Type XIII collagen is widely expressed in the adult and developing human eye and accentuated in the ciliary muscle, the optic nerve and the neural retina.
The distribution of mRNAs coding for type XIII collagen, a novel nonfibril-forming collagen, was studied by Northern and in situ hybridizations of adult and fetal human eyes and the corresponding protein was localized by indirect immunofluorescence in frozen sections of 12 and 17 week human fetal eyes using a polyclonal antipeptide antibody to type XIII collagen. Type XIII collagen was found to be widely expressed in ocular tissues when studied at both the mRNA and protein levels in fetal and adult human tissues. No major differences were observed in the expression patterns between fetal and adult tissues. Surprisingly, the strongest signals seen in in situ hybridizations and immunofluorescence stainings occurred in the optic nerve bundles and in the ganglion cell layer of the retina. Other notable locations containing type XIII collagen included the developing ciliary smooth muscle, the posterior two-thirds of the corneal stroma and the striated extraocular muscles. Low level signals were also detected in the blood vessel walls and mesenchymal cells of the other ocular tissues. All immunosignals detected were adherent to cells, and the extracellular matrices appeared to be devoid of type XIII collagen. Our results are in concert with the presumed plasma membrane location of type XIII collagen, and it is hypothesized that this molecule could be involved in cell-matrix and perhaps cell-cell interactions. The wide expression of type XIII collagen in the eye, and especially in the neural structures, warrants future studies on type XIII collagen in other nerve structures and in pathological conditions affecting the eye. Due to its wide expression, type XIII collagen is likely to be an important factor for the normal development and functioning of the eye.