It remains to be determined whether the reflective bands associated with outer retinal tubulation can even be correlated with reflective bands arising from normal retina since the anatomy is so deranged.
Band #2 has been given various names in the past. In the early years of optical coherence tomography (OCT) imaging, the band was mislabeled and confused with adjacent hyperreflective bands,1 or it wasn’t named at all,2 or was just called it “the red line” because of its high reflectivity in the false color display mode.  

Soon the correlation to anatomical structures was narrowed down to the inner segment/outer segment (IS/OS) junction or ellipsoid zone (EZ). Throughout the literature, depending on research group, both terms were used and associated with band #2. Even within the same paper, band #2 was correlated to the EZ in one figure and named the IS/OS in another.3

Despite the confusion of what to call band #2 and the anatomical structure correlated with it, consensus at least exists on the appearance of this band: It reflects photoreceptor integrity and health, and its absence is a poor prognostic sign. Here, we explore the anatomy of the region and discuss two of the terms that have been used to describe it.

Anatomy of Band #2 Region

The inner segment of each photoreceptor is comprised of an IS myoid and an IS ellipsoid (ISel). The ISel is the distal-most portion of the IS and adjacent to the IS/OS junction. About 75 percent of the content
of the ISel is tightly packed with mitochondria. In a healthy retina, ISel mitochondria are long, thin and bundled parallel like uncooked spaghetti.4

The IS and OS is connected by a cilium, measuring approximately 0.25 μm by 1 μm, which is comprised of microtubules.4 A narrow gap (50-200 nm), can be seen on electron microscopic (EM) imaging and separates the IS from the OS.5 It’s not quite clear if this gap is real or an artifact. This passage, which includes the connecting cilium, is called the IS/OS junction.

Nomenclature: Pro-Ellipsoid Zone

The term “Ellipsoid Zone” arises from the reflectivity source of OCT band #2 correlating with the ISel. When authors compared OCT band #2 with histologic images, the apparent source of the signal appeared to correlate with the ISel, presumably arising from the ISel mitochondria.6,7 The ovoid mitochondria with multiple internal membranes (cristae) were thought to contribute to a high refractive index in this region and optically serve as microlenses.8

Courtesy Christine Curcio; modified from Schaal and Jonnal.
Figure. A: Red arrows indicate the inner segment/outer segment (IS/OS) ellipsoid band (band #2) in an spectral-domain optical coherence tomography cross-sectional B scan through the foveal center of a healthy eye. B: Toluidine blue-stained histologic section of photoreceptors at 2.5 mm temporal to the foveal center in a healthy macula. IS myoid (-my) is pale; inner segment ellipsoid (-el) has darkly stained vertical streaks indicating closely packed thin mitochondria; inner plexiform layer (IPL); Müller cell nuclei (M) in the inner nuclear layer (INL); outer plexiform layer (OPL); Henle’s nerve fiber layer (HFL); rods (R) and cones (C) in the outer nuclear layer (ONL); external limiting membrane (ELM); outer segments (OS); scale bar = 25 µm. C: Magnified histologic image of a cone photoreceptor with brackets indicating IS myoid (ISmy) and inner-segment ellipsoid (ISel). D: Transmission electron microscopic image of photoreceptors at 2.5 mm temporal to the foveal center in a healthy macula. Green arrow highlights ELM. Electron-dense mitochondria (red arrowhead) are thin and tightly packed in the ISel (inset). Scale bar = 5 µm; inset scale bar = 1 µm. E: Electron microscopic image of a partial human cone photoreceptor showing transition between the inner and outer segments. ISel is densely packed with mitochondria, arranged with long axes parallel to the optical axis of the cone. Yellow arrow highlights a narrow gap (50–200 nm; unclear if a region of interstitial fluid or an artifact due to tissue preparation).  (Courtesy Christine Curcio, MD, (B-D), modified from Schaal KB et al.12; E modified from Jonnal RS et al.5)
The work of physicists, through the use of adaptive optics (AO) ultra-high resolution OCT (UHR-OCT) imaging, supported these conclusions.3,6,8 UHR-OCT with pancorrection seemed to elongate band #2, resembling the true axial morphology of cones, and correlated to the ISel.8 Because photoreceptors have high metabolic needs and require an excess of large mitochondria, their orientation in the ISel could suggest an arrangement that would yield optical benefit beyond their role in energy production.4,9

Genetic studies in which the age-related maculopathy susceptibility 2 (ARMS2) gene plays a role in age-related macular degeneration (AMD) have implied the importance of mitochondria and band #2, possibly through a mitochondria-related pathway. ARMS2 encodes for a mitochondrial protein found with ISel.10 A reported decreased intensity of the band #2 signal in AMD patients further supported this theory,11 but this may reflect a more general finding of photoreceptor dysfunction.

Moreover, electron micrographic images of photoreceptor degeneration, as in outer retinal tubulation (ORT), revealed morphologic changes in the ISel mitochondria. The mitochondria in these photoreceptors lost their well-structured arrangement and were randomly scattered, shortened and displaced toward the nucleus.12 In a 1:1 correlation between OCT and histology of a patient with ORT, a hyper-reflective band was seen on OCT and the reflectivity source was correlated with histology and EM images.

The images revealed short, randomly scattered mitochondria on both sides of the external limiting membrane and an absent OS. In this case, the hyper-reflective band on OCT could not represent the IS/OS junction, because OS were not present.13 However, it remains to be determined whether the reflective bands associated with ORT can even be correlated with reflective bands arising from normal retina since the anatomy is so deranged.

A 2011 paper correlated band #2 to the EZ based on distance measurements to adjacent hyper-reflective bands and thickness measurements of band #2 itself using Spectralis SD-OCT (Heidelberg Engineering).4 These authors correlated OCT findings to a photoreceptor model, which they created based on histologic measurements they obtained in a broad literature review. They found the thickness of band #2 to be similar to the anatomical measurements of the ISel portion.

In an attempt to reach a consensus on the terminology for retinal layers and the bands that appear on SD-OCT imaging, a panel of retinal specialists proposed a nomenclature for the posterior segment, choosing to call band #2 the EZ.14 However, controversy continues to surround band #2, with a recent publication using AO-OCT5 challenging the EZ-band #2 correlation point by point.

Nomenclature:  Pro IS/OS Junction

This terminology revolves around the reflectivity source of OCT band #2 correlating with the IS/OS junction. Recent reports on AO-OCT imaging have focused on photoreceptor cellular details, and the light reflected within single cone photoreceptors was located at the IS/OS junction and correlated with band #2.15 Furthermore, thickness measurements of band #2 using AO-OCT imaging were found to be three to four times narrower than in the corresponding clinical OCT images and did not correspond to the thickness of the ISel (16-20 µm).5,16

Dr. Rosenfeld is a professor at Bascom Palmer Eye Institute, University of Miami Miller School of Medicine. He has been the principal investigator and study chair for several clinical trials involving wet and dry AMD. Dr. Schaal is a retina researh fellow at Bascom Palmer.
Since conventional OCT averages over multiple cells, which leads to an overestimation of layer thickness, OCT imaging can lead to incorrect band measurements.5 An additional concern has been that the IS/OS junction would be too small to visualize on OCT. However, the sensitivity of the current commercially available OCT instruments can detect a refractive index mismatch between IS/OS.5

Another argument is that the ISel serves as a waveguide,2,6 and its contribution to the Stiles-Crawford effect is widely accepted.5 This is not surprising when having a closer look at the arrangement of the mitochondria in the ISel (long, thin and bundled), which resembles fiber optics and increasing in number with eccentricity.9  The OCT back-reflection from the IS/OS region seems to arise from the abrupt change in optical index of refraction at the IS/OS junction, and the ISel mitochondria (lipid rich) are thought to contribute to the high refractive index of the inner segment.17

Which Sides to Take?

So, which nomenclature is correct? Does band #2 correspond to the high reflectivity of the ISel mitochondria or is it the contribution to a refractive index change between IS and OS that’s responsible for generating band #2 at the level of the IS/OS junction? Both waveguiding and scattering contribute to this photoreceptor hyperreflective band.

There’s no correct answer at this time and band #2 is referred to interchangeably as the IS/OS band or the EZ band. Regardless of your terminology, it’s important to acknowledge the research and theories from both sides and appreciate the significance of retinal anatomy and the OCT band toward understanding the principles of OCT imaging and outer photoreceptor pathology.  RS

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