In vivo visualization of macrophage-like cells within the retina has been reported previously. Liu et al.
36 were the first group to image star-shaped cells on the surface of the ILM in living human retina using AO-OCT and hypothesized they were microglia or astrocytes. Following this, two groups imaged the retinal star-shaped cells, one of them on mice using adaptive optics - scanning light ophthalmoscopy (AO-SLO) fluorescence
46 and the other with AO-OCT on humans,
37 and labeled them as microglia. Histologically, microglia within the healthy retina remain in a dormant state while they actively survey their environment.
47–50 In their resting state, they have a quiescent phenotype, characterized by small somata and ramified filodopia-like processes.
23,51 The description of dormant microglia matches with the star-shaped cells described by both studies as well as the appearance of the cells revealed within our own images. Additionally, time-lapse confocal microscopy has demonstrated their dynamic behavior as evidenced by the continuous movement of their processes.
9 They appear to be in constant motion, extending and retracting with the purpose of surveying their environment.
51 The motility of their processes is supported by our own observations of shifts in location and orientation of these macrophage-like cells between images at different time points of the same control subject (
Fig. 5). This finding is also consistent with several animal studies that reported microglial motility in the brain and retina.
52,53 This cell motility over time could therefore explain the relatively low intervisit repeatability of NND measurements in the current study. Other studies hypothesized that hyperreflective spots seen on a commercial OCT B-scan could be activated microglia, and they were able to find these spots in some of their controls on the ILM.
34,54 Two studies that specifically harvested human ILM using surgical procedures demonstrated the presence of cells of glial origin in the surgical specimens using histologic staining.
55,56 These studies, conducted on the maculae of diabetics, suggested that the cells were of glial origin, specifically microglia, within this retinal layer. Similarly, when compared to histologic studies, our own findings, which showed wide variation between individuals in cellular density, appeared to closely correspond to data previously reported.
30 Additionally, our NND calculations were similar to results reported on mice.
9