Morphological and histochemical characterization of eyelids and orbital glands in the crocodile monitor (Varanus salvadorii)

Abstract

Abstract The crocodile monitor is an arboreal predator, and its ocular adnexa remain poorly characterized. This study provides the first detailed histochemical characterization of the ocular adnexa in this species, including the upper and lower eyelids, the nictitating membrane, the Harderian gland, and the lacrimal gland, based on two adult individuals. Because the available material was limited, the findings should be interpreted as descriptive baseline observations. Histological slides were analyzed using microanatomical assessment, histochemistry, and Picrosirius Red staining under polarized light. Both eyelids were covered by scales and contained a pigmented corneous β-protein–rich epidermis. A collagen-rich dermis with a thick stratum compactum and an osteoderm located between the dermis and hypodermis were identified. The posterior palpebral epithelium was non-keratinized and contained abundant goblet cells producing mainly acidic mucins. The lower eyelid also contained a dense regular tarsal plate and lymphoid tissue. The nictitating membrane was folded and pigmented, contained hyaline cartilage, and showed conjunctiva-associated lymphoid tissue. Birefringence patterns suggested Type I–like collagen in the perichondrium and Type III–like collagen in the cartilage matrix. The Harderian gland, a large acinar gland, opened onto the bulbar surface of the nictitating membrane and secreted mainly acidic mucins with a moderate neutral mucin component. The small lacrimal gland produced predominantly acidic mucins without detectable neutral mucins. These findings provide a baseline for the protective and secretory ocular structures of the crocodile monitor and represent the first detailed histochemical characterization of the studied orbital glands. In addition, this study contributes to comparative anatomy, veterinary medicine, and exotic animal conservation.