Anatomical, scanning electron microscopic, and histological studies of the ocular lids and muscles of the Egyptian agama Trapelus mutabilis

Abstract

The present study aimed to describe the anatomical, scanning electron microscopic, and histological features of the ocular eyelids and muscles of the Egyptian agama, Trapelus mutabilis. The eye of the Egyptian agama is characterized by mobile upper and lower eyelids and a small, immobile, nictitating membrane. The upper eyelid appears shorter than the lower one, and the head skin above the upper eyelid extends laterally to form a superior extension. The scales of the eyelids are arranged in the same order and are equipped with micro-ornamentation and sense organs at their tips. In contrast, the histological structure of the upper eyelid is similar to that of the lower eyelid. The external surface of both eyelids is composed of keratinized stratified squamous epithelium of 2-4 cell layers, while cuboidal stratified epithelium lines their internal surface. Both eyelids contain iridophores and melanophores. The immobile nictitating membrane (third eyelid) appears as a small protrusion with a concave surface that connects with the lacrimal gland posteriorly at the anterior canthus of the eye. Its external surface is covered with stratified squamous epithelium, while its internal surface is lined by one or two layers with rounded nuclei attached to the conjunctival epithelium. Video recordings of eye movement in the laboratory revealed that the movement of the eyelid is synchronized with the eyeball movements, allowing all eye movements to be observed. The placement of the eye inside the skull, its protection by two eyelids equipped with hard scales and sensory organs, and the presence of a superior extension of broad scales acting as an umbrella for the eyes, in addition to the presence of two types of pigment cells, enhance eye protection against ultraviolet rays. All these characteristics are well-suited for the remote desert environment in which the agama lives.