EVIDENCE FOR A DIMINISHED MATURATION OF PREOSTEOBLASTS INTO OSTEOBLASTS DURING AGING IN RATS - AN ULTRASTRUCTURAL ANALYSIS

Bone is subject to continuous remodeling throughout life. The age-rela ted loss of (trabecular) bone, leading to senile osteopenia, is mainly due to impaired bone formation. Osteoblasts (OB) and osteoclasts (OC) have been identified as playing a crucial role in the process of bone turnover, but the contribution made by their precursors is not well d ocumented. We analyzed the cells of the osteoblast and osteoclast cell lineage along the trabecular bone of tibiae and the stromal cells in the marrow of aging BN/Bi Rij rats using electron microscopy. It appea red possible to distinguish preosteoblasts (pre-OB), OB, preosteoclast s (pre-OC), OC, and inactive bone-lining cells. Periods of increase, t he maximal peak, and the decrease in trabecular bone volume were defin ed by means of morphometric measurements of trabecular bone volume. We found a decrease of more than ill-fold in the number of OB with age, but the numbers of pre-OB, pre-OC, and OC expressed per unit bone leng th, although variable, were age independent. The relative bone resorpt ion and formation surface, expressed as a percentage of the total bone surface, decreased 2- and 15-fold, respectively. In 2-year-old animal s the total volume of stromal cells, part of which constitutes the ste m cell compartment of the osteogenic lineage, was a quarter of that fo und in 1-month-old animals and a third of that found in 6-month-old an imals. The loss of trabecular bone is concomitant with a sharp increas e in the ratio of pre-OB/OB, the ratio of OC/OB, and in the ratio of r esorption to formation surfaces. There was no relation between the rat io of pre-OC/OC with age. These data lead to the conclusion that the m ain factor causing bone loss with age is a diminished maturation of pr e-OB into OB.