Eyelids and orbital glands of the Komodo dragon (Varanus komodoensis): Gross anatomy, histology, and histochemistry.

Abstract

Morphological data on the eyelids, the third eyelid (nictitating membrane), and the orbital glands remain limited for many Anguimorpha, including the Komodo dragon (Varanus komodoensis). The ocular adnexa contribute to eye protection, lubrication, and immune defense, and they may also reflect ecological and phylogenetic variation within Squamata. The objective of this study was to describe the gross anatomical, histological, histochemical, and polarized-light microscopy characteristics of the upper and lower eyelids, third eyelid, Harderian gland, and lacrimal gland in the Komodo dragon. A panel of histological and histochemical stains was used to evaluate connective tissues (collagen, elastic fibers, and reticulin), muscles, cartilage, pigmentation, and mucins/glycoconjugates. The eyelids showed osteoderms and a multilayered keratin arrangement (β-keratin layer/fibrous intermediate layer/β-keratin layer/α-keratin layer). Intense palpebral scale pigmentation and folded conjunctivae with mucin-producing goblet cells may contribute to periocular protection, although this functional interpretation requires confirmation in free-ranging animals. The third eyelid contained hyaline cartilage with Type I collagen-associated birefringence in the perichondrium, Type III collagen-associated birefringence in the cartilage matrix, and conjunctiva-associated lymphoid tissue. The Harderian gland was the dominant orbital gland, but its histochemical profile showed only weak and uneven evidence of acidic mucosubstances, mainly in luminal secretory material; therefore, its secretory phenotype should be interpreted cautiously, whereas the lacrimal gland showed a clearer mucous profile dominated by acidic mucosubstances. By documenting a large non-spectacled anguimorph, these findings provide comparative data relevant to eyelid support, nictitating-membrane organization, lacrimal drainage, and orbital gland variation across Squamata.