Spicules are tiny spike-like structures of diverse origin and function found in many organisms, such as the copulatory spicules of certain nematodes or the grains on the skin of some frogs.

This article discusses the skeletal spicules that occur in most sponges. They provide structural support and deter predators. Large spicules, visible to the naked eye are referred to as megascleres, while smaller, microscopic ones are termed microscleres.
Spicules have four major symmetry types: Monaxon (simple cylinders with pointed ends), triaxon, tetraxon, and polyaxon. Sponges can be calcareous, siliceous, or composed of spongin. The meshing of many spicules serves as the sponge’s skeleton. The composition, size, and shape of spicules is one of the largest determining factors in sponge taxonomy.
Spicules are formed by sclerocytes, which are derived from archaeocytes. The sclerocyte begins with an organic filament, and adds silica to it. Spicules are generally elongated at a rate of 1-10 μm per hour. Once the spicule reaches a certain length it protrudes from the sclerocyte cell body, but remains within the cell’s membrane. On occasion, sclerocytes may begin a second spicule while the first is still in progress.