Within the lens of the vertebrate eye, macromolecules can move from cell to cell through a poorly characterised process known as the large molecule diffusion pathway (LMDP). To explain this movement, it has been proposed that the lens contains syncytia (the product of multiple cell-cell fusion events) – but how are these syncytia structured, and what might be their role in light focusing in the eye? To address these questions in living, intact lenses, Steven Bassnett and colleagues () express GFP in lens epithelial cells in young mice, and track its movement between cells. The authors show that macromolecules preferentially traffic between lens cells that lie within a single stratum of the lens cortex (and are, therefore, of a similar age). They next examine the role of the lens plasma-membrane protein Lim2, which has been proposed to mediate intercellular communication in the lens. They show that, in mice lacking Lim2, the LMDP is not established and syncytia do not form. Moreover, regions of partial cellular fusion are common in wild-type lenses, but are rare or absent in Lim2-deficient mice. The authors conclude that the lens contains several overlapping Lim2-dependent syncytia that form a unique `stratified syncytium', and propose several ways in which intercellular protein diffusion within the strata might promote light focussing by the lens.
