Histological investigation of the nictitating membrane ofElanus caeruleus
The nictitating membrane of Elanus caeruleus is divided into two regions; the head and body. Further, the head of the membrane can be distinguished into two parts: the plica marginalis (free margin) externally and the leading edge internally (Fig.10 a& b), and both are covered by stratified epithelium (Fig. 10 b). This epithelium changes into stratified cuboidal epithelium to form the leading edge (Fig. 10 b). This stratified cuboidal epithelium has short cytoplasmic extensions (Fig. 10 b), which decrease gradually and then disappear toward the fornix conjunctiva bulbi (Fig. 10 c). Numerous pigmented granules are interspersed in the connective tissue of the plica (Fig. 10 b). The body of the nictitating membrane possesses two surfaces: the palpebral and bulbar surfaces. Both surfaces are covered by stratified cuboidal epithelium with an irregular apical membrane (Fig. 10 c). while This epithelium changes into the stratified columnar type containing goblet cells toward the fornix conjunctiva bulbi (Fig.10 d).
The high magnification of the epithelium of the bulbar surface of the nictitating membrane of Elanus caeruleus revealed four nucleated cell layers; the stratum basal has cells with irregular basal lamina forming basal finger-like fine protrusions (Fig. 11 b). The apical membrane of the superficial cells located between the folds has a short cytoplasmic extension. Moreover, there are numerous scattered goblet cells, which increase in the fornix conjunctiva bulbi (Fig. 11 b). The palpebral surface is composed of six nucleated cell layers. The superficial cells have pale cytoplasm and irregular apical membrane (Fig. 11 a)
Ultrastructure investigation of the nictitating membrane ofElanus caeruleus
The ultrastructure of the nictitating membrane of Elanus caeruleus revealed the flattened cells of the palpebral surface appear loosely connected with each other by complex interdigitations of the cell membranes. Furthermore, the cytoplasm contains many cytoplasmic vacuoles of varying size (Fig. 12 a).
The Immature M cells observed in the bulbar surface are lighter compared to the neighboring enterocytes and abundant various sizes of vacuoles in the apical cytoplasm. M cells exhibited shorter and irregular microvilli on their apical membrane compared to the neighboring enterocytes. In addition, goblet cell with free edge (Fig. 12 b & c) and enterocyte-like columnar cells were observed (Fig. 12 b & c)