Fluctuations in follicular structures of rat thyroid glands during 24 hours: fine structural and morphometric studies

Microscopic Focus on the Thyroid Follicles of the One-Humped Camel (Camelus dromedarius)

The microstructure of the thyroid gland of the one-humped camel (Camelus dromedarius) was described using morphometric, histological, immunohistochemical staining, and ultrastructural standard techniques. The follicular secretory units of the thyroid glands displayed comparable and variable sizes where the large follicles were situated at the peripheral margins; however, the small ones were located in the central region. Semisquamous epithelium (low cuboidal) lined the large thyroid follicles, while high cuboidal (columnar) epithelium lined the small ones. Our electron microscopic findings revealed that the low-sized cuboidal follicular cells lack organelles and are hypoactive. The high cuboidal follicular cells are active cells and rich in cellular organelles such as cisternae of rough endoplasmic reticulum, mitochondria, colloid droplets, scrolled Golgi apparatus, and secretory vesicles. A few degenerate follicular cells appeared on rare occasions. The parafollicular cells appeared with a more prominent and conspicuous nucleus than the follicular cells. The follicular cells were classified as active, inactive, or degenerated using transmission electron microscopy. The follicular and parafollicular cells showed calcitonin-positive immunoreactivity. Overall, the presented results showed particular convergences of the morphostructural aspects of the thyroid gland of C. dromedarius to that of other mammals with some distinctive features to cope with their physiology and harsh niche.

Age-dependent effects on radiation-induced carcinogenesis in the rat thyroid

Childhood radiation exposure is a known thyroid cancer risk factor. This study evaluated the effects of age on radiation-induced thyroid carcinogenesis in rats irradiated with 8 Gy X-rays. We analyzed cell proliferation, cell death, DNA damage response, and autophagy-related markers in 4-week-old (4W) and 7-month-old (7M) rats and the incidence of thyroid tumors in 4W, 4-month-old (4M), and 7M rats 18 months after irradiation. Cell death and DNA damage response were increased in 4W rats compared to those in controls at 1 month post-irradiation. More Ki-67-positive cells were observed in 4W rats at 12 months post-irradiation. Thyroid tumors were confirmed in 61.9% (13/21), 63.6% (7/11), and 33.3% (2/6) of irradiated 4W, 4M, and 7M rats, respectively, compared to 0%, 14.3% (1/7), and 16.7% (1/6) in the respective nonirradiated controls. There were 29, 9, and 2 tumors in irradiated 4W, 4M, and 7M rats, respectively. The expression of several autophagy components was downregulated in the area surrounding radiation-induced thyroid carcinomas in 4W and 7M rats. LC3 and p62 expression levels decreased in radiation-induced follicular carcinoma in 4W rats. Radiosensitive cells causing thyroid tumors may be more prevalent in young rats, and abrogation of autophagy may be associated with radiation-induced thyroid carcinogenesis.

Macrophage infiltration into thyroid follicles: an immunohistochemical study using donated elderly cadavers

To describe and discuss the morphology of the aged thyroid gland, with particular reference to the contribution of macrophages.With the aid of immunohistochemistry, we examined 1) macrophage accumulation, 2) infiltration of lymphocytes, and 3) the size and density of follicles in the unilateral lobe of the thyroid gland obtained from elderly donated cadavers (mean age, 84 years) without macroscopic malignancy. Each almost entire unilateral lobe of the thyroid showed 2554-9910 follicles per section, and each of the follicles ranged in area from 0.014-0.072 mm². We often found evidence suggesting absorption and fusion of follicles to provide a larger colloidal lumen, containing small follicles and/or epithelial fragments. In addition to dendritic perifollicular macrophages, large and round macrophages often formed clusters in the colloid. Colloidal lumina with weak macrophage immunoreactivity were intermingled with those showing strong reactivity. Notably, a greater number of macrophage foci in the colloid was usually associated with a lower density of perifollicular macrophages. Likewise, perifollicular macrophages were not always associated with lymphocyte infiltration. In the elderly, the initial appearance of colloidal macrophages does not appear to be associated with perifollicular infiltration of mononuclear cells. Macrophage invasion into a follicle might depend on the functional state of each follicle. After destruction of a follicle, a macrophage cluster appears to remain in the perifollicular tissue, and perhaps lymphocyte infiltration occurs secondarily. This course is likely to represent the process of degeneration of the thyroid gland structure with age.

Pituitary-thyroid axis development during the larval-juvenile transition in the pejerrey Odontesthes bonariensis

The morphological development of the thyroid gland of pejerrey Odontesthes bonariensis during larval-juvenile transition was studied and related to whole-body concentrations of thyroxine (T₄ ) and tri-iodothyronine (T₃ ). A complementary (c)DNA fragment of the thyroid-stimulating hormone β-subunit (tshb) was sequenced and transcript levels quantified during this period. Follicles with eosinophilic and T₄ -immunoreactive colloids were detected at hatching together with tshb transcript levels and whole-body concentrations of T₄ and T₃ hormones. Thyroid follicles were located in the subpharyngeal region associated with the ventral aorta below the hyoid bone. Follicle structure switched from the rounded form at hatching to oval in juveniles. Significant increase of follicle number per larva, mean colloidal area and total colloidal area was observed throughout development with maximum values at the end of the larval-juvenile transition. A significant decrease of tshb expression together with a significant increase in T₄ and T₃ whole-body concentrations was observed prior to achieving the juvenile phenotype. These results are in accordance with a negative feedback regulation of tshb expression by thyroid hormones and a possible association between thyroid hormone levels and the acceleration of metabolic processes necessary to complete metamorphosis.

