Fas-ligand expression on nucleus pulposus begins in developing embryo

Study Design. The expression of Fas ligand in the notochord or nucleus pulposus was examined immunohistochemically using rat embryos. Objective. To clarify at which stage of embryo development the expression of Fas ligand begins in the nucleus pulposus. Summary of Background Data. The nucleus pulposus has been reported to be an immune-privileged site. Immune-privileged characteristics in other tissues, such as the retina and testis, have been attributed to the local expression of Fas ligand, which acts by inducing apoptosis of invading Fas-positive T cells. The authors reported previously on the expression of Fas ligand in nucleus pulposus cells of mature rats and humans, which could play a key role in the potential molecular mechanism of maintaining immune privilege of the disc. However, it is unknown at which stage of the developing embryo Fas ligand expression begins in the nucleus pulposus. Methods. Female adult Sprague-Dawley rats were housed with males for one night and monitored the next morning for the appearance of a vaginal plug. Later, whole sequential embryos were dissected and fixed immediately. Immunohistochemical staining for Fas ligand was performed for sagittal sections of notochords or nucleus pulposus using standard procedures. The sections were observed using light microscopy. Results. In the 14.5-day-old embryo, the notochord appeared as a continuous structure with a uniform diameter, and there was no positive staining for Fas ligand. In the 16.5-day-old embryo, the notochord became compressed at the centers of the vertebral bodies and expanded in the presumptive nucleus pulposus areas. At this stage, some of the nucleus pulposus cells exhibited weak positive staining for Fas ligand. In the 18.5-day-old embryo, the nucleus pulposus enlarged in fusiform, and most of the nucleus pulposus cells exhibited intense positive staining for Fas ligand. Conclusions. The present results demonstrated that Fas ligand expression is not detected in the notochord, but at the time of intervertebral disc formation, Fas ligand expression develops in the nucleus pulposus. These results indicate that the immune privilege of the intervertebral disc may begin in the very early stages of disc formation. Moreover, Fas ligand may play an important role in the formation of the intervertebral disc.