Hypha

A hypha (plural hyphae) is a long, branching filamentous cell of a fungus, and also of unrelated Actinobacteria. In fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium.

A hypha consists of one or more cells surrounded by a tubular cell wall. In most fungi, hyphae are divided into cells by internal cross-walls called septa (singular septum). Septa are usually perforated by pores large enough for ribosomes, mitochondria and sometimes nuclei to flow among cells. The structural polymer in fungal cell walls is typically chitin (in contrast plants have cellulosic cell walls, and animal cells lack walls). Some Fungi however, have non septate hypha, meaning their hypha are not separated by septa.

Hyphae grow at their tips. During tip growth, cell walls are extended by the external assembly and polymerization of cell wall components, and the internal production of new cell membrane. The Spitzenkorper is an intracellular organelle associated with tip growth. It is composed of an aggregation of membrane-bound vesicles containing cell wall components. The vesicles travel to the cell membrane via the cytoskeleton, and dump their contents outside the cell by the process of exocytosis. Vesicle membranes contribute to growth of the cell membrane while their contents form new cell wall. As a hypha extends, septa may be formed behind the growing tip to partition each hypha into individual cells. Hyphae can branch through bifurcation of a growing tip, or by the emergence of a new tip from an established hypha.

Hyphae may be modified in many different ways to serve specific functions. Some parasitic fungi form haustoria that function in absorption within the host cells. The arbuscules of mutualistic mycorrhizal fungi serve a similar function in nutrient exchange, so are important in assisting nutrient and water absorption by plants. Hyphae are found enveloping the gonidia in lichens, making up a large part of their structure. In nematode-trapping fungi, hyphae may be modified into trapping structures such as constricting rings and adhesive nets. Cords can be formed to transfer nutrients over larger distances.

Types of Hyphae

 * septate (with septa)
 * Pseudohyphae are not true septate hyphae and are distinguished from "true hyphae" by their method of growth, relative frailty and lack of cytoplasmic connection between the cells. They are most often found in yeasts as the result of a sort of incomplete budding where the cells remain attached after division.
 * aseptate or coenocytic (without septa)

Characteristics of hyphae can be important in fungal classification. In basidiomycete taxonomy, hyphae that comprise the fruiting body can be identified as generative, skeletal, or binding hyphae.
 * Generative hyphae are undifferentiated and can develop reproductive structures. They are thin-walled, usually have frequent septa, and may or may not have clamp connections.
 * Skeletal hyphae are thick-walled and very long and straight, with little cell content (they are "dead" once they have formed). They have few septa and lack clamp connections.
 * Binding hyphae are generally thick-walled and are bendy, with very frequent junctions and ramifications.

Based on these hyphal types, in 1932 E. J. H. Corner applied the terms monomitic, dimitic, and trimitic to hyphal systems, in order to improve the classification of polypores.
 * Every fungus must contain generative hyphae. A fungus which only contains this type, as do fleshy mushrooms such as agarics, is referred to as monomitic.


 * Skeletal and binding hyphae give leathery and woody fungi such as polypores their tough consistency. If a fungus contains all three types (example: Trametes), it is called trimitic.


 * If a fungus contains generative hyphae and just one of the other two types, it is called dimitic. In fact dimitic fungi almost always contain generative and skeletal hyphae; there is just one exceptional genus, Laetiporus, which includes only generative and binding hyphae.