AN ULTRASTRUCTURAL-STUDY OF CELL-CELL CONTACT BETWEEN MOUSE SPLEEN-CELLS AND CALVARIA-DERIVED OSTEOBLASTIC CELLS IN A COCULTURE SYSTEM FOR OSTEOCLAST FORMATION

We have previously established a mouse coculture system of osteoblasti c 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-r esistant acid phosphatase (TRAP)-positive mononuclear and multinucleat ed 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 t ouched adjacent osteoblastic cells. The adjacent osteoblastic cells an d spleen cells adhered to each other by forming electron-dense cytopla smic materials on their inner leaflets of plasma membranes at cell-cel l contact sites. Some adjacent osteoblastic and spleen cells made cont act on the plasma membranes, forming an extracellular microenvironment . Coated pits were also formed on the plasma membranes facing this mic roenvironment. These morphological features of the cell-cell contact b etween osteoblastic cells and spleen cells indicate that there is inte rnalization of organic components, i.e., receptor-mediated endocytosis in the contact sites between the two types of cells.