Histological investigation of the upper and lower eyelids ofElanus caeruleus
The upper and lower eyelids of
black-winged kite Elanus caeruleus have an outer skin and an
inner conjunctival surface. Histologically, the conjunctival surface of
the upper eyelid is defined into two regions, the marginal and orbital
regions (fig.4), While in the lower eyelid, it is composed of the
marginal, tarsal, and orbital regions (Fig.5).
The skin of both eyelids is
composed of keratinized stratified squamous epithelium. The skin of the
upper lid consists of six to seven nucleated cell layers (Fig.4a), while
it consists of three to four nucleated- cell layers in the lower eyelid
(Fig.4b, 5a). These epithelial layers increase gradually towards the
palpebral margin to become ten nucleated cell layers in both eyelids
(Fig.5a). The skin of both eyelids is covered by a thin layer of
keratin, which is lacking at the MCJ and the conjunctival surface
(Fig.4b&c, 5b&c).
Semi-thin sections of the skin of
both eyelids of Elanus caeruleus revealed the details of
different layers. The stratum basal, which is composed of cuboidal cells
with rounded nuclei, contains melanocytes with spherical and oval nuclei
and numerous melanin granules in their cytoplasm; and the stratum
spinosum, where the keratinocytes have rounded nuclei and adhere to each
other by desmosomes. Within these layers, it was noticed that the
Langerhans cells have light cytoplasm, lack melanin granules, and their
nuclei are kidney-shaped or lobulated. The stratum granulosum forms two
layers of polyhedral keratinocyte cells, which are flatter and more
irregular in shape than those in the stratum spinosum. Finally, the
stratum corneum which lacks nuclei is formed of wavy keratin
(Fig.6a&c).
The conjunctival surface of the
upper eyelid in the orbital region consists
of
stratified cuboidal epithelium
bearing a short cytoplasmic
extension at the apical margin of cells (Figs.4d).
This epithelium toward the fornix
conjunctiva palpebral converted to stratified columnar type and appeared
as a cells-cluster with small pockets/space in between them. These
clusters lack the cytoplasmic extensions and have irregular surfaces
(Figs.4e). In addition, the epithelium of conjunctiva containing
numerous goblet cells which exhibit purple color with PAS reaction, the
goblet cells increase in density toward
the fornix conjunctiva palpebral.
(Fig.4e). Meanwhile, in the lower eyelid, the conjunctival at the MCJ
and the marginal region, is composed
of
nine layers of non-keratinized
stratified squamous epithelium with scattered single mucous cells
(Fig.5c&d). Whereas, only three to five layers of stratified cuboidal
epithelium were seen in the tarsal and orbital region, respectively
(Fig.5e). The goblet cells scattered in the lower eyelid of the orbital
region and increased toward the fornix (Fig.5e).
Semi-thin sections of the conjunctival surface of both eyelids ofElanus caeruleus revealed
the presence of M like cells in
the superficial layer. They are characterized by euchromatic nuclei,
pale cytoplasm filled with vesicles of various sizes, and surrounded by
intraepithelial lymphocytes (IEL) Moreover, neighboring
enterocytes and goblet cells with
pale cytoplasm were observed (Fig.6 b & d).
The stroma of both eyelids ofElanus caeruleus contained
a scattered network of collagen
fibers and contained many feather
follicles in the lower lid which were not seen in the upper eyelid
(Figs. 4b and 5c& e). Moreover, a sparse number of histiocytes, blood
vessels and few heavy endothelial
venues (HEV) were observed (Fig.4c&5e). The stroma of the marginal
region of both eyelids is highly vascularized (Fig.4b& 5b). Numerous
diffuse lymphocytes were scattered in the connective tissue of the lower
eyelid beneath the orbital region of the conjunctival epithelium, which
became more extensive near the tarsal plate (Fig. 5e). Numerous blood
vessels and heavy endothelial
venule (HEV) were observed around the inferior tarsal plate (Fig.5b, d).