The podosome is a structure on the surface of some cells discovered on macrophages in the 1980s. They are shaped as an invagination inside of which they contain finger shaped extensions.
Each of these protuberances is formed from a core of actin surrounded by a number of proteins forming a ring.
Other structures that only differ in architectural detail have been seen in other types of cells and are called invadopods. The distinction between these two types of structures is still uncertain.