Young female rats were fed with normal (1.18%) or low (0.05%) calcium diet for 3, 7, 15 or 30 days. The morphology of the parathyroid glands was studied together with serum calcium, parathyroid hormone (PTH), calcitonin and bone mineral density (BMD). As compared to the animals fed with the normal calcium diet, BMD of whole body of the rats fed with the low calcium diet was significantly decreased, whereas the serum PTH level was increased. The parathyroid glands in the rats fed with the low calcium diet were markedly enlarged. In the parathyroid chief cells of the rats fed with the low calcium diet, the Golgi complexes and the cisternae of the granular endoplasmic reticulum were well developed, while the large granules and large vacuolar bodies decreased. Some secretory granules located near the plasma membrane. A proportionally larger increase of the cytoplasm was estimated in the rats fed with the low calcium diet for three and seven days. Enlargement of the cytoplasm and rather frequent mitoses of the chief cells were observed in the rats fed with the low calcium diet for 15 and 30 days. These findings suggest that the rapid bone loss in young rats induced by the low calcium diet is essentially due to stimulated activity of the parathyroid gland. The stimulated gland may be a result of hypertrophy at the early stage and a combination of hypertrophy and hyperplasia at the later stage of calcium deficiency.

Osteoporosis is a major public health problem among elderly characterized by a progressive loss of bone mass and subsequent structural weakness of bone. Calcium is one of the nutrients required for normal skeletal growth and mineralization, which plays a very important role in terms of its influence on bone mass (Matkovic and Heaney, 1992). Low calcium intake certainly causes osteoporosis in animals, presumably as the result of exaggerated bone resorption produced by the increased parathyroid activity resulting from hypocalcemia (Stauffer et al., 1973). Considerable studies have showed that low calcium diet stimulates secretory activity of the parathyroid gland (Lee and Roth, 1975; Naveh-Many and Silver, 1990; Wernerson et al., 1991). The influence of low calcium diet on the morphology of the parathyroid glands is s subject of controversy. Some reported that low calcium diet increased the rate of DNA