Microdosimetric analysis of 211At in thyroid models for man, rat and mouse

BACKGROUND

The alpha particle emitter 211At is proposed for therapy of metastatic tumour disease. 211At is accumulated in the thyroid gland in a similar way as iodine. Dosimetric models of 211At in the thyroid are needed for radiation protection assessments for 1) patients receiving 211At-labelled pharmaceuticals where 211At may be released in vivo and 2) personnel working with 211At. Before clinical trials, preclinical studies are usually made in mice and rats. The aims of this study were to develop thyroid models for mouse, rat and man, and to compare microdosimetric properties between the models.

METHODS

A thyroid follicle model was constructed: a single layer of 6 to 10-μm thick follicle cells with centrally positioned 4 to 8 μm (diameter) spherical nuclei surrounded a 10 to 500 μm (diameter) spherical follicle lumen. Species-specific models were defined for mouse, rat and man. The source compartments for 211At were the follicle lumen, follicle cells and follicle cell nuclei. The target was the follicle cell nucleus. Simplified species-specific thyroid models were used to investigate the contribution from surrounding follicles. Monte Carlo simulations were performed using the general purpose radiation transport code MCNPX 2.6.0.

RESULTS

When 211At was homogeneously distributed within the follicle lumen, the mean specific energies per decay, 〈z〉, to the follicle cell nucleus were 2.0, 1.1 and 0.17 mGy for mouse, rat and man, respectively. Corresponding values for the single-hit mean specific energy per decay, 〈z1〉, were 1.3, 0.61 and 0.37 Gy. Assuming a homogeneous 211At concentration in the follicle lumen, <0.5%, 7%, and 45% of the emitted alpha particles were fully stopped within the follicle lumen for the respective models.

CONCLUSIONS

The results clearly show the influence of the follicle size, alpha particle range and 211At location within the thyroid follicle on the dosimetric parameters. Appropriate thyroid models are required for translation of dosimetric parameters between species.

Light and electron microscopic study of the thyroid gland of the camel (Camelus dromedarius)

The thyroid gland of sexually immature dromedary camels was studied using both light and electron microscopy. The thyroid gland contained follicles of different sizes in both summer and winter. However, most of the follicles were large in summer and small in winter. The large follicles were lined by very low cuboidal or semi-squamous follicular cells whereas the small ones were lined by high cuboidal or low columnar follicular cells. Electron microscopy showed that the very low cuboidal follicular cells were poor in organelles and were considered hypoactive. High cuboidal follicular cells on the other hand, were rich in organelles that included mitochondria, cisternae of rough endoplasmic reticulum, secretory vesicles, colloid droplets, heterosomes and autophagic vacuoles; they were considered to be very active. The possible role played by these organelles is synthesis of thyroglobulin and liberation of tri- and tetraiodothyronine is discussed. A few degenerate follicular cells were infrequently encountered in the camel thyroid. Parafollicular (C) cells were not seen in this study either with light or electron microscopy.

Deactivation of TSH receptor signaling in filter-cultured pig thyroid epithelial cells

Thyrotropin [thyroid-stimulating hormone (TSH)] receptor on-off signaling was studied in polarized monolayers of pig thyrocytes cultured on permeable support. Transepithelial resistance (R) and potential difference (PD) were used as parameters to monitor the effect of altered TSH concentrations on vectorial electrolyte transport. TSH induced rapid but long-lasting changes in R (decrease) and PD (increase) that were cAMP-dependent and related to enhanced transcellular conductance of sodium and chloride. Withdrawal of TSH from cultures prestimulated with TSH (0.1 mU/ml) for 48 h resulted in restitution of R to control level within 30 min. Such deactivation was markedly accelerated by mild trypsinization, which degraded receptor-bound ligand without affecting TSH receptor responsiveness or ion transporting capacity. Small alterations in the TSH concentration (0.01-0.1 mU/ml) were followed almost instantaneously by adjustments of R. In contrast, the reversal of R after acute TSH stimulation (30 min) and subsequent TSH washout was delayed for several hours independently of cell surface trypsinization. The observations indicate that, during continuous exposure to physiological concentrations, TSH exerts a close minute-to-minute surveillance of thyroid function and the rate-limiting step of deactivation is the dissociation of ligand from the TSH receptor at the cell surface. TSH-deprived cells briefly exposed to TSH are refractory to rapid deactivation, probably because of altered metabolism downstream of TSH receptor signal transduction.

The epitheliomeral concept of the thyroid gland's structural organization

BACKGROUND

The permanent increase in the number of cases of different forms of thyroid pathology requires a deeper study of the thyroid structure and its morphogenetic processes.

METHODS

The thyroid glands of Wistar rats were used in a composite study that involved standard light microscopy, semithin sections, silver staining, immunohistochemistry, morphometry, three-dimensional analysis, and computer-assisted three-dimensional reconstruction.

RESULTS

The absolute majority of follicles have from one to seven interfollicular contacts (C) with other neighbouring follicles. The surface of C makes up 32% of the follicular basal surface. In the central zone of 1-micron-thick sections, the relative quantity of C per follicle (0.44 +/- 0.05) is 36% more than in the peripheral zone (0.28 +/- 0.07; P < 0.05). The analysis of serial sections shows that 134 follicles have 133 C. Only three follicles are separate; four form pair groups, and others form groups that include up to 92 follicles. Follicles form linear chains with numerous branches. A three-dimensional reconstruction demonstrates a spatial configuration of the parenchyma's elements. In C, the basement membrane is absent. Consequently, a group of interconnected follicles form a parenchymal compartment with a common basement membrane. We named these parenchymal compartments "epitheliomers." Both the formation of new follicles and follicular fusion take place within the boundaries of the epitheliomer without violating the integrity of the compartment. The fusion of follicles is brought on by the destruction of the epithelium of C.

CONCLUSIONS

The authors put forward a new "epitheliomeral" concept with regard to the structural organization of the thyroid gland.

The age at onset of diabetes influences functional and structural changes in the pituitary-thyroid axis of streptozocin-diabetic male rats

Severe structural changes leading to marked alterations in secretory activity are known to occur in the pituitary-thyroid axis 1 month after induction of postpuberal streptozocin (SZ)-diabetes. However, SZ-diabetic rats of different age groups have not been compared, nor has the maturity of the pituitary and thyroid glands at the onset of diabetes been correlated with the type and evolution of functional and structural changes. We thus induced diabetes in 1-month (prepuberal of 3-month (postpuberal) old male rats and compared diabetic with control groups 4 and 8 months after SZ or saline injection. We determined: 1) pituitary and thyroid weights, 2) the basal plasma TSH, T3, and T4 concentrations, and 3) several morphometrical measurements in the pituitary and thyroid glands. After 4 months, 1) the pituitary and thyroid weights were decreased, 2) plasma TSH and T3 were unchanged, plasma T4 was reduced. and 3) the number of thyrotropes, degenerative changes of follicle cells, and colloid area were increased, the follicle cell height as well as the number of fused cold follicles decreased, and the follicle area was unchanged in diabetic compared with control rats. The lesions were more conspicuous in pre- than in postpuberal diabetic animals. After 8 months, plasma TSH, T3, and T4 were decreased in diabetic compared with control rats. Except for the increased colloid area, all other lesions were similar, though more severe in prepuberal diabetic rats after 8 than 4 months. Few changes were found in postpuberal diabetic rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Circadian rhythmicity in the rat exocrine pancreas: chronomorphological patterns

Circadian rhythmicity of the structural morphometric model of rat endocrine pancreas has been studied in 24 Wistar female rats, four months old, kept in LD 12:12. The following parameters were evaluated: the volume fractions of nucleus and cytoplasm of exocrine cells, the size distribution and number in unit tissue volume of acinar cell nuclei, the mean nuclear diameter, the shape coefficient of glandular acini (that is the ratio acinar area/perimeter2 which indicates the shifting of structures from circularity). A statistically significant circadian rhythm was demonstrated for the shape coefficient of glandular acini. Results obtained in the present experiment are compared with data recorded in a previous study.

Variations in immunocytochemical localization of cathepsin B and thyroxine in follicular cells of the rat thyroid gland and plasma TSH concentrations over 24 hours

Immunocytochemical localization of cathepsin B and thyroxine (T4) in follicular cells of the rat thyroid gland and plasma concentrations of thyroid stimulating hormone (TSH) were examined at six evenly spaced times over 24 h. By light- and electron microscopy, immunodeposits for cathepsin B were localized in cytoplasmic granules of various sizes, whereas those for T4 were detected mainly in larger granules of the cells and in the colloid lumen. The size and location of cytoplasmic granules showing immunoreactivity for cathepsin B and T4 in the cells varied over 24 h, corresponding to a change in plasma TSH concentrations. These immunopositive large granules appeared in the apical cytoplasm at 12.00 h, when the level of TSH was highest. At 20.00 h when the level of TSH was lowest, T4-positive granules almost disappeared, and cathepsin B-positive small granules were abundantly seen in the basal region. From 00.00 h to 08.00 h, these positive granules changed in the same manner as those seen from 12.00 h to 20.00 h, associated with an increase in plasma TSH levels. These results suggest that newly formed colloid droplets migrate from the apical to the basal regions. Cathepsin B may play a role not only in the degradation of thyroglobulin but in the maturation of thyroid hormones during the migration of the granules.