of D1 protein in the internal granule cells of the cerebellum in 1-day-old chicks, which was confirmed by the presence of in vitro D1 activity. Western blot analysis of hepatic and cerebellar extracts revealed a band of 27 kDa. In hypothyroid embryos D1 was expressed in both the internal and external granule cell layer and the signal diminished with more severe hypothyroidism, which is in agreement with the expected downregulation of D1 activity during hypothyroidism.

Corticotropin-releasing hormone (CRH)-induced thyrotropin release is directly mediated through CRH receptor type 2 on thyrotropes.

CRH is known as the main stimulator of ACTH release. In representatives of all nonmammalian vertebrates, CRH has also been shown to induce TSH secretion, acting directly at the level of the pituitary. We have investigated which cell types and receptors are involved in CRH-induced TSH release in the chicken (Gallus gallus).

In vitro study of corticotropin-releasing hormone-induced thyrotropin release: ontogeny and inhibition by somatostatin.

Recent research has shown that in the chicken important interactions take place between the adrenal and the thyroidal axis both at the central and the peripheral level. In vivo as well as in vitro experiments showed that ovine corticotropin-releasing hormone (oCRH) clearly increases thyrotropin (TSH) secretion in late embryonic and early posthatch chicks. In vivo experiments in older chickens, however, suggested that this response might disappear at a later stage.

Effects of dexamethasone treatment on iodothyronine deiodinase activities and on metamorphosis-related morphological changes in the axolotl (Ambystoma mexicanum).

In amphibians, there is a close interaction between the interrenal and the thyroidal axes. Hypothalamic corticotropin-releasing hormone or related peptides stimulate thyroidal activity by increasing thyrotropin synthesis and release, while corticosterone accelerates both spontaneous and thyroid hormone-induced metamorphosis. One of the mechanisms that is thought to contribute to this acceleration is a corticosterone-induced change in peripheral deiodinating activity.

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.

Thyroid hormone deiodinases during embryonic development of the saltwater crocodile (Crocodylus porosus).

All tissues of the embryonic saltwater crocodile (Crocodylus porosus) gradually increased in weight during development except for lung tissue, which had a peak weight of 1.09 g at day 67, thereafter decreasing in weight. The brain was a relatively large organ.

Internalization of the chicken growth hormone receptor complex and its effect on biological functions.

In the chicken, as in mammals, GH is a pleiotropic cytokine that plays a central role in growth differentiation and metabolism by altering gene expression in target cells. In the growing and adult chicken it stimulates gene expression of IGF-I and inhibits gene transcription of the type III deiodinating enzyme (D3) and by doing so also increases T(3) concentrations. GH binding to its receptor leads to internalization of the GH-GHR complex to the Golgi apparatus.

Characterization of outer ring iodothyronine deiodinases in tissues of the saltwater crocodile (Crocodylus porosus).

The distribution and characterization of outer ring deiodination (ORD) using reverse triiodothyronine (rT3) and thyroxine (T4) as substrates is reported in microsomes of liver, kidney, lung, heart, gut, and brain tissues from juvenile saltwater crocodiles (Crocodylus porosus). In lung and heart only small amounts of rT3 ORD and T4 ORD were detected, while in brain only a small amount of T4 ORD was detected. More detailed characterization studies could be performed on liver, kidney, and gut microsomes.