Controlling cell position in complex heterotypic 3D microtissues by tissue fusion

Tissue fusion and cell sorting are processes fundamental to developmental biology with applications in tissue engineering. We have designed a fusion assay to investigate the factors governing the fusion of microtissues and the cell sorting that occurs after fusion. Normal human fibroblast (NHF) spheroids were self-assembled and cultured for 1, 4, or 7 days, then combined in trough shaped recesses. Over a 24-h period the spheroids fused to become a rod shaped microtissue and the kinetics and extent of fusion could be quantified by assessing rod contraction. By varying the amount of spheroid culture time prior to fusion (1-7 days), the rate of fusion, the coherence of the building units (as measured by fusion angle) and the steady state length of the structure could be easily controlled. Longer pre-culture times for the spheroids resulted in slower fusion, less coherence and increased length of rod microtissues. The fusion kinetics and steady length of rods formed by smaller versus larger spheroids ( approximately 100 vs. 300 microm diameter) were indistinguishable, even though smaller spheroids had twice the surface area and greater numbers of contacts between units. Both small and large spheroids were strongly influenced by spheroid pre-culture time. Pre-culture time could also be used to control cell sorting and cell position when combinations of NHFs and H35s, a rat hepatocyte cell line, were fused to form heterotypic microtissues. Control of fusion and cell position are important parameters for creating functional heterotypic microtissues as well as the use of microtissues as building units to create larger tissue structures.