pH-dependent multilamellar structures in fetal mouse bone: possible involvement of fatty acids in bone mineralization.

pH-dependent multilamellar structures in fetal mouse bone: possible involvement of fatty acids in bone mineralization. Am. J. Physiol. 266 (Cell Physiol. 35): C590-C600, 1994.--Multilamellar structures (MLS) were found inside and outside osteoblasts in cultured and uncultured fetal mouse parietal bone fixed at pH 7.3 with glutaraldehyde solution containing tannic acid (TA). Electron-lucent areas (up to 1.5 microns in diameter) surrounded by thin lamellar structures were found in place of MLS in bone and bone cell cultures fixed at pH 6.0 in the presence of TA. Large lipid droplets were found, in place of electron-lucent areas and MLS, in specimens fixed at pH 7.4 in the absence of TA and dehydrated with a procedure that did not extract neutral lipid. Freeze-fracture studies showed phosphatidylcholine formed MLS at both pH 8.1 and pH 6.0, whereas oleic acid formed MLS at pH 8.1 and lipid droplets at pH 6.0. Thus fatty acids probably formed the pH-dependent MLS found in bone. The data suggest that osteoblasts synthesize and secrete fatty acids, as droplets, into the extracellular space. The close association of MLS with calcifying osteoid in specimens processed with TA suggests that fatty acids are directly involved in bone mineralization.