Cutaneous receptors in the bill of the Platypus (Ornithorhynchus anatinus)

Abstract

The pattern of cutaneous innervation of the bill of the platypus has been examined using standard light and electron microscope techniques. These revealed that the external surfaces of the bill were covered with thick, keratinised epidermis containing numerous receptor complexes or 'rod organs'. Each rod organ consisted of epidermal cells associated with three types of peripheral nerve terminals: (i) free nerve endings running axially in the epidermal rod, (ii) Merkel endings lying at the base of the rod, (iii) a cluster of 4-6 simple, encapsulated terminals with inner cores lying within the vascular subepidermal connective tissue beneath the rod. Rod organs were distributed over the entire surface of the bill and the frontal and gular shields but were found in greater numbers at the border of the upper bill. The unit design and array of receptor complexes in the platypus bill is similar to that found in the naked rhinaria of other mammals where such innervation patterns are associated with tactile function.