An ultrastructural study of cell-cell contact between mouse spleen cells and calvaria-derived osteoblastic cells in a co-culture system for osteoclast formation

We have previously established a mouse coculture system of osteoblastic cells and spleen cells for examining osteoclast differentiation. In the present study, we examined the morphological features of the cell-cell contact between mouse spleen cells and osteoblastic cells in the co-cultures. Light microscopic investigations revealed that tartrate-resistant acid phosphatase (TRAP)-positive mononuclear and multinucleated spleen cells appeared in the vicinity of alkaline phosphatase (ALP)-positive osteoblastic cells. Ultrastructurally, spleen cells extended long cytoplasmic filopodia in all directions, by which spleen cells touched adjacent osteoblastic cells. The adjacent osteoblastic cells and spleen cells adhered to each other by forming electron-dense cytoplasmic materials on their inner leaflets of plasma membranes at cell-cell contact sites. Some adjacent osteoblastic and spleen cells made contact on the plasma membranes, forming an extracellular microenvironment. Coated pits were also formed on the plasma membranes facing this microenvironment. These morphological features of the cell-cell contact between osteoblastic cells and spleen cells indicate that there is internalization of organic components, i.e., receptor-mediated endocytosis in the contact sites between the two types of cells.