Morphological study of the upper, lower and third eyelids in the African black ostrich (Struthio camelus camelus L., 1758) (Aves: Struthioniformes) during the embryonic and postnatal period

Abstract The upper, lower and third eyelids are accessory organs of the eye. The aim of this study was to describe the development of the upper, lower and third eyelids in ostriches in the embryonic and postnatal period and to characterise the conjunctiva-associated lymphoid tissue (CALT) in the eyelids. The study was performed on 59 African black ostriches from the 28th day of incubation up to 3 years of age. Hematoxylin and eosin, Azan trichrome, van Gieson trichrome and Mallory’s trichrome stainings were used to demonstrate the structure of the eyelids. The connective tissue structure transformed from loose connective tissue into dense connective tissue during the development of the eyelids. In the third eyelid, the network of collagen fibres was rebuilt and the density of the collagen fibres decreased with age. As the animals grew, there were clearly visible changes in the structure of the upper and lower eyelids, especially in the stratified squamous epithelium of the skin surface, the conjunctival epithelium and the tarsal plate. The lymphatic follicles were observed only in the lower eyelid of adult ostriches. A diffuse CALT system with scattered lymphatic cells was observed within the connective tissue of the third eyelid, mostly under the conjunctival epithelium in the group of adult birds.