Merkel cells in vitro: Production of nerve growth factor and selective interactions with sensory neurons

Abstract

A method has been developed for obtaining mixed primary cultures of dissociated epidermis enriched in Merkel cells. Merkel cells obtained from embryonic rat buccal pads were grown in serum-free medium and identified in vitro using a variety of histological and immunohistochemical markers. Quinacrine, a fluorescent amine, which has been used to identify Merkel cells in situ, labeled a morphologically distinct population of cells in vitro. Cells labeled with quinacrine had a large, phase bright nucleus with prominent nucleoli, surrounded by a phase dark perinuclear ring. Antibodies directed against neuron-specific enolase, another marker for Merkel cells in situ, and antibodies against a well-characterized neuroendocrine vesicle antigen also labeled this population of quinacrine fluorescent cells. Electron microscopic examination of our cultures indicated that cells containing characteristic features of Merkel cells including cytoplasmic dense-cored granules were present. A small but significant increase in the number of Merkel cells was observed over time in culture. Merkel cells supported the survival and outgrowth of both trigeminal ganglion sensory neurons and sympathetic neurons from the superior cervical ganglion in serum-free medium in the absence of exogenous nerve growth factor (NGF). Immunoblots probed with antibodies directed against NGF demonstrated that NGF was present in the medium taken from these cultures. NGF-like immunoreactivity colocalized to cells containing quinacrine fluorescence in situ and in vitro. Addition of antibodies directed against NGF to cocultures of Merkel cells and neurons decreased survival of sympathetic neurons by 90% and decreased survival of sensory neurons by 60%. These results suggest that Merkel cells are capable of providing trophic support for their normal complement of sensory neurons by producing NGF. Selective recognition of these targets was studied in vitro by characterizing the interactions between Merkel cells and growth cones from sensory or sympathetic neurons using both time-lapse videomicroscopy and standard morphometry of fixed cocultures. The majority of trigeminal ganglion sensory neurons (approximately 60%) extended growth cones onto clusters of Merkel cells. Neurites which contacted clusters of Merkel cells were significantly more highly branched than those growing on collagen. In contrast, the majority of sympathetic neurons (greater than 90%) failed to grow onto Merkel cells. Growth cones of sympathetic neurons often “collapsed” and retracted when contact was made with a cluster of Merkel cells. Fixation of Merkel cells with paraformaldehyde prior to coculture did not affect this difference between sensory and sympathetic neurite extension onto the Merkel cells. However, prior fixation of Merkel cells eradicated the apparent Merkel cell-induced branching of sensory neurites. These results suggest that innervation of Merkel cells in vivo may be limited to sensory neurons, in part, by molecules on the surface of these target cells. Furthermore, the extensive arborization of sensory neurons which occurs in the vicinity of these targets in vivo may be due to the production of a target-derived agent.