A histological study of the facial hair follicles in the pygmy sperm whale (Kogia breviceps).

In the pygmy sperm whale (Kogia breviceps, Blainville 1838), vibrissae are present in neonates, but within a few months the hairs are lost, and the structures remain as empty vibrissal crypts (VCs). In this work, we have studied histologically the facial vibrissal follicles of two juveniles and one adult specimens stranded dead. A few VCs with no visible hairs were found grouped in a row rostral to each eye. The follicular lumen, covered by a simple squamous epithelium, showed invaginations in the most superficial part. Beneath the epithelium, the follicle walls were made of loose connective tissue and were encircled by a thick capsule of dense connective tissue. In juveniles, a dermal papilla was found basally and, from it, a non-keratinized pseudo hair grew upwards but did not reach the skin surface. The VCs were richly innervated and irrigated. Many lamellated corpuscles were identified in the subluminal connective tissue of the crypt walls. A large venous cavernous plexus was located beneath and around the hair papilla. The main differences observed in the adult specimen were the degeneration and calcification of both the dermal papilla and the pseudo hair, and the absence of the venous cavernous plexus, albeit maintaining a rich vascularization and innervation. Our study revealed that VCs of the pygmy sperm whale possess features of fully functional sensory structures, with a microanatomy different from those described in other species. In addition, they undergo a postnatal morphological transformation, which implies functional differences between the VCs of neonates and adults.