ABSTRACT. Merkel cells have been assumed to guide nerve fibers to the skin. However, there has been little in vitro evidence that supports this hypothesis, because there is no suitable established culture system of Merkel cells. Here we show that Merkel cells isolated from rat footpad skin were successfully cultured in a monolayer with keratinocytes. Keratinocytes did not affect any structural changes in Merkel cells. When nerve cells (NG108-15 or PC12) were added to the culture system, both nerve fibers and cytoplasmic processes of Merkel cells outgrew and cooperatively organized synapse-like structures at their contact points. Nerve cells promoted Merkel cell survival, compared with keratinocytes only. Merkel cell proliferation was not detected in all conditions, even with nerve growth factor, neurotrophin-3, interleukin-6 and tumor necrosis factor-α. The data suggest, firstly, that Merkel cells may guide nerve fibers to the skin by interacting with nerve cells; and, secondly, that nerve cells, but not keratinocytes, may produce some survival factors other than the cytokines above for Merkel cells, although Merkel cells may be a terminally differentiated cell type. Our method could open a way to study Merkel cell biology.

The data suggest, first, that direct contact between MCs and keratinocytes may be critical for retaining MCs in vitro; second, that nerve cell-affected maintenance of keratinocyte differentiation, but not NT-3 and NGF, may contribute to MC maintenance; and third, that MCs are not able to grow, at least in our system. Our method would be useful for studying MC biology.