The internal limiting membrane: Roles in retinal development and implications for emerging ocular therapies

Abstract

Basement membranes help to establish, maintain, and separate their associated tissues. They also provide growth and signaling substrates for nearby resident cells. The internal limiting membrane (ILM) is the basement membrane at the ocular vitreoretinal interface. While the ILM is essential for normal retinal development, it is dispensable in adulthood. Moreover, the ILM may constitute a significant barrier to emerging ocular therapeutics, such as viral gene therapy or stem cell transplantation. Here we take a neurodevelopmental perspective in examining how retinal neurons, glia, and vasculature interact with individual extracellular matrix constituents at the ILM. In addition, we review evidence that the ILM may impede novel ocular therapies and discuss approaches for achieving retinal parenchymal targeting of gene vectors and cell transplants delivered into the vitreous cavity by manipulating interactions with the ILM.

Keywords: Barrier; Basement membrane; Cell replacement; Cell signaling; Inner limiting membrane; Neurodevelopment; Retinal ganglion cell; Transplantation.

Figure 1.. Transmission electron microscopy demonstrating internal limiting membrane (ILM) integrity during ex vivo organotypic retinal explant culture.

Vitreoretinal interface of adult organotypic mouse retinal explants demonstrates ILM (arrows) separating posterior vitreous cortex (asterisk) and neuroretina. Müller glial footplates underlie the ILM. The ILM remains intact in culture from day 0 (A) to day 7 (B). Scalebars: 1μm (A, B); 400nm (A’, B’).

Figure 2.. Effects of Pronase-E digestion on ILM laminin.

Adult organotypic mouse retinal explants treated with BSS (A-C) or Pronase-E at a concentration of 0.6 U/mL (D-F) were cultured for 7 days. Fixed tissue stained with laminin (green) shows ILM organization. Tiled confocal microscopy of flat mount retina quadrants are shown (A, D). Three-dimensional reconstruction of confocal microscopy z-stacks detail ILM surfaces in control (B) and Pronase (E) treated retinas. Retinal explant cryosections exhibit intact (C) and linear interruptions (F) in ILM, with preserved retinal layers indicated by DAPI counterstain of retinal cell nuclei in blue. Scale bars: 500μm (A, D); 100μm (B, E); 50μm (C, F).