Lateral diffusion in plasma membrane of mouse egg is restricted after fertilisation

Te functional properties of egg membranes change rapidly at fertilisation¹. A physical basis for the earliest changes may be inferred from the observed rapid alterations in ion permeability and from the transient and prolonged changes of electrical potential^2,3. Subsequently, new membrane is added to the egg surface during exocytosis of cortical granules, and some surface is selectively shed in polar and midbodies^4,5. The physical changes occurring in the egg membrane, and the changed functional properties consequent upon them, may involve alterations of the mobility and association of lipids and proteins. Here, we present the results of studies on a general physical property, apparent lateral diffusion rate, of lipids and proteins in the membranes of unfertilised and fertilised mouse eggs. The single-cell photobleaching technique developed for measurement of diffusion in the surface membrane of cultured cells^6-8 has been used to measure diffusion of the fluorescent dye 3,3-dioctadecylindocarbocyanine iodide (diI-C18) dissolved in surface membrane lipids and of fluorescent-labelled antibody fragment bound to a spectrum of surface membrane proteins. Our results indicate that diffusion of the lipid probe and of proteins in the membrane of unfertilised eggs occurs at about the maximum rate expected for these molecules. In fertilised eggs, both markers seem to diffuse at greatly reduced rates.