Integrating single-cell and spheroid strategies in tissue engineering: Comparative insights from pancreas, cartilage, heart and brain.

Tissue engineering (TE) remains a cornerstone of regenerative medicine, aiming to bypass the limitation of organ transplantation through the fabrication of functional tissue substitutes. Traditionally, TE has followed two primary paradigms: the top-down approach, utilising single cells seeded on a scaffold, and the bottom-up approach, employing cell spheroids as building blocks. While top-down offers architectural and structural control, bottom-up promotes self-assembly, native-like extracellular matrix deposition, and intercellular signalling. However, modern techniques increasingly blur this dichotomy, creating a spectrum of cell-based fabrication approaches. This review evaluates the diverse approaches across four major tissue classes: epithelial (pancreas as an example), connective (cartilage), muscle (heart), and nervous (brain) tissues. For each tissue, we examine notable studies to evaluate how different assembly methods recapitulate native tissue properties. By reviewing case studies across diverse tissue types, we highlight the relative strengths and limitations of various fabrication strategies. Although this review is limited by a selective cross-section of literature within a rapidly advancing technological landscape, it provides critical insights into optimising next-generation tissue constructs. In conclusion, we posit that there is no universal fabrication strategy; rather, the future of the field depends on tailoring approaches along this single-cell-to-spheroid spectrum based on the specific architectural and functional demands of the target tissue.