Filopodier

Filopodier are slender, finger‑like membrane protrusions that extend from the surface of many cell types. They are rich in filamentous actin (F‑actin) and dynamically extend and retract at the leading edge of migrating cells or growth cones, enabling environmental sensing and exploratory behavior.

Each filopodium is a narrow, rod‑shaped projection typically 0.1–0.3 µm in diameter and several micrometers long, consisting of tight bundles of F‑actin aligned with the growth direction. The actin core is cross‑linked by bundling proteins such as fascin and stabilized by actin‑binding proteins that regulate elongation. Growth is powered by actin polymerization at the tip; retrograde flow and membrane tension influence length and stability. Filopodia often emerge in clusters and can coordinate with lamellipodia and focal adhesions to probe the substrate.

Filopodium formation is regulated by Rho‑family GTPases, especially Cdc42, which activate actin‑nucleating pathways. Two major routes contribute: formin‑mediated actin assembly (e.g., mDia/formins) that generates unbranched filaments, and Arp2/3‑mediated pathways that can feed into filopodial formation via regulators such as IRSp53 and Ena/VASP proteins that promote filament elongation and antagonize capping. The balance of Rac1/Cdc42 signaling and adhesion cues from integrins influences initiation and persistence.

Filopodier function as sensory and exploratory structures, sampling the extracellular matrix, soluble cues, and cell–cell signals. They guide cell migration, influence neurite outgrowth, and participate in immune cell surveillance. In development and disease, filopodial dynamics affect tissue morphogenesis, wound healing, and invasion. Some pathogens can hijack filopodial machinery to facilitate infection.