1. Moving Membrane up to the Front of Migrating Cells
Mark S Bretscher 
Cell 
Volume 85, Issue 4, Pages (May 1996)
DOI: /S (00)

2. Figure 1
The Distribution of a Rapidly Circulating or of a Noncirculating Receptor May be Determined by a Polarized Endocytic Cycle
(A) The rapidly recirculating red receptor is efficiently collected by coated pits in the plasma membrane, transported through the cell and returned to the cell surface at the leading edge. During the time it resides on the cell surface—typically 1–4 minutes—it may diffuse rearwards about 10–20 μ before being recaptured and returned to the leading edge. Thus, on a cell large compared to 20 μ, say 100 μ in length, the receptor is concentrated towards the leading tip. The paucity of red receptors at the trailing end may mean that coated pits here take longer to form, or they may contain a different selection of receptors.
(B) The noncirculating blue receptor on the cell surface does not enter coated pits: it would be expected to be almost randomly distributed, being slightly swept rearwards by the polarized flow in the plasma membrane. This is indicated by the less intense blue colour at the leading edge. The extent to which it is swept backwards will depend on the length of the cell (or axon) and the rate of lipid flow. In a stationary cell, the rate of endocytosis is undiminished, but the internalized membrane is returned to the cell surface randomly.
Cell  , DOI: ( /S (00) )