Anatomy, histology, and histochemistry of the harderian and lacrimal glands in the great spotted woodpecker (dendrocopos major)

Abstract

Abstract The Harderian and lacrimal glands are the main orbital glands of the avian lacrimal apparatus and contribute to the tear film, nictitating membrane function, and local protection of the ocular surface. Although the ocular adnexa have been well described in domestic birds, the morphology and function of these organs remain poorly characterized in free-living species. This study aimed to characterize the morphology and histochemistry of these glands in the Great Spotted Woodpecker. The material consisted of 18 individuals found dead in the natural environment in Poland. Gross anatomical, morphometric, histological, and histochemical analyses were performed. Both glands occupied typical orbital positions. The secretory units of the Harderian gland consisted of one predominant epithelial cell type, contrasting with the more heterogeneous epithelial organization reported in some other avian species. Both glands were compound tubuloacinar structures. The Harderian gland opened into the lower conjunctival sac, whereas the smaller lacrimal gland drained into the dorsal conjunctival sac. The Harderian gland had a more homogeneous secretory parenchyma and a well-developed excretory duct system. The overall morphology of both glands was comparable to that described in other birds. However, the histochemical secretory profile differed from that reported in several species: the Harderian gland produced mainly acidic mucosubstances, whereas the lacrimal gland showed a mixed secretory profile. The immune component of both glands had a diffuse and periductal distribution, which contrasted with the pattern described in some birds. These findings show that the Harderian and lacrimal glands are structurally and histochemically distinct and suggest complementary roles in ocular surface maintenance in this species. At the same time, the results indicate selected morphological and functional differences compared with both domestic and other free-living birds.