The protocol presented describes organotypic explant cultures of mouse retina with intact RPE in defined R16 medium, free of serum and antibiotics. This protocol was originally developed starting in the late 1980s7,28 and since then it has been continuously refined6,11,12. Notable applications include studies into the mechanisms of hereditary retinal degeneration and the identification of retinoprotective drugs23,29,30.
For a successful experiment, some important considerations need to be taken into account. Here are some important troubleshooting points to help enhance the quality of cultures. First, the retinal cultures may display excessive folding and/or rosette formation31. This can be caused by touching the retina with a forceps during the explantation procedure. Moreover, the ciliary body must be completely removed from the explant, as this can increase retinal folding during culture. Second, during the transfer of the retina to the well plate in a hanging drop, if the retina faces the membrane the wrong side down, keep it in the drop hanging from the pipette tip and very gently push the medium in and out of the tip (without detaching the hanging drop) to flip the retina around. Finally, if the RPE remains attached to the sclera and detaches from the retina, it is most likely caused by an insufficient predigestion of the sclera. This problem could be especially important when working with eyes from older animals or non-rodent species (e.g., pigs) and may be resolved by increasing the proteinase K concentration.
Conducting organotypic retinal explant cultures is a complex procedure that requires adequate training and experience. Lack of training can lead to variability in the quality of the retinal explants. For these reasons, it is important to monitor and verify viability and reproducibility, characterizing, for instance, the rate of cell death with the TUNEL assay. The use of an antibiotic-free medium makes the retinal explants vulnerable to contamination by bacteria and fungi. To minimize this risk, we recommend that particular care is taken to work under truly aseptic conditions. Another limitation of in vitro retinal culturing are differences in physiochemical environment when compared to the in vivo retina (e.g., choroidal and retinal blood supply, oxygen and glucose levels, intraocular pressure, composition of the vitreous). A continuous perfusion system, perhaps embedded into a dedicated bioreactor32 could make this model closer to the in vivo condition. Furthermore, the axotomy of the optic nerve during retinal dissection will lead to ganglion cell death, that can induce stress responses8. Therefore, it is recommendable that the explant be left to adapt to culturing conditions for at least 2 days in vitro before it is subjected to a specific manipulation or treatment.
The described method is usually performed on immature retinal tissues, which may survive well for 4 weeks in vitro7,33. However, the procedure is tailorable to a variety of applications, including culturing of adult retina. Although different published approaches describe the isolation of the adult retina without its RPE34,35, the incubation with papain solution for up to 1 h at 37 °C before dissection allows the RPE to stay attached to the retina even when derived from an adult mouse36.
The serum-free medium and the chemically defined in vitro environment provide for an entirely defined and reproducible manipulation of the experimental conditions. Therefore, organotypic retinal explant cultures are valuable tools in the field of ophthalmology and neuroscience, and have been used for studying retinal diseases37, retina development38,39, retinal stem cell therapy40, genetic modifications41, and pharmacological screening. As a specific example of drug testing, here we used retinal explant cultures to test a cGMP analogue (CN003), known to reduce photoreceptor cell death in animal models for inherited retinal disease23 (Figure 3B). Another possible application of the technique is described in Figure 3C, which illustrates how the precise control of the tissue environment can be exploited to emulate diabetic conditions24. Because of the preservation of tissue architecture over the entire culturing period, organotypic retinal explant cultures are also suitable for electrophysiological studies. Neuronal functionality on retinal explants have been investigated using patch-clamp recording42 and multi-electrode-array (MEA) recording33,43. The latter allows recording of electrical activity of neuronal populations at the same time and has been exploited to characterize photoreceptor and ganglion cell functionality in culture conditions. In a broader perspective, the organotypic explant culture systems can also be applied in pre-clinical research, where explant cultures were used to test the therapeutic efficacy of hypothermia44.
The organotypic explant culturing technique is relatively simple to perform and, when compared to corresponding in vivo experiments, is less expensive and time-consuming, and avoids the ethical concerns related to live animal studies. The precise control over experimental conditions and the preservation of RPE and tissue complexity make the method a valuable tool to improve our knowledge on retinal physiology and pathophysiology and enable numerous experimental applications.