Serial block face scanning electron microscopy reveals novel organizational details of the retinal pigment epithelium.

Abstract

Advances in imaging have led to the development of several new types of microscopes such as serial block face scanning electron microscopy (SBF-SEM), lightsheet microscopy, as well as X-ray micro-computed tomography (micro-CT), which enables the study of samples in fundamentally different ways. Significantly, these are now commercially available, which facilitates their widespread use in research. With SBF-SEM, fixed and resin-embedded specimens can be serially sectioned and imaged to construct a 3D dataset of the ultrastructure of cells and tissues at high resolution. We used this technique on perfusion-fixed C57BL/6 mouse eyes to image the outer retina. Our findings revealed novel organizational details of the retinal pigment epithelium (RPE) (Keeling et al., 2020b); a specialized cell monolayer that maintains the overlying photoreceptors and also forms the outer blood-retinal-barrier. RPE cells were found to look after far more photoreceptors than was widely assumed. 3D-data enabled measurements of the RPE cytoplasmic and nuclear volumes, the length and angle of microvilli on the apical RPE surface, as well as sub-RPE spaces under the basolateral membrane. The study also compared between mono-nucleate vs. bi-nucleate RPE cells, whilst the use of computing microinstructions (macros) provided information on interactions between adjacent cells in the RPE monolayer. Analysis of SBF-SEM stacks showed several hundred mitochondria which were rendered in 3D, providing information on their volume and spatial distribution in healthy RPE. Mitochondria were found in varying shapes and sizes, and predominantly localized to the mid and basal-zones of cells. The capabilities of SBF-SEM alongside other imaging techniques are being increasingly harnessed by investigators to gain novel insights into the organization of cells and tissues in the eye. These findings also help improve the current understanding of pathology linked with common blinding conditions such as age-related macular degeneration (AMD), as well as rare forms of retinopathy which leads to irreversible sight-loss