Discussion
The eyelids serve as integumentary duplications or folds of skin around
the eyes, acting as a physical barrier in terrestrial vertebrates to
protect against mechanical damage and desiccation while regulating light
passage through the pupil (Johnson, 1927; Walls, 1942). Studies by Yasui
et al. (2006) have confirmed that eyelids protect the eyeball and
maintain a moist surface\RL.
The anatomical structure of the upper eyelid of T. mutabilis resembles that of the lower eyelid. However, the lower eyelid in this
agama species is more mobile and can move downward through the
contraction of the depressor palpebrae inferioris muscle, attaching to
its base. Additionally, the lower eyelid appears larger than the upper
eyelid, a common trait observed in reptiles and birds compared to
mammals, where the upper eyelid is typically more mobile and larger than
the lower one (Wyneken, 2012; Klećkowska-Nawrot et al., 2019)\RL.
Eyelids are movable structures supported by a connective tissue plate
called the tarsal plate or tarsus (tarsus palpebralis). In most taxa,
including lizards, birds, and mammals, the tarsal plate is described as
a dense fibrous connective tissue, possibly containing cartilage, found
within one or both of the upper and lower eyelids (Rieppel, 2000).
Wyneken (2012) noted the presence of a tarsus in the eyelid of most
lizards and crocodilians, with a bony structure in the upper eyelid of
crocodilians and a cartilaginous structure in the lower eyelid of most
lizards. However, turtles and tortoises lack these structures. In birds,
the tarsal plate is located only in the lower eyelid and consists of
dense connective tissue with elastic fibers (Klećkowska-Nawrot et al.,
2016). In mammals, the tarsal plate is present in the upper and lower
eyelids, comprising dense fibrous connective tissue (Klećkowska-Nawrot
et al., 2019).
The eyelids of reptiles are covered by epidermal scales, displaying
various patterns, shapes, thicknesses, and degrees of overlap among
different species (Guerra-Fuentes et al., 2014; Wegener et al., 2014;
Toni et al., 2007; Calsbeek et al., 2006). In T. mutabilis , the
external surface of both upper and lower eyelids features highly
polygonal scales of varying sizes and forms. The scales on the superior
extension are notably large and plate-like acting as an umbrella for the
eyes, serving as heat exchangers and aiding in water conservation in
arid environments\RL.
Integument sense organs were first described in reptiles by Leydig
(1868) in the lizard genera Lacerta and Angus and the
snake genus Coronella . He identified small depressions on the
surface of their scales, referred to as ”organs of the
6th sense,” and compared them to the taste buds of
fishes and amphibians. Scortecci (1941) examined these receptors in
detail in agamids, iguanids, and other lizards, distinguishing two
principal types based on their structure: the first type has the
Oberhäutchen with a bristle (or seta, or hair), while the second lacks
bristles. The first type is typical of the integument of pygopodids,
agamids, and gekkonids (Landmann, 1975)\RL.
The sense organs with the Oberhäutchen and bristle appear on the tips of
some scales of the Egyptian agama’s eyelids. These organs may serve as
points of contact with the external environment. Maclean (1980) and
Sherbrooke and Nagle (1996) suggested that these sense organs might work
in conjunction with visual and other sensory systems to detect, locate,
and apprehend prey, as noted by Barrett et al. (1970) in Crotaline and
Boid snakes\RL.
The present study reveals that the structure of the two eyelids inT. mutabilis is very similar. The outer skin surface of the
eyelid consists of two to four nucleated cell layers of keratinized
stratified squamous epithelium, while the internal surface comprises two
to five nucleated cell layers of stratified squamous epithelium.
Additionally, the distribution of pigment cells shows significant
variability within the dermal layer, with large black melanophores
(light-absorbing pigment) and dendrites that invaginate into the stratum
basal of the upper and lower eyelids. These melanophores increase in
number within the superior extension of the upper eyelid.
Brownish-yellow iridophores (reflecting pigments) are scattered in large
numbers within the dermis of the upper eyelid, its extension, and the
lower eyelid. Saenko et al. (2013) and Teyssier et al. (2015) pointed
out that these types of pigmented cells (iridophores) in reptiles
participate in skin coloration and thermoregulation.
The third eyelid, known as the nictitans eyelid or nictitating membrane,
is a prominent semilunar fold of the conjunctiva in Elasmobranchii fish,
amphibians, reptiles, birds, and mammals (except humans)
(Klećkowska-Nawrot et al., 2019). The nictitating membrane is sometimes
lost in various taxa due to ecological differences, such as in whales,
echidnas, opossums, marsupial moles, largely burrowing squamates like
pygopodids, amphisbaenians, and dibamids, as well as chameleons and
snakes (Caprette et al., 2004)\RL.
According to Schramm et al. (1994), the size of the nictitating membrane
is inversely proportional to an animal’s ability to remove foreign
bodies from its eyes. This membrane, known as the haw, is well developed
in ungulate mammals such as horses and cows but relatively small in
carnivores like canines and felines. Cats can efficiently remove foreign
bodies from their eyes using their paws. In aquatic animals, such as
fishes and some amphibians, the anterior portion of the eye is cleaned
by surrounding water, making the nictitating membrane unnecessary. The
most prominent nictitating membranes are found in reptiles and birds
exposed to wind, storms, dust, and sand (Schramm et al., 1994). A
well-developed nictitating membrane is essential for keeping the corneal
portion of the eye clean in species that cannot do so otherwise. In
lizards, a well-developed nictitating membrane, associated with
nictitating cartilages, completely covers the cornea, as seen inColobosaura modesta and Tretioscincus oriximinensis (Guerra-Fuentes et al., 2014). Conversely, this membrane is reduced to a
small projection at the base of the cornea and eyelid inAlopoglossus angulatus , Cercosaura ocellata , andMicrablepharus maximiliani (Guerra-Fuentes et al., 2014), similar
to T. mutabilis . Rehorek et al. (2000) reported that reptilian
species with a spectacle have lost the nictitating membrane, as inNothobachia ablephara and Calyptommatus
sinebrachiatus \RL.
The nictitating membrane in squamates is a movable fold that slides over
the cornea (Walls, 1942). The exocrine secretion of the Harderian gland,
attached to the medial region of the orbit (Guerra-Fuentes et al.,
2014), lubricates this sliding movement (Payne, 1994). Oria et al.
(2015) stated that the spreading of the tear film (orbital secretion) is
accomplished by nictitating membrane movement rather than eyelid
movement.
Crocodiles possess a well-developed nictitating membrane that moves
obliquely backwards and slightly upwards. These animals often move the
nictitating membrane across the eye without closing their eyelids. When
closing the eye, the lower lid closes after the nictitating membrane has
moved across the eye, unlike most other reptiles, where the membrane and
eyelids move simultaneously (Johnson, 1927). In anura, the nictitating
membrane moves from below upwards rather than horizontally or obliquely
from the inner angle across the eye, as seen in all birds and many
mammals (Johnson, 1927). Stibbe (1928) noted that birds require the
fastest method for this movement, while mammals prioritize thoroughness
over speed\RL.
Hiller (1995) reported that the conjunctival outer morphology of agamid
species is mainly characterized by epithelial folds running parallel to
the eyelid margins. These folds are present in all agamid species and
contain scattered goblet cells that release secretory products onto the
conjunctival surface. The goblet cell content in T. mutabilis is
mucus, similar to the iguanid conjunctiva that mentioned by Hiller
(1995)\RL.
Goodrich (1988) stated that extraocular muscles, responsible for eyeball
movement, are consistent in number, arrangement, and innervation across
vertebrates. El Hassni et al. (2000) noted that chameleons and higher
vertebrates use the same musculature, but the performance of each type
of movement varies according to lifestyle. Reptiles have six extrinsic
eye muscles (medial rectus, lateral rectus, superior rectus, inferior
rectus, inferior oblique, and superior oblique) responsible for rotating
the eyeball within the orbit (Kardong, 2012). These muscles are
characterized by fine movement and coordinated control of the eyes with
the vestibular system (Kardong, 2012; Schwab, 2012). Eye movement is
essential to prevent photoreceptor fatigue (Schwab, 2012). Turtles,
crocodilians, and most lizards (except Heloderma) have mobile eyes,
while snake eyes are immobile (Underwood, 1970).
The dorsalis rectus muscle and dorsalis oblique muscle in T. mutabilis are divided into two parts with different origin sites
on the orbitosphenoid and parabasisphenoid bones but share the same
insertion in the sclera and upper eyelid. The eye of T. mutabilis lacks levator muscles (levator palpebrae superioris), so the dorsalis
rectus muscle and dorsalis oblique muscle are responsible for elevating
the upper eyelid.
In reptiles, the protective reflex caused by the retraction of the
eyeball in response to corneal stimulation (Oelrich, 1956; Tansley,
1965) seems to be accomplished by simultaneous contractions of two
retractor muscles, the retractor bulbi and bursalis, as observed in the
lizard Varanus (Barbas-Henry and Lohman, 1988; El Hassni et al.,
2000). Retractor oculi and protractor oculi, known as the levator bulbi
or retractor bulbi muscle in reptiles, are inserted on the sclera just
adjacent to the optic nerve and are responsible for inward and outward
eye movement within the socket (Wyneken, 2012). The present results
showed that the retractor bulbi and bursalis muscles of T.
mutabilis are inserted on the scleral cartilage, with their origin
sites lying ventral to the optic nerve. These muscles pull the eyeball
inward and outward within the socket.
Retractor bulbi muscles are synapomorphic for tetrapods and are
subsequently lost in snakes and birds. Losing these muscles in snakes
results in a condition convergent with primitive aquatic animals. This
loss implies visual reduction, similar to the loss observed in
large-eyed forms with great visual acuity, like birds (Caprette et al.,
2004).
Bour et al. (2000) stated that in the absence of retractor bulbi
muscles, retraction is produced by the simultaneous contraction of
multiple extraocular muscles. In some reptiles, the retractor bulbi is
assisted by one of two additional muscles: the quadratus (in lizards) or
the pyramidialis (in crocodiles and turtles). In birds, the globe of the
eye fits so tightly into the eye socket that little room is left for
retraction. Therefore, the retractor bulbi is not present and is
replaced by both quadratus and pyramidalis muscles. These muscles slide
the nictitating membrane over the cornea without retraction of the eye
(Butler and Hodos, 2005).
In terrestrial mammals, well-developed retractor muscles are usually
associated with the presence of a nictitating membrane (Duke-Elder,
1958; Spencer and Porter, 2006). Retractors are absent in some mammals,
such as the aardvark, flying fox, and primates. In mammals, the
retractor bulbi can be divided into two (mouse), three (dog), or four
(cat) muscle bellies (Matheus et al., 1995; Meshida et al., 2020).
In conclusion, all these characteristics that mentioned above are
well-suited for the remote desert environment in which the agama lives.