Morphology and morphogenesis of hereditary microphthalmia in Texel sheep.

The study presents a morphological description of inherited microphthalmia in Texel sheep and the morphogenesis of the lesion during embryonal development. The eyes of 28 newborn and young lambs with microphthalmia were examined morphologically. Blind rams with microphthalmia were crossed with ewes known to be carriers of the "microphthalmia factor", to obtain embryos of different age with and without microphthalmia for morphogenesis studies. The microphthalmic eyes showed an anteriorly located conus-shaped mass composed of connective tissue with cartilage, smooth muscle and fat tissue, particularly in the posterior part of the conus, and often islets of lacrimal glands and cystic structures lined with squamous epithelium in the anterior part. The iris, ciliary body and ciliary processes were incorporated in the periphery of the broad base of the conus. The conus was covered by a detached and folded but normally developed retina. The morphogenetic study showed an abnormal development of the lens vesicle. In embryos aged 24 days, the lens consisted of a solid unorganized proliferation of epithelial cells without formation of a lens capsule. An extensive overgrowth of primitive mesenchymal tissue followed, with infiltration and disintegration of the lens. In a later stage (45 days) a conus-shaped mass was seen in the anterior part of the eyes, which at 56 days consisted of mesenchymal tissue with focal differentiation to cartilage and epithelial structures in the anterior part of the conus. The primary event proved to be abnormal development of the lens vesicle, with disintegration of the lens and subsequent overgrowth of mesenchymal tissue. The mesenchymal tissue later differentiated in various directions, whereas the epithelial structures found in the microphthalmic eyes at days 56 and 132 and in the newborn lambs seemed to be remnants of the epithelial lens vesicle.