Electron microscopy of human melanopsin retinal ganglion cells

Abstract

Purpose: Melanopsin-expressing retinal ganglion cells (mRGC) subserve a variety of functions, particularly pupil light responses and circadian photoentrainment, and are peculiar in their resistance to neurodegeneration in mitochondrial optic neuropathies. We here present the first ultrastructural images of a human mRGC.

Methods: Serial retinal sections from a normal 24-year-old male control were immuno-stained for melanopsin (rabbit polyclonal antibody) and mitochondrial antigens (mouse monoclonal antibody against mitochondrial extract) using diaminobenzidine (DAB). One example of an mRGC was then processed into plastic for ultrathin sectioning for transmission electron microscopy. The subtype of this mRGC could not be indentified due to twisting of the retinal section during antigen retrieval.

Results: Melanopsin was identified with electron-dense DAB immunostaining, particularly in cell processes and along cell membranes, and some DAB was seen within the cell, associated with membrane-bound structures presumed to be endoplasmic reticulum, Golgi and vesicles. The mRGC was large, with subjectively more electron lucent and vesicular cytoplasm than other ganglion cells and surrounding glia. Antigen retrieval (with heat and citrate buffer) limited the resolution of small ultrastructural features, so the mitochondrial phenotype was not characterized. Immunostaining for melanopsin and mitochondrial antigens for light microscopy has indicated high mitochondrial density in mRGCs.

Conclusions: These first electron micrographs of a human mRGC resemble previous findings in the mouse [Belenky et al. J Comp Neurol 2003;460:380-93]. Our attempt to describe the structural mitochondrial phenotype was limited by the trade off between antigen retrieval and ultrastructural preservation. Immunostaining for mitochondria at light microscopy suggests an abundant population of these organelles in the large cytoplasm of mRGCs, which may help to explain their resistance to death after injury.