at plus end ~0.06: at minus end ~0.6 μM
||Koestler, S.A., K. Rottner, F. Lai, J. Block, M. Vinzenz, and J.V. Small. 2009. F- and G-actin concentrations in lamellipodia of moving cells. PLoS ONE. 4:e4810. doi:10.1371/journal.pone.0004810 p.1 left column top paragraphPubMed ID19277198
|| Pollard TD, Blanchoin L, Mullins RD (2000) Molecular mechanisms controlling actin filament dynamics in nonmuscle cells. Annu Rev Biophys Biomol Struct 29: 545–576. DOI: 10.1146/annurev.biophys.29.1.545  Le Clainche C, Carlier MF (2008) Regulation of actin assembly associated with protrusion and adhesion in cell migration. Physiol Rev 88: 489–513. doi: 10.1152/physrev.00021.2007.PubMed ID10940259, 18391171
||P.1 left column top paragraph: "Eukaryotic cells move by the extension of a leaf-like structure, the lamellipodium, at the cell front [ref 1]. Protrusion occurs by polymerization of actin filaments at the tip of the lamellipodium, thereby pushing the membrane forward [ref 2]. Actin filaments are polar, with the barbed, fast growing ends pointing towards the direction of protrusion [ref 3]. Under steady state conditions the network of actin filaments in lamellipodia maintains a constant breadth by coordinated depolymerization from the filament pointed ends towards the rear, in a treadmilling regime [ref 2], [ref 4]–[ref 6]. Treadmilling relies in the first instance on inherent differences of critical concentration for growth at the two filament ends, measured in vitro as around 0.06 µM and 0.6 µM at the plus and minus ends, respectively [primary sources]. Regulation can take place on several levels: actin filament nucleation, elongation and depolymerization, monomer sequestration and filament end capping [primary source 8], [ref 9]. For an understanding of the basic principles of actin turnover and for simulating the molecular scenarios underlying protrusion [ref 10] the biochemical parameters in vivo and, not least, the concentrations of F- and G-actin in the lamellipodium need to be established."