One of the biological characteristics that distinguish multicellular fungi from other organisms is their constitutional cells, or hyphae (singular, hypha). Hyphae are nucleated cells in the shape of thin tubes, externally enveloped in a rigid chitin-rich cell wall and presenting an internal plasmatic membrane. They contain cellular organelles such as mitochondria, Golgi apparatus, ribosomes , endoplasmatic reticulum, which is also found in other Eukaryotes as well as cytoplasmatic vesicles bound to the plasmatic membrane. Hyphal cytoskeleton is organized by microtubules. Hyphae are separated by walls termed septae (singular, septum), usually bearing pores and regulatory structures that prevent cellular leaking due to cell disruption. For instance, the septum of Ascomycetes contains the Woronin body, an oily structure that blocks the pore if cell disruption occurs, whereas Basidomycetes have a dolipore septum, with the hyphae containing distinctly layered wall structures and endoplasmatic reticulum next to the pore. Hyphal growth and proliferation form structures similar to fine branches, which form the mycelium or vegetative hyphal network. However, Zygomycetes and Chytridiomycetes have non-septate vegetative mycelium, except for the reproductive structures.
As fungi grow, the older layers of hyphae gradually die because growth occurs through the proliferation and branching of the apical cell of the mycelium (i.e., cell at the mycelium tip). Growth takes place when two cytoplasmatic vesicles bound to the internal membrane fuse at the apical hypha, enlarging the hyphal tip because of the accumulation of biomass, leading to septum formation and branching. Branching is due to the growth of another apical cell inside the sub apical region of the mother cell. The growth process so far described is directed into new regions of the organic substrate, from which the fungus is feeding, and is termed the extension zone. The aerial part of the fungus, consisted of older hyphae forming aerial mycelia, may develop and differentiate to form structures bearing spores, and is termed the productive zone.
For most fungi, the haploid spore is the starting point from which the haploid hypha will develop and form the monokaryotic mycelium. The joining of two haploid mycelia leads to diploidization of the apical cells, resulting in two cells containing two separate nuclei, known as a dikaryotic mycelium. These cells are capable of producing spores and in Basidomycetes, are termed basidia (singular, basidium) whereas in Ascomycetes, they are termed ascus. Usually, a typical basidium produces four sexual spores and the ascus can produce eight spores, although the amount of spores can vary among the species of a given phylum. In the basidium, for instance, the two nuclei are duplicated and then merged when the cell is about to undergo meiosis twice, thus resulting in the formation of four haploid spores. Some Ascomycetes as well as a few Basidomycetes may also produce asexual spores; and asexual reproduction is the way Zygomycetes and Chytridiomycetes reproduce themselves.
See also Fungal genetics; Fungi; Mycology; Parasites; Yeast