The mechanoreceptors in hatchling and adult Elasmobranch skin.

The skin is the most extensive organ in vertebrates, composed of two layers: the epidermis and the dermis. Sensory axons originating from the dorsal root ganglia innervate the skin mechanoreceptors in the dermis. Elasmobranchs, which appeared 380 million years ago, are characterized by rough skin composed of dermal denticles.