FIGURE 4.

Cell communication in the NVU. (A) In healthy tissues, retinal ganglion cell (RGC) neurons, astrocytes and endothelial cells communicate through gap junctions which permit the movement of electrical conductance (Na⁺, K⁺, and Ca²⁺) and small molecules such as ATP/ADP, glutamate, and glucose, and second messengers. Gap junctions exist in the retina and ONH between neurons, glia and between vascular cells. (B) In glaucoma, several changes occur that affect communication between the cells of the NVU. (1) RGCs are vulnerable to stressors that lead to apoptosis; (2) cell–cell communication is lost as astrocytes and other glial cells become reactive and gap junction expression is reduced; (3) a reduction in gap junction expression, and loss of tight junctions between vascular cells leads to a leaky blood–retinal-barrier (BRB), allowing the infiltration of circulating monocytes into the retina and ONH.