Recent work by a number of investigators has demonstrated that the process of bone matrix formation and mineralization is under the influence of growth factors and cytokines present in the local environment. Utilizing primary and established osteoblast cell culture systems, these studies have examined the regulation of bone matrix protein synthesis and deposition into the extracellular matrix (ECM) and subsequent mineralization. In previous studies, we have utilized the human osteoblastic cell line, HOS TE85, to study the effects of Tumor Necrosis Factor - alpha (TNF-α) on the regulation of matrix proteins and proteolytic function in monolayer cultures as well as during the development and calcification of ECM formed by HOS TE85 cells during extended culture. Our studies demonstrate that TNF-α inhibited formation and mineralization of nodules. In the study reported here, we evaluated the ultrastructural morphology of the cell-matrix complex formed by HOS TE85 cells in the presence and absence of TNF-α at selected time points during the matrix development process utilizing both transmission electron microscopy and light microscopy. In the presence of TNF-α, the cell-matrix complex does not develop normally, with a lack of organization and mineralization, when compared to untreated cells. The lack of mineralization appears to result from the lack of normal collagen fibril deposition and formation of an appropriate ECM essential for the mineralization process. These results support our previous observations that TNF-α inhibits HOS TE85 cells from forming a mineralizing ECM by inhibiting incorporation of collagen into the ECM and inducing the synthesis of proteolytic enzymes capable of degrading collagen in the ECM.