Effect of low phosphate diet on the structure of the rat parathyroid and thyroid glands

Dietary phosphate restriction in patients with chronic renal insufficiency has been demonstrated to prevent the increase in serum parathyroid hormone [PTH] levels clinically. Besides, the effect of phosphate on serum PTH levels is reported to be independent of changes in the level of both serum calcium and vitamin D. To investigate the structure of the parathyroid and the thyroid glands in rats fed restricted phosphate diet. Three groups of adult Wistar rats were used in this study. Group I, served as control. They were fed with regular balanced diet containing normal [0.6%] phosphate, normal [0.6%] calcium diet. Groups II and Ill [the experimental groups], were fed low [0.02%] phosphate, normal [0.6%] calcium and low [0.02%] phosphate, low [0.02%] calcium diets respectively for 30 days. The morphology of the parathyroid and thyroid glands was studied, using light and electron microscopic techniques. The parathyroid glands of groups II and Ill were atrophied with connective tissue invasion and darkening of the chief cells. The chief cells revealed well developed Golgi and RER cisternae, and the cytoplasm contained numerous small granules containing particulate material. The thyroid follicles in group Ii animals appeared lighter in staining, whereas in group Ill, most of the follicular cells were darkened. However, the activation was focal in distribution. Active interaction between the thyroid follicular and the adjacent parafollicular cells was evident. Both the plasma and nuclear membranes of the follicular cells exhibited signs of interactions. The thyroid parafollicular cells in group H appeared dark and contained numerous secretory granules, whereas in group III they were pale and contained few granules. It can be concluded that phosphate restriction induces a direct atrophic effect on the parathyroid gland, independent of calcium level. On the other hand, phosphate restriction influences the thyroid parafollicular cells indirectly, most probably through modulating the blood calcium level. It can also be concluded that a mutual cooperation exists between the thyroid follicular and parafollicular cells