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.