Filopodium

Filopodia are thin, temporary, actin-rich protrusions that extend from the cell membrane. They are found on a variety of cell types, including fibroblasts, neurons, and immune cells. Filopodia play crucial roles in cell migration, sensing the environment, and cell-to-cell communication.

These structures are typically 1-2 micrometers in length and 50-100 nanometers in diameter. Their dynamic nature allows cells to explore their surroundings. Filopodia are constantly extending and retracting, acting as cellular "fingers" that probe for physical or chemical cues.

The formation of filopodia is driven by the assembly of actin filaments at the cell's leading edge. Proteins such as formins and Arp2/3 complex are involved in nucleating and elongating these actin filaments. The extension of filopodia is often regulated by signaling pathways that respond to extracellular stimuli, like growth factors or extracellular matrix components.

Once extended, filopodia can serve several functions. They can act as sensory organelles, detecting mechanical forces or the presence of signaling molecules. They can also mediate cell adhesion, forming transient contacts with other cells or the extracellular matrix. In some cases, filopodia can guide cell migration by establishing a direction of movement. They are also implicated in processes like synapse formation in neurons and the recognition and engulfment of pathogens by immune cells. The dynamic behavior and sensing capabilities of filopodia make them essential for cellular motility and communication in multicellular organisms.