Histogenesis of the cornea in sheep fetuses

In this study, 36 samples of sheep fetuses at different ages were used. After preparation and determination of age, the samples were immediately fixed and subjected to gross anatomy and histology examination. In addition to routine H&E staining, two specific staining methods, Verhoeff and Masson's trichrome, were used in the histological study. The results showed that the corneal forming layer completely loses its contact with the lens and posterior layers and the eyelid ridges start to emerge at around 30 days of gestation. However, the cornea becomes fully differentiated in the second month of gestation and only undergoes growth stages in subsequent ages. From the beginning, the cornea in sheep fetuses is formed as a horizontal ellipse, with its inner curvature larger than its outer curvature. The growth process in terms of dimensions, including longitudinal diameter, transverse diameter, and corneal height, shows a regular ascending trend with increasing age. The upper and lower eyelids fuse in early second month and separate again in late third month. The third eyelid appears at approximately 120 days of gestation in the inner angle of the eye. The outer epithelial covering of the cornea changes from simple cuboidal to stratified squamous epithelium. With increasing age, the matrix is strengthened both in terms of cellular components (fibroblasts) and fibers (collagen). Additionally, an increase in elastic fibers is noticeable in the late fetal stages. The Descemet's membrane appears as an inner limiting membrane beneath the inner epithelial covering in early second month and also undergoes growth with increasing age. Descemet's membrane lacks elastic fibers. Bowman's layer or the outer limiting membrane was not observed in this animal. Masson's trichrome staining confirms the growth of collagen fibers, while Verhoeff staining confirms the growth of elastic fibers with increasing age.