Crosstalk of osteoblast and osteoclast precursors on mineralized collagen--towards an in vitro model for bone remodeling.

Bone remodeling and, therefore, integration of implant materials require the coordinated regulation of osteoblast and osteoclast activity. This is why the in vitro evaluation of biomaterials for bone regeneration should involve not only the analysis of osteoblast differentiation but also the formation and differentiation of osteoclasts. In the present study, we applied a material made of mineralized collagen I that mimics extracellular bone matrix to establish a culture system, which allows the cocultivation of human monocytes and human mesenchymal stem cells (hMSCs), which were differentiated into osteoclast-like cells and osteoblasts, respectively. Both cell types were cultivated on membrane-like structures from mineralized collagen. Transwell inserts were used to spatially separate the cell types but allowed exchange of soluble factors. The osteoclastogenesis and osteogenic differentiation were evaluated by analysis of gene expression, determination of alkaline phosphatase (ALP), and tartrate-resistant acidic phosphatase (TRAP) activity. Furthermore, cell morphology was studied using scanning electron and transmission electron microscopy. Osteogenically induced hMSC showed an increased specific ALP activity as well as increased gene expression of gene coding for alkaline phosphatase (ALPL), when cocultivated with differentiating osteoclasts. Adipogenic differentiation of hMSCs was suppressed by the presence of osteoclasts as indicated by a major decrease in adipocyte cell number and a decrease in gene expression of adipogenic markers. The formation of multinucleated osteoclasts seems to be decreased in the presence of osteogenically induced hMSC as indicated by electron microscopic evaluation and determination of TRAP activity. However, gene expression of osteoclast markers was not decreased in coculture with osteogenically induced hMSC.

In vitro investigations of bone remodeling on a transparent hydroxyapatite ceramic.

The light microscopic examination of cells directly on bioceramic materials in the transmission mode is impossible because many of these materials are opaque. In order to enable direct viewing of living cells and to perform time-lapse studies, nearly transparent bioceramic materials were developed. A dense and fine-grained transparent hydroxyapatite (tHA) was processed by a gel-casting route followed by low-temperature sintering (1000 degrees C). By virtue of its transparency, direct visualization of cellular events on this material is possible in transmitted light. In this study, the interaction of different bone cell types with the tHA ceramic was envisaged. Investigation of rat calvaria osteoblasts (RCO) cultured on tHA by means of transmission light microscopy indicated good cytocompatibility of tHA. Microscopic analysis of osteogenic-induced human bone marrow stromal cells (hBMSC) on tHA and quantitative analysis of their lactate dehydrogenase (LDH) activity at different time points of culture revealed favorable proliferation as well. An increase of the alkaline phosphatase (ALP) activity indicated the differentiation of osteogenic-induced hBMSC towards the osteoblastic lineage. In addition, the differentiation of human monocytes to osteoclast-like cells could also be demonstrated on tHA and was confirmed by fluorescent microscopy imaging of multinucleated cells on the transparent material.

Germ cell-less expression in medaka (Oryzias latipes).

The gene germ cell-less (gcl) plays an important role in the early differentiation of germ cells in Drosophila. We isolated the gcl homolog of the model teleost medaka (Oryzias latipes) using degenerated primers and an ovary cDNA bank. The predicted amino acid sequence of medaka gcl showed 92, 68 and 31% overall identity to mouse, human and Drosophila gcl respectively. RT-PCR revealed stronger expression in the ovary and weaker expression in testis, brain, heart, liver and muscle tissue. Expression in early embryos indicates the presence of maternal mRNA. By in situ hybridisation (ISH), gcl could not be detected in embryos. In contrast to vasa, ISH revealed expression of gcl in the ovary but not in the testis. Mol. Reprod. Dev. 67: 15-18, 2004.