Amoeboid movement is a type of movement accomplished by protrusion of cytoplasm of the cell involving the formation of pseudopodia. The cytoplasm slides and forms a pseudopodium in front to move the cell forward. This type of movement has been linked to changes in action potential; the exact mechanism is still unknown. This type of movement is observed in amoeboids, slime molds and some protozoans, as well as some cells in humans such as leukocytes. Sarcomas, or cancers arising from connective tissue cells, are particularly adept at amoeboid movement, thus leading to their high rate of metastasis. Locomotion of amoeba occurs due the sol-gel conversion of the cytoplasm within its cell. The ectoplasm is called the plasma gel and the endoplasm the plasma sol. The conversion of the endoplasm to ecto and vice versa is called sol-gel conversion.
All cells do not use cilia or flagella are for movement. Some, such as Amoeba, Chaos (Pelomyxa) and human leukocytes (white blood cells), employ pseudopodia to move the cell. Unlike cilia and flagella, pseudopodia are not structures, but rather are associated with actin near the moving edge of the cell. They are temporary projections of eukaryotic cells. Pseudopodia extend and contract by the reversible assembly of actin subunits into microfilaments. Filaments near the cell's end interact with myosin which causes contraction. The pseudopodium extends itself until the actin reassembles itself into a network. This is how amoebas move, as well as some cells found in animals, such as white blood cells.
Pseudopods can be classified into several types:
1. Lobopodia is bulbous, short and blunt in form as in Amoebozoa . These finger-like, tubular pseudopodia contain both ectoplasm and endoplasm.
2. Filopodia is more slender and filiform with pointed ends, consisting mainly of ectoplasm. These formations are supported by microfilaments as in Euglypha .
3. Reticulopodia is complex formations where individual pseudopods are blended together and form irregular nets. The primary function of reticulopodia, also known as myxopodia, is the ingestion of food, and the secondary function is locomotion.
4. Axopodia are thin pseudopods of complex arrays of microtubules enveloped by cytoplasm. They are mostly responsible for phagocytosis by rapidly retracting in response to physical contacts.
- The first detailed chemical analysis of the protein components of the cilia of Tetrahymena pyriformis was conducted by I. R. Gibbons in 1963.
- In Chlamydomonas several mutational defects have been studied in the axoneme of flagellum which may lead to paralysis of the flagellar function.
- The cilia are modified into a variety of structures such as the rods and cones of the retina, crown cell of saccus vasculosus of third ventricle of fishes, primitive sensory cells of the pineal eye and cnidocil of the nematocysts of the coelenterates.