Nuclear chromatin-concentrated osteoblasts in renal bone diseases.

The morphological appearance of an osteoblast largely alters with its differentiation and maturation, along with the change of cell function. We quantitatively observed the osteoblast morphology and compared it with bone metabolism. Biopsied iliac bone samples obtained from 77 dialysis patients (14 mild change, 37 osteitis fibrosa, 2 osteomalacia, 8 mixed, and 16 adynamic bone) were included in the study. Osteoblast appearances were classified into three groups: (i) type II and III osteoblasts, namely, active osteoblasts characterized by cuboidal or columnar shapes with or without a nuclear clear zone; (ii) type IV osteoblasts, lining osteoblasts characterized by extremely thin cytoplasm; and (iii) type V osteoblasts, apoptotic osteoblasts characterized by nuclear chromatin concentration. The results were quantitatively expressed as the length of bone surface covered by each type of osteoblasts. The type II and III osteoblasts were predominant in osteitis fibrosa, mixed, and mild change. The type IV osteoblasts were overwhelmingly predominant in adynamic bone. The type V osteoblasts appeared most frequently in osteitis fibrosa, followed by mixed and mild change. Both absolute and relative lengths of bone surface covered by the type V osteoblasts were significantly higher in the high-turnover bone group (osteitis fibrosa and mixed) than the low-turnover bone group (adynamic bone and osteomalacia). The type V osteoblasts were slightly correlated with serum intact parathyroid hormone levels. In conclusion, a high bone-turnover condition seems to be associated with the promotion of osteoblastic apoptosis in dialysis patients. This finding may explain the fact that osteopenia develops faster in CKD patients with high turnover of bone.