Cellular isolation, culture and characterization of the marrow sac cells in human tubular bone

The goal of this study is to characterize the epithelioid-like human marrow sac cells that separate the myeloid and osteoblast populations in situ and to determine if they express osteoblast cytoplasmic markers. Tubular segments of femoral diaphyseal bone were obtained from healthy young (4-8 yr) male and female patients undergoing femoral shortening surgeries. The interface between bone and marrow was examined by scanning (SEM) and transmission electron microscopy (TEM). The marrow sac cells were isolated and cultured in a-MEM medium with and without dexamethasone, glycerophosphate, and ascorbic acid [DGPA]. Alkaline phosphatase (ALP), bone morphogenetic protein-2 (BMP-2) and osteocalcin were evaluated. In the SEM, the marrow sac presented a distinctive pattern of large overlapping cells. TEM studies showed that marrow sac was one or two cells thick, which were attenuated with elongated nuclei, few cellular organelles, and appeared to display intercellular gap junctions. In culture, the marrow sac cells stained positively for ALP and BMP-2, and their expression was enhanced two- to three-fold when the cells were grown in DGPA. DGPA did not enhance osteocalcin expression. The cells of the human marrow sac reside proximate to endosteal osteoblasts and express osteoblastic markers. It is possible that these stromal cells constitute an osteoprogenitor pool from which replacement osteoblasts are recruited, and that they are involved in normal bone formation and in bone diseases (e.g., osteoporosis and osteopenia).