Ultrastructural observations on the three-dimensional reorganization and growth of newly-formed candidate stromal cells and residual hemopoietic cells in culture.

Gelfoam sponges were used as three-dimensional culture supports to allow the migration, reorganization and growth of regenerative cells derived from bone fragments previously depleted of marrow by curettage. The explants and sponges, cultured in 0.3% agar-medium, were analysed ultrastructurally throughout a 14 day culture period. By day 5 the gelfoam sponge was infiltrated by large cells with extensive cytoplasmic processes (stromal cells) and primitive round cells. The stromal cells were pleomorphic and varied in their content of rough endoplasmic reticulum, mitochondria, cytoplasmic granules, microfilaments, lipid droplets and glycogen. The cytoplasmic processes made multiple intercellular contacts with other stromal cells thereby forming a stromal network. Close and direct associations of stromal cells with primitive hemopoietic cells and various stages of myeloid cells were identified. The identification of proliferating granulocytes and mononuclear phagocytes on days 7--14 in these unstimulated cultures suggests that the organization of stromal cells is able to maintain primitive cells and supports the proliferation and differentiation of myeloid cells. It is proposed that this three-dimensional culture technique (i) allows the development of primitive stromal precursors and primitive residual hemopoietic cells; (ii) provides a model for the analysis of early stromal cell-hemopoietic cell interactions.