Meeting Abstract
Orbital glands are found in many tetrapod vertebrates, and are usually separate structures: with individual glands lying in the eyelids and both canthi of the orbit. In cetaceans, however, the orbital glandular units are less distinct and have been described by numerous authors as a single, periorbital mass. It is presumed that these orbital glands produce the ocular fluid, whose primary role in cetaceans may be defense. Though histochemical data is available for only a few species, it is restricted solely to the periorbital glands, and does not mention any eyelid glands. In this study, we examined the orbital region of both a bowhead whale (Balaena mysticetus : Mysticeti) and a beluga whale (Delphinapterus leucas: Odontoceti). Both species examined possess a periorbital glandular mass, which have slightly larger masses in the rostral (Harderian) and cranial (lacrimal) poles. The bowhead whale also possessed a smaller glandular mass in the middle of the upper eyelid (glands of Wolfring) and a series of diffuse glandular units in the rim of the eyelid (Meibomian glands). Histochemically all bowhead whale orbital glands possess the same four distinct secretions. However these glands exhibit differences in glandular activity level, density of glandular material and presence of Pacinian corpuscles. Beluga orbital glands additionally possessed lipid granules, but lack the Pacinian corpuscles. Despite this histochemical homogeneity, there is chemical heterogeneity between the glands, specifically in the form of regionalized bactericidal agents. This is the first time such chemical heterogeneity has been shown in orbital glands of any tetrapod.
