Crop Morpho-Histological Peculiarities in Domesticated Pigeons (Columba livia domestica), Cattle Egret (Bubulcus ibis) and Domesticated Ducks (Anas platyrhynchos domestica)

The crop architecture is varied among different species according to their feeding habits. Therefore, twenty-one mature healthy pigeons (granivorous), cattle egrets (carnivorous), and ducks (omnivorous); n= 7 per species were utilized for the histological, immunohistochemical, and ultrastructural analyses. Histologically, the mucosal folds were covered by non-keratinized stratified squamous epithelium in the three investigated species. The mucosal glands are only peculiar to the crops of the cattle egrets as well as the ducks, while the pigeons' crops were devoid of any secretory units. Morphometrically, the optical densities of the AB and PAS reactions in the secretory glands, the area % of collagen fibers, the thickness of the tunica musculosa were measured and declared significant differences among the three studied avian species. Immunohistochemically, the ki-67 immunopositive reactivity was obviously detected in the nuclei of basal cell layers of the crop epithelium in pigeons only. Ultrastructurally, the covering epithelium of all studied species had been shown to be the basal layer of cuboidal to tall columnar cells with desmosomal junctions at the level of their cellular interdigitations, intermediate layers of large irregular polygonal cells with obviously increased cytokeratin filaments especially in the ducks and cellular interdigitations in between. The superficial layer of flat-shaped cells in the three investigated species manifested by desmosomal junctions in between but at the level of the surface squamous cells of the superficial layer, a fine lateral process is only inspected in the pigeons. The mucus secretory endpieces' of tall columnar cells were characterized apically by secretory granules of variable electron densities in the cattle egrets and electron-lucent in ducks.