Morphological and functional changes in several endocrine glands induced by hypothyroidism in the rat

Effect of thyroxin on cell morphology and hormone secretion of pituitary grafts in rats

INTRODUCTION

Growth hormone and prolactin secretion is affected by thyroid hormones. To see if this influence is subsidiary to the hyptothalamus, we investigated the effects of thyroxin (T4) on hormone secretion and histology of sellar pituitaries and pituitary grafts detached from the hypothalamus (autografted or allografted under the kidney capsule).

MATERIALS AND METHODS

Male Wistar rats were divided into eight groups: control, thyroidectomised, pituitary autografted, pituitary allografted, and four additional groups that were injected with T4 for two weeks, starting four weeks after surgery. At sacrifice, adenohypophysial hormone blood levels were assessed, and tissue from sellar and grafted pituitaries were investigated by histology and electron microscopy.

RESULTS

Growth hormone and prolactin blood levels, as well as the number of growth hormone immunopositive cells increased in T4-treated groups. Both pituitary auto- and allo-grafts showed lactotroph hyperplasia and displayed spongiform areas containing cells with vesicles in their cytoplasm resembling thyroidectomy cells. This phenomenon was minimized in their respective T4-treated group. Thyroidectomy cells were identified in pituitary grafts, indicating that hypothalamic control was not essential to induce them.

DISCUSSION AND CONCLUSION

It is intriguing that the pituitary allografted group, even maintaining normal T4 blood levels, developed thyroidectomy cells in their grafts, suggesting that a long- term deficit of vascularization (>4 weeks) prevented T4 from reaching the graft. After 6 weeks, post T4 treatment of two weeks seemed to be the determining factor to minimize thyroidectomy cells in both pituitary autografted + T4 and pituitary allografted + T4 grafts compared to the untreated groups, although more time and/or higher T4 doses may be required to fully restore the euthyroid morphology.

T3 fails to restore mitochondrial thiol redox status altered by experimental hypothyroidism in rat testis

Oxidative stress impaired sperm function might lead to infertility. The objective of this study was to evaluate the effects of altered thyroid hormone levels on regulation of mitochondrial glutathione redox status and its dependent antioxidant defense system in adult rat testis and their correlation with testicular function. Adult male Wistar rats were rendered hypothyroid by administration of 6-n-propyl-2-thiouracil in drinking water for six weeks. At the end of the treatment period, a subset of the hypothyroid rats was treated with T(3) (20 μg/100g body weight/day for 3 days). Mitochondria were isolated from euthyroid, hypothyroid and hypothyroid+T(3)-treated rat testes, and sub-fractionated into sub-mitochondrial particles and matrix fractions. Mitochondrial respiration, oxidative stress indices and antioxidant defenses were assayed. The results were correlated with daily testicular sperm production and epididymal sperm viability. Increased pro-oxidant level and reduced antioxidant capacity rendered the hypothyroid mitochondria susceptible to oxidative injury. The extent of damage was more evident in the membrane fraction. This was reflected in higher degree of oxidative damages inflicted upon membrane lipids and proteins. While membrane proteins were more susceptible to carbonylation, thiol residue damage was evident in matrix fraction. Reduced levels of glutathione and ascorbate further weakened the antioxidant defenses and impaired testicular function. Hypothyroid condition disturbed intra-mitochondrial thiol redox status leading to testicular dysfunction. Hypothyroidism-induced oxidative stress condition could not be reversed with T(3) treatment.

The role of thyroid hormone in testicular development and function

Thyroid hormone is a critical regulator of growth, development, and metabolism in virtually all tissues, and altered thyroid status affects many organs and systems. Although for many years testis has been regarded as a thyroid hormone unresponsive organ, it is now evident that thyroid hormone plays an important role in testicular development and function. A considerable amount of data show that thyroid hormone influences steroidogenesis as well as spermatogenesis. The involvement of tri-iodothyronine (T(3)) in the control of Sertoli cell proliferation and functional maturation is widely accepted, as well as its role in postnatal Leydig cell differentiation and steroidogenesis. The presence of thyroid hormone receptors in testicular cells throughout development and in adulthood implies that T(3) may act directly on these cells to bring about its effects. Several recent studies have employed different methodologies and techniques in an attempt to understand the mechanisms underlying thyroid hormone effects on testicular cells. The current review aims at presenting an updated picture of the recent advances made regarding the role of thyroid hormones in male gonadal function.

Effect of thyroidectomy on the histology of rat sublingual gland

This study was carried out to investigate the effect of thyroidectomy on the histology of rat sublingual gland. Twenty-eight male Wistar albino rats, aged 4 weeks and weighing between 45-55 g, were used. The rats were divided into two experimental groups (control and thyroidectomy), each containing 14 animals. Total thyroidectomy of rats was performed under ether anesthesia in thyroidectomy group. The rats in the control group were sham operated without having the thyroidectomy. Seven rats randomly selected from both groups were fixed using the perfusion fixation technique 2 and 6 weeks after thyroidectomy, and their sublingual glands were harvested for histological investigation. No histological difference was observed between the two groups 2 weeks after thyroidectomy. However, 6 weeks after thyroidectomy considerable cytoplasmic vacuolization of the epithelial cells of the mucous tubules was seen in the thyroidectomy group compared to the controls. Enlargement of mucous tubules was also observed, and the lumina in most of the tubules was quite dilated. In the stroma surrounding the parenchymal tissues, increased lipid tissue mass was observed. In addition, increased connective tissue mass and mononuclear cell infiltrations were evident. Furthermore, the number of mast cells was significantly higher in the thyroidectomy group than in the controls 6 weeks after thyroidectomy. It was concluded that the thyroid gland and hormones might have an influence on the histology of the sublingual gland.

Selective vulnerability of the hippocampal pyramidal neurons to hypothyroidism in male and female rats

Thyroid hormone deficiency has long been considered to affect profoundly such cognitive functions as learning and memory, which are known to depend on the structural integrity of the hippocampal formation. Since we previously found that the number of granule cells of the dentate gyrus is reduced in hypothyroid animals, we decided to extend our observations to the pyramidal cells of the hippocampus in order to gain further insight into the effects of hypothyroidism upon the other neuronal links of the hippocampal trisynaptic circuitry, inasmuch as CA1 neurons are known to be particularly vulnerable to aggressive agents. Groups of 6 male and 6 female rats aged 30 and 180 days were analysed separately after being treated as follows: (1) hypothyroid from day 0 until day 30 (30-day-old hypothyroid group); (2) respective 30-day-old control; (3) hypothyroid from day 0 until day 180 (180-day-old hypothyroid group); (4) hypothyroid until day 30 and thenceforth maintained euthyroid (recovery group); (5) hypothyroid since day 30 (adult hypothyroid group); and (6) respective 180-day-old control. The volume of the pyramidal cell layer of the CA1 and CA3 regions and the numerical density of the respective neurons were evaluated, thereby allowing us to estimate the total number of pyramidal cells in each hippocampal region. The areal density and the mean nuclear volume of CA1 and CA3 pyramidal cells were also estimated. In the CA3 region, we found that hypothyroidism, whatever its duration and time of onset, induces a reduction in the volume of the pyramidal cell layer and a parallel increase in the numerical density of its neurons, without interfering with the total number of pyramidal cells. Conversely, in the CA1 region, thyroid hormone deficiency started either neonatally or during maturity was found to lead to a decrease in the total number of pyramidal cells. Reductions ranging between 14.2 and 22.5% were found in 30 and 180-day-old hypothyroid groups. The reestablishment of a euthyroid state did not ameliorate the referred neuronal loss. The present results support the view that hypothyroidism induces small alterations in the structural organization of the hippocampal CA3 region, contrary to what happens in CA1 in which neuronal death occurs. Furthermore, the data presented herein demonstrate that the total number of CA1 pyramidal cells displays sexual dimorphism that is not affected by thyroid hormone manipulations.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of hypothyroidism upon the granular layer of the dentate gyrus in male and female adult rats: a morphometric study

The effects of hypothyroidism upon the structure of the central nervous system of adult rats are poorly understood in spite of evidence that the mature brain is vulnerable to this condition. Existing developmental studies show that the morphological changes induced by thyroid hormone deficiency are related to alterations in neurogenesis. We studied the granular layer of the dentate gyrus under different experimental conditions of hypothyroidism, because in rodents the neurogenesis of the granule cells continues during adulthood. The following groups of rats were analysed: 1) control; 2) hypothyroid from day 0 until day 180 (hypothyroid group); 3) hypothyroid until day 30 and henceforth maintained euthyroid (recovery group); and 4) hypothyroid since day 30 (adult hypothyroid group). Groups of 6 male rats and 6 female rats were analysed separately. The volume of the dentate gyrus granular layer and the numerical density of its neurons were evaluated, so we were able to estimate the total number of granule cells. Because in the experimental groups the volume of the granular layer and the numerical density of its neurons were reduced, the total number of granule cells was decreased. In the hypothyroid and recovery groups the alterations were identical and more striking than in the adult hypothyroid groups. The total number of granule cells displayed sexual differences in all groups studied except in the hypothyroid groups. The present results support the view that thyroid hormone deficiency interferes with the process of cell acquisition by reducing neuronal proliferation and that it also leads to increased cell death. These events underlie the irreversible morphological changes observed in the brain of hypothyroid rats, either during development or at maturity. The referred structural alterations are probably related to the functional deficits observed in this condition.

The supraoptic nucleus in hypothyroid and undernourished rats: an experimental morphometric study

The supraoptic nuclei of both male and female 30-day-old rats rendered hypothyroid by daily subcutaneous injection of propylthiouracil were studied and the results were compared with age- and sex-matched rats fed ad libitum (control rats) and with undernourished rats. Morphometric methods were used to evaluate the volume of the supraoptic nucleus and the areal and numerical densities of its neurons. These parameters allowed us to estimate the total number of neurons of this nucleus. In addition, the mean cross-sectioned area and mean nuclear diameter of the same neurons were also evaluated. The volume of the supraoptic nucleus was reduced both in hypothyroid and undernourished animals when compared with normal controls. The areal and numerical densities of neurons from the former groups were increased and the volume density of the neuropil was reduced. As a consequence, the total number of neurons was found to be identical in all groups studied. Furthermore, the mean nuclear diameters and cross-sectioned areas of the supraoptic neurons were similar in all groups. The volumetric differences and the increased packing found were always more marked for hypothyroid than for undernourished rats. Differences were not detected between male and female groups. The present results support the view that the increased cell packing in hypothyroid animals depends upon a reduction in the neuropil of the nucleus, as has been described under similar conditions in other central nervous system areas displaying identical patterns of neurogenesis. In addition, it was found that the effects of undernourishment cannot be discriminated from those dependent on neonatal hypothyroidism.