Purpose: To localize the cell adhesion-related HNK-1 carbohydrate epitope in the human retina at cellular and subcellular levels.
Methods: Retinas were obtained from seven normal human eyes at autopsy (age, 43-78 years). The specimens were embedded in medium-grade resin and studied by postembedding immunoelectron microscopy using the primary mouse mAb HNK-1 (Leu 7) to the HNK-1 epitope and secondary antibodies conjugated to 10-nm colloidal gold particles.
Results: Prominent immunolabeling with mAb HNK-1 was observed on the outer surface of the entire plasma membrane of Müller radial glial cells, including their microvilli between the inner segments of rods and cones, on the plasma membranes of astrocytes in the ganglion cell layer, in bipolar cells in the inner nuclear layer, and in photoreceptor cells in the outer nuclear layer. Fewer gold particles were present on plasma membranes of other main types of retinal neurons, including ganglion cells. Only the outer segments of rods and cones and the endothelial cells of retinal capillaries were never labeled. In the ciliary epithelium, gold particles localized to the basement membrane of the nonpigmented and pigmented layers and to the cytoplasm of the pigmented epithelium.
Conclusions: Unlike in many other species, the HNK-1 epitope in the human retina is found on both glial and neuronal cells, including photoreceptors. This epitope potentially contributes to neuron-to-neuron and glia-to-neuron adhesion of human retinal cells.