Morphology and distribution of touch cell terminals in the skin of the leech

1. The receptor terminals of individual mechanosensory neurones responding to light touch (T cells) have been visualized directly in the skin of the leech by injecting horseradish peroxidase (HRP) into their cell bodies in the central nervous system. The axons of injected cells could be followed from their origin in the neuropile of the ganglion to their terminals in the skin. 2. The axons of T cells run through ipsilateral nerve roots in the body wall to the base of the layer of epithelial cells in the skin. Here axons branch extensively and turn between the epithelial cells to end a few microns from the skin surface. These terminals are situated in intercellular spaces immediately below the junctional complex joining the outer ends of the epithelial cells. 3. The T cell terminals are free nerve endings with a beaded appearance; they contain large mitochondria and clusters of vesicles. 4. An individual T cell makes about 100 endings within a square millimetre of skin in the centre of its territory, and is estimated to make a total of several hundred endings. 5. The distribution of T cell endings observed directly agrees with physiological studies of receptive field organization and emphasizes the high degree of specificity of connexions of these neurones with their peripheral targets.