Morphology and Histology of the Orbital Region and Eye of the Asiatic Black Bear (Ursus thibetanus)-Similarities and Differences within the Caniformia Suborder.

Abstract

Simple SummaryThe aim of this study was the anatomical description of the orbital region, eye tunics, upper and lower eyelids, superficial gland of the third eyelid with the third eyelid, and lacrimal gland of the Asiatic black bear. The eyeball morphometry, the orbital region analysis, macroscopic, morphometric and histological analysis of the eye tunics and anatomical analysis of the accessory organs of the eye with histochemical methods were used. The anatomical structures within the orbital region of Asiatic black bear were typical of only the family Ursidae and were similar to the family Canidae. The macroscopic and histological structure of the eye and eyelids was comparable to the structure of the eye and eyelids in other representatives of the Ursidae family, although features typical of only Canidae or terrestrial Mustelidae and Pinnipedia were also observed. The Bowman’s membrane of the cornea was absent, similar to all domestic dogs and some wild dogs. The pupil was similar to other bear species subfamilies, Tremarctinae and Ursinae, and also to domestic and wild dogs. The lens was biconvex round, similar to the Canidae. The retina was composed similarly to the diurnal terrestrial carnivores. In both eyelids were observed very well-developed tarsal glands, ciliary glands and sebaceous glands. The third eyelid was T-shaped and composed of the hyaline tissue and contained conjunctival-associated lymphoid tissue (CALT), which was also the case in Canidae. The obtained results indicate that the features of the anatomy of the eye and orbital region in the Asiatic black bear are also typical of the Ursidae family. In this study, we present first data concerning the morphological observations of the orbital region, eye tunics, upper and lower eyelids, superficial gland of the third eyelid with the third eyelid, and lacrimal gland in captive adult male Asiatic black bear. The following research methods were used in the work: the eyeball morphometry, the orbital region description, macroscopic description, morphometric and histological analysis of the eye tunics and selected the accessory organs of the eye (Fontana–Masson, hematoxylin & eosin (H&E), Methyl-green-pyronin Y (MGP Y), Movat pentachrome, and picro-Mallory trichrome) as well as histochemical examination (PAS, AB pH 1.0, AB pH 2.5, AB pH 2.5/PAS and HDI). The eyeball of the Asiatic black bear was a spherical shape, while the periorbita was funnel/conical-shaped and the eye socket was of the open type. The cornea was absent of the Bowman’s membrane similar to all domestic dogs and some wild dogs. There were palisades of Vogt in the corneal limbus epithelium similar to the Canidae. Degenerative choroidal tapetum lucidum similar to ranch mink (Mustelidae) has been found. The pupil was big and round in shape. The ciliary muscle, dilatator and sphincter muscle were well developed, similar to the pinnipeds. The lens was biconvex round, similar to the Canidae. The retina was composed similarly to the diurnal terrestrial carnivores. In both eyelids were observed very well-developed tarsal glands, ciliary glands and sebaceous glands. The orbital zone in the eyelids was characterized by lymphoid follicles, diffuse lymphocytes and specialized high endothelial venules. In the anterior palpebral margin of the upper eyelid, soft and short eyelashes were observed, while in the lower eyelids they were absent. The third eyelid was T-shaped and composed of the hyaline tissue, and it contained CALT, similar to that in Canidae. The superficial gland of the third eyelid was a multilobar alveolar branched complex with seromucous nature, while the lacrimal gland was also a multilobar acinar branched complex gland, but producing a mucous–serous secretion. The results of our research indicate that the features of the anatomy of the eye and orbital region in Asiatic black bear are also typical of the Ursidae family. Moreover, a detailed analysis of the morphological eye region may be useful in comparative studies and veterinary diagnostics in this bear species.