Studies of electron micrographs of pigeon optic nerve showed numerous unmyelinated fibers (29%of total) and more than twice as many total fibers as previously reported. The 2.4 million axons had a mean diameter below 1 F , close to the limit of resolution of light microscopy. This probably accounts for the large discrepancy between light a n d electron microscopic counts. Studies of retinal whole mounts showed an irregular distributicn of cells i n the ganglion cell layer including a minor posterior fovea and an area of increased density in the posterior-suyerior quadrant. Sample counts produced E n estimate of over 4.8 million cell bodies in this layer. Cytological investigation indicated that over 85% of these cells were neuronal. The considerable excess of neuron cell bodies over axons is thought to be explained by the presence of large numbers of aniacrine cells, cells which do not project axons into the optic nerve.

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Ectopic calcification of diseased tissues or around prosthetic implants can lead to serious disability. Therefore, calcification of implants of glutaraldehyde-cross-linked collagenous tissues and reconstituted collagen was compared with mineralization induced by demineralized bone matrix (DBM). Whereas implants of DBM accumulated large amounts of calcium and a bone-specific gamma-carboxyglutamic acid protein (BGP or osteocalcin) following implantation in both young and older rats, implants of cross-linked pericardium calcified with only traces of BGP. Glutaraldehyde-cross-linked DBM failed to calcify after implantation in 8-month-old rats for 2-16 weeks. Implants of cross-linked type I collagen exhibited small calcific deposits 2 weeks postimplantation but calcium content eventually dropped to levels equal to those of soft tissues as the implants were resorbed. The calcium content of DBM implanted in 1- and 8-month-old rats reached comparable levels after 4 weeks, but the BGP content was approximately twice as high in the younger animals than in the older ones. Glutaraldehyde-cross-linked implants of DBM, tendon, and cartilage calcified significantly in young but not in old animals. This form of dystrophic calcification was associated with only trace amounts of BGP. Alkaline phosphatase activity was high in implants of DBM and undetectable in implants of cross-linked collagenous tissues. These results show that implants of glutaraldehyde-cross-linked collagenous tissues and reconstituted collagen calcify to different extents depending upon their origin and the age of the host, and that the mechanism of dystrophic calcification differs significantly from the process of mineralization associated with bone induction as reflected by alkaline phosphatase activity and BGP accumulation.