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).