IntroductionIn bone remodeling the activities of osteoblasts, the bone-forming cells, and osteoclasts, cells of hematopoietic origin capable of resorbing bone, must be balanced carefully in order to maintain skeletal integrity (1). The importance of understanding the factors controlling these activities is highlighted by metabolic bone disorders such as osteoporosis, in which the imbalance of bone formation and resorption leads to bone loss. Parathyroid hormone (PTH), a major regulator of calcium homeostasis, plays an important role in both bone formation and resorption. While PTH excess in hyperparathyroidism (2) and in continuous administration of PTH (3) is characterized by large numbers of osteoclasts, rapid bone turnover, and low cortical bone mass, it has long been known that intermittent dosing of PTH can lead to increased trabecular bone mass (4,5). This anabolic effect is due to increased bone formation (6, 7). Interestingly, histomorphometric studies in patients with mild hyperparathyroidism also show an increase in cancellous bone (2). Although osteoblasts likely mediate both the anabolic and catabolic actions of the peptide, the molecular mechanisms underlying this dual effect are incompletely understood.The PTH/PTH-related protein (PTH/PTHrP) receptor (PPR), a G protein-coupled receptor, is believed to mediate many of the actions of both PTH and PTHrP in bone, as shown by mutations in mice and humans. Mice in which the PPR has been ablated by homologous recombination have decreased trabecular bone and increased thickness of cortical bone during fetal development (8). These skeletal abnormalities are similar to those observed in patients with Blomstrand lethal chondrodysplasia, a rare autosomal recessive disorder caused by inactivating PPR mutations (9, 10). Consistent with this crucial role of the PPR in cells of the osteoblast lineage, expression of the mRNA encoding this receptor is detectable in relatively mature osteoblasts and in adjacent stromal cells likely to be osteoblast precursors (11).Jansen's metaphyseal chondrodysplasia (JMC) is a rare form of short-limbed dwarfism caused by activating mutations of the PPR leading to ligand-independent cAMP accumulation (12). Histomorphometric analysis of bone from a patient with this disorder shows exaggerated loss of cortical bone and preservation, or even augmentation of trabecular bone, as is seen in mild primary hyperparathyroidism (13). Parathyroid hormone (PTH), an important regulator of calcium homeostasis, targets most of its complex actions in bone to cells of the osteoblast lineage. Furthermore, PTH is known to stimulate osteoclastogenesis indirectly through activation of osteoblastic cells. To assess the role of the PTH/PTHrelated protein receptor (PPR) in mediating the diverse actions of PTH on bone in vivo, we generated mice that express, in cells of the osteoblastic lineage, one of the constitutively active receptors described in Jansen's metaphyseal chondrodysplasia. In these transgenic mice, osteoblastic function was increased in the t...

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The majority of chronic hemodialysis patients have elevated serum iPTH levels and bone disease characterized by osteitis fibrosa. However, a small group of patients develop osteomalacic bone disease associated with normal or slightly elevated iPTH values and a tendency to hypercalcemia which occurs either spontaneously or after treatment with small doses of vitamin D sterols. To examine the causes of the relatively low iPTH levels, we evaluated the change in serum iPTH levels that occurred in response to acute hypocalcemia, produced by dialysis using a low calcium dialysate, in 11 patients with osteomalacia and 8 control hemodialysis patients. Dialysis against a dialysate free of calcium for 60 to 90 min led to a fall in serum calcium to 7.5 +/- 0.2 and 7.2 +/- 0.2 mg/dl in the osteomalacic and control patients, respectively. Serum iPTH rose in controls from 1380 +/- 287 to 1960 +/- 287 pg/ml (P less than 0.01), whereas in patients with osteomalacia it rose from 360 +/- 58 to 507 +/- 104 pg/ml (P less than 0.05), a value only slightly above normal for this PTH assay. These data suggest that the relatively low basal levels of serum iPTH do not arise as a consequence of physiologic suppression of parathyroid gland function. This reduction in parathyroid function could contribute to the pathogenesis of low turnover osteomalacia.

At the Fourth ISCERG Symposium, held in Hakone, Japan, in I966 Straub (I966) reviewed the subject of unilateral retinitis pigmentosa and made a number of important observations. Up to then only 34 cases of unilateral retinitis pigmentosa had been reported. Only one of them was of unilateral disease without pigment (Jacobson and Stephens, I962). Rickers and Domarus (I974) stated that nearly ioo cases of unilateral retinitis pigmentosa had been described since the first report in I865. They thought that the diagnosis could be verified in only about 20 cases.Straub emphasized the clinical value of electroretinography (ERG) in establishing the truly unilateral nature of such cases, indicating that the ERG response must be normal in the unaffected eye. Others have long recognized that many cases of supposed unilateral retinitis pigmentosa were actually instances of highly asymmetric bilateral disease in which the 'normal' eye eventually showed the disorder, but only much later. Franceschetti, Frangois, and Babel (I963) showed that the ERG response in such cases was impaired in both eyes. This significant clinical feature distinguishes genuine unilateral disease from 'pseudounilateral' (that is, asymmetric bilateral) retinitis pigmentosa.Henkes (I964) found that the electro-oculogram was bilaterally disturbed in many instances of presumed unilateral retinitis pigmentosa but normal in the unaffected eye in truly unilateral cases. Thus measuring the resting potential of the eyes as well as the ERG is of value.If unilateral retinitis pigmentosa is uncommon, cases without pigmentation are even rarer. We report here a case of unilateral retinitis pigmentosa sine pigmento. OphthalmoscopyThe fundus of each eye was examined through pupils dilated with i per cent tropicamide and io per cent viscous phenylephrine hydrochloride ophthalmic solutions (Fig. i)

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