Fungal Morphology and Structure
Fungi are eukaryotic cells. They are heterotrophs, meaning they acquire food by absorbing dissolved molecules and do not photosynthesise because they do not have chlorophyll. Fungi have rigid cell walls. Medically important fungi belong to their own kingdom.
Medically important fungi are often classified in different ways, including by major classes and by morphology.
In addition to the major classes, fungi are also classified based on their morphology,
Basic Morphological Forms:
There are two basic morphological forms observed in fungi: the hypha and the yeast.
- Yeast: These are single-celled (unicellular) fungi. They are round to oval and are typically 3–10 μm in diameter. Yeast reproduce by budding or binary fission. A new organism grows from the parent's body and detaches. Budding starts with an outgrowth that develops into a daughter cell or blastoconidium, separated by a septum. Yeast cells can also develop into long forms (chains) when buds do not separate.
- An example is Cryptococcus neoformans, which is a spherical yeast and grows via budding. Candida species are also described as polymorphic yeasts.
- Hyphae: