A comparative study of elasmobranch corneal and scleral collagens

Abstract

Corneas, dissected from young and adult spiny dogfish sharks ( Squalus acanthias), were prepared for transmission electron microscopy and immunofluorescence. In the latter case, tissues were fixed in formaldehyde solutions, sectioned with a cryostat, incubated with antibodies specific for collagen types I and II, and examined by indirect immunofluorescence. Collagen α- and β- chains were separated by sodium dodecylsulfate-slab gel electrophoresis and characterized by two-dimensional mapping of 125I-labeled peptides generated by tryptic and chymotryptic digestion. The corneal stroma, the sutural fibers which span the stroma, and the surrounding limbus were positive for type I collagen, as judged by immunofluorescence. The corneal stroma was negative for type II collagen. Scleral cartilage matrix was intensely positive for type II collagen, but was negative for type I. In addition, the perichondrium of the scleral cartilage was positive for type I collagen. In confirmation of these results, slab gel electrophoresis revealed α1, and α2-like bands from shark corneal stroma, but only an α1-like band from shark cartilage collagen. Two-dimensional peptide mapping revealed some degree of resemblance between the α1 band of shark corneal stroma and the α1 band of chick type I collagen. Likewise, the α1 band of shark cartilage collagen somewhat resembled the α1 band of chick type II collagen. The α2-like band of shark corneal stroma did not closely resemble the α2 band of chick type I collagen. The most prominent β band of shark corneal stroma appeared to be a dimer composed of one α1 chain and one α2 chain. The collagen of shark corneal stroma was very susceptible to degradation by pepsin, whereas that from shark cartilage was much less susceptible.