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podosome

n. The primary site of integrin-stimulated actin polymerization in some leukocytes

Wikipedia
Podosome

Podosomes are conical, actin-rich structures found on the outer surface of the plasma membrane of animal cells. Their size ranges from approximately 0.5 um to 2.0 µm in diameter. While usually situated on the periphery of the cellular membrane, these unique structures display a polarized pattern of distribution in migrating cells, situating at the front border between the lamellipodium and lamellum. Their primary purpose is connected to cellular motility and invasion; therefore, they serve as both sites of attachment and degradation along the extracellular matrix. Many different specialized cells exhibit these dynamic structures such as invasive cancer cells, osteoclasts, vascular smooth muscle cells, endothelial cells, and certain immune cells like macrophages and dendritic cells.