Anatomical, scanning electron microscopy, histological and histochemical studies of the orbital glands of the Egyptian agama Trapelus mutabilis

Abstract

Abstract Background The orbital gland of the group agama (Agamidae) is not well-studied. Here is a first description of anatomical and histological studies on the orbital gland of the Egyptian agama Trapelus mutabilis. Results The Harderian gland (HG) of the Egyptian agama is well-developed, with a tongue shape, appearing whitish in color, and reaching approximately 1.47 mm in length. The gland can be divided into head and tail regions and lies at the medial corner of the orbit ventral to the ventralis oblique muscle and dorsal to the medialis rectus muscle. Secretory product of the HG is released into the conjunctival fornix, which is deep into the nictitating membrane. The HG is a compound tubuloacinar type, consisting of columnar and pyramidal glandular cells with rounded nuclei and lumina that are variable in size and containing pigments in the connective tissue capsule that envelops the tail portion of this gland. The glandular cells of the HG react positively to periodic acid-Schiff (PAS) and alcian blue staining at pH 2.5. In contrast, lacrimal gland (LG) of Egyptian agama is a reduced mucous gland that appears whitish in color, opens on the surface by numerous orifices at the base of nictitating membrane, and reaches approximately 0.1 mm in length. The lacrimal gland is composed of a simple tubulo-acini of columnar cells with narrow lumen and basal oval-shaped nuclei and exhibits a weak response to PAS but a strong response to alcian blue at pH 2.5. Conclusions Morphology of LG and HG isn't impacted by features of the orbit, such as the existence or absence of nictitating membrane and/or fixed or movable eyelid. The small size of the LG of the Egyptian agama does not affect the performance of its main function in lubricating the surface of the cornea. The pigmentation envelope of the HG of the Egyptian agama allows heat to be absorbed in order to increase its secretion activity, thereby increasing the protection of the eyes against mechanical damage. Finally, both orbital glands may play a secondary function in digestion via indirect connection with the oral cavity.