Epithelial-mesenchymal interactions play a crucial role in morphogenesis of various tissues and organs. In the development of hair follicles, extensive interactions between epithelial and mesenchymal layers in the hair follicle germs (HFGs) trigger follicular morphogenesis. Recently, a research group succeeded in regenerating hairs by imitating the process during in vivo hair development. However, the preparation technique for thousands of HFGs which have high hair regenerative ability is required for human treatments. In this study, we propose a scalable and automated approach using a microgel bioprinter for large-scale preparation of HFGs.

　Two types of hair follicle stem cells, epithelial and mesenchymal cells, were isolated from the skin of mice, and were then suspended in collagen gel solution. Collagen drops were placed adjacent to each other using an electromotive pipette or bioprinter to prepare collagen-containing HFGs. During 3 days of suspension culture, the two microgel-merged constructs spontaneously shrunk by cell attraction forces and formed HFG-like structures. The HFGs efficiently regenerated hair follicles and shafts when transplanted to mice compared to the HFGs without collagen gel. However, most of generated hair shafts remained under the skin epithelial layer. We therefore prepared HFGs with a nylon thread guide and examined its effects on hair regeneration. Nylon thread guides were inserted in the HFGs by placing each collagen drop on the guides. HFGs with nylon thread guides after 3 days of culture were transplanted into the back skin of mice. After 3 weeks of transplantation, the nylon thread guide lead to outgrowth of hair shafts (~80% of HFG per hair-regenerated HFG).