Estudo morfológico da superfície ocular Bradypus variegatus (Schinz, 1825)

Abstract

Sloths (Bradypus variegatus) are directly or indirectly affected by human action, for example: car accidents with collisions, deforestation, hunting for consumption of exotic meat and pesticide poisoning occur frequently. Several studies on ecology, reproduction and nutritional aspects of wild animals have already been published, however, most anatomical characteristics are described in a generalist way or remain unknown. Veterinary ophthalmology is an expanding field and research in comparative ophthalmology produces a range of knowledge that allows the understanding of different evolutionary processes among species. The ocular surface is of vital importance for vision, being in direct contact with the environment, it is vulnerable to dryness, injuries and pathogens and therefore has a complex mechanism for maintaining its homeostasis. Sloths (Bradypus variegatus) may be affected by several ocular diseases, like humans, due to different etiologies, thus the importance of characterizing the normal tissues of the ocular surface in these animals since studies on the sloth ocular anatomy and physiology are still scarce. The present study aimed to characterize the normal anatomy of the ocular surface of this species. For the development of the histomorphometric study of the ocular surface, eyes were collected from animals that died for reasons unrelated to this research. The eyes were processed for inclusion in paraffin blocks and stained by the Hematoxylin-Eosin (HE), Schiff's Periodic Acid (PAS), alcian blue, tricromic masson, and then submitted to evaluation by light microscopy. Anatomical, histological, and histochemical characteristics of the lacrimal gland, the superficial and profound glands of the third eyelid, palpebral glands, cornea and limbus were described. Histologic examination revealed tarsus, tarsal glands and cilia were not present.Instead, well-developed sebaceous glands associated with accessory hairs along the palpebrae were present. The inner face of the eyelids is lined with the palpebral conjunctiva, which consists of a thin layer of loose connective tissue with a stratified epithelium. Mixed-secreting accessory lacrimal glands were noted in the deep stroma of both palpebrae. The lacrimal gland was situated in the dorsolateral angle of the orbit between the dorsal rectus and the lateral rectus muscles. Histologically, the gland is a tubuloalveolar type. The third eyelid protrudes from the medial canthus over the anterior surface of the globe, its cartilage resembles an anchor in shape. The stroma of the nictitans consists of loose to dense connective tissue that supports glandular and lymphoid tissue. Histological and histochemical examination revealed a superficial and a deep (harderian) glandular tissue. The deep gland is large and situated in the anteroventral region of the orbit. It is a compound branched, tubulo-alveolar, with a single layer or columnar cells. The cornea is elliptical in shape, with a horizontal diameter greater than the vertical. The anterior epithelium consists of a single cell layer of basal cells, which are columnar in shape and lie on a thin basement membrane; two three layers of polyhedral cells; and three layers of nonkeratinized squamous cells. The Bowman membrane is absent, and vessels are present in the stroma reaching the central cornea.