Translated from the original German language version
The choroid supplies the light-sensitive photoreceptors in the outer retina. The retinal blood supply is responsible for nourishing the cells of the inner retinal layers. Blood circulation within these supply systems depends on the size of the blood vessels and is subject to strict regulation processes. However, like any system, this one too is susceptible to disruptive factors. Flickering lights provide us with an interesting example of how peripheral blood vessel size is regulated. On exposure to flickering light, the arterioles respond with vasodilatation to accommodate the increased neural activity.
The inner retinal layers can now be supplied with an increased amount of blood. The response is a well-defined hyperaemic reaction. In contrast, the vascular tonus of the retinal blood vessels is not modulated by direct neuronal input. Here, other factors come into play. Macroglial cells (Müller cells and astrocytes) are assumed to actively regulate the size of these vessels and to adapt to changes in neuronal activity.2
Regulating retinal blood flow is key to the eye's functionality. Vascular dysfunctions can lead to retinal diseases. One of these is diabetic retinopathy. In the early stages of this condition, we see reduced retinal blood flow and even retinal ischemia.2
This video shows the contact between microglial cells and pericytes in a retinal sample from a NG2-DsRed reporter mouse. The pericytes (DsRed, red), microglia (Iba-1, green), and endothelial cells (CD31, blue) were labeled immunohistochemically.2
Let us now turn to the idea of the neurovascular unit. The video presented by the research group led by Mills made it difficult to deny the existence of this unit of intertwined cells. The neurovascular unit encompasses the endothelial cells, the pericytes, and the microglial cells. It may be one of the complex regulatory pathways of the retina. In previous studies, Müller cell-dependent calcium signaling alone was thought to influence the vessel diameter of the retinal vasculature. However, this is contradicted by the observation that a light stimulus can induce both vasoconstriction and vasodilatation. In addition, calcium signalling in glial cells solely controls the capillaries within the intermediate vascular plexus.2
For a long time, researchers assumed that microglia were only responsible for the release of proinflammatory and neurotoxic cytokines and the diseases associated with them. More recent theories suggest that microglia are capable of much more. In the retina and brain, microglia's inflammation-independent roles include dynamic synaptic surveillance and synaptic pruning. It is also now known that microglia can influence vessel development.
References:
1. University of Pennsylvania School of Medicine. "Penn Researchers Calculate How Much The Eye Tells The Brain." ScienceDaily. ScienceDaily, 28 July 2006. .
2. Mills S. A. et al. (2021). Fractalkine-induced microglial vasoregulation occurs within the retina and is altered early in diabetic retinopathy, Proceedings of the National Academy of Sciences (2021).