Definition: "Type of neuron that is a primary mechanosensory cell, with peripheral neurites innervating the skin with free nerve endings. It first appears in trunk levels at NF stage 33/34, and at NF stage 35/36 has somas that are the largest in the spinal cord. It forms a nearly continuous double row of cells along the cord's medial dorsal surface and contributes ascending and descending axons to the dorsal tract. It appears well back in the tail by NF stage 39, begins to decline at NF stage 50 and is few in number, with pycnotic nuclei, at NF stage 55."