Human scalp skin and hair follicles (HF) (most prominently at the level of the bulge) are densely innervated by sensory peripheral nerve fibers (NF) suggesting that neuromediators released by cutaneous NFs impact on human epidermis and HF activities and vice versa. While imbalance of this complex bi-directional communication, associated e.g. with neurogenic skin inflammation, may contribute to the development of skin and hair disorders, it has not yet been possible to dissect these bi-directional interactions in a fully humanized re-innervated ex vivo model. For this purpose, we have placed full-thickness human scalp skin punches directly onto a layer of  human iPSC-derived neural stem cells (nSCs), or human iPSC-derived sensory neuron progenitors (snPs) and cultured them in neurotrophin-supplemented serum-free culture medium at the air-liquid interface. While no remaining  NFs were detected in human scalp skin after 10 days ex vivo, PGP9.5 NFs were observed after 10 to 15 days of skin co-cultures with human nSCs or snPs.
Interestingly, some of newly developed NFs around HFs were NF200+, indicating the presence of myelinated NFs. While skin re-innervation did not induce major changes in overall epidermal and HF morphology, the presence of neurons up-regulated the proliferation and apoptosis of epidermal and hair matrix keratinocytes and K15+ bulge cells at the different days of culture. We are currently characterizing the phenotype of the axons generated, the  neuromediators released by the newly generated NFs, and their interactions with mast cells. Once fully established, this assay provides unparalleled preclinical research opportunities for dissecting and manipulating the bi-directional communication between human sensory NFs with defined skin  cell populations in native human skin under physiological and pathological conditions.

Towards the development of a completely humanized model for the investigation of sensory nervous system in scalp skin ex vivo