Absence of type IV collagen in the centre of the corneal epithelial basement membrane.

Type IV collagen is the basic structural component of all basement membranes (BM), and forms the backbone to which other BM components attach. We have found that in the centre of the adult human cornea the epithelium does not display a type IV collagen immunoreactive BM. In fetal corneas (14 and 22 weeks of gestation), however, the epithelial BM shows uninterrupted type IV collagen immunoreactivity. In similar experiments laminin immunoreactivity was observed in the entire corneal epithelial BM, in fetal as well as adult corneas. Ultrastructurally, a normal BM with a lamina lucida and a lamina densa can be observed in the conjunctiva. The adult corneal centre, however, shows epithelium without a lamina densa. Focal deposits of electron-dense material are observed in conjunction with hemidesmosomes and anchoring fibres. These observations indicate that in the development of the eye, the cornea is initially covered with an epithelium which attaches to a normal BM. Later on, however, the BM type IV collagen disappears from the corneal centre. Assuming that highly differentiated epithelium cannot produce a BM, this could be due to the high level of differentiation of central corneal epithelium, which is generated in the limbal proliferation zone. Alternatively, the acellular Bowman's layer might lack triggers to induce type IV collagen production by the epithelial cells.