axopodia

Axopodia are slender, ray-like pseudopodia produced by some single-celled eukaryotes, especially within the protist groups Heliozoa and Radiolaria. They extend from the cell surface and are supported internally by a rigid axial bundle of microtubules that runs along the length of each projection. The microtubule core provides stiffness for long, protruding filaments, which can reach several times the diameter of the cell.

Each axopodium consists of a central microtubule axis surrounded by cytoplasm. The cytoplasm is typically divided into an outer clear ectoplasm and an inner granular endoplasm, and is often coated by a glycoprotein layer or, in radiolarians, by a siliceous skeleton from which axopodia radiate. The microtubules originate at a microtubule-organizing center near the cell body and extend into the projection, with cross-bridging elements that help maintain shape.

Axopodia function primarily in prey capture and sensory reception. By extending into the surrounding water, they can trap small prey such as bacteria or microalgae. Cytoplasm is drawn into the proximal part, delivering captured material to the cell body for digestion. Axopodia may also play a role in locomotion and environmental sensing through cytoplasmic streaming and changes in projection length.

Axopodia are a diagnostic feature of heliozoans and radiolarians. In radiolarians the axopodia radiate from a silica or siliceous skeleton; in heliozoans they project from the cell membrane without a rigid skeletal framework. Their presence and morphology aid in identification and classification within these groups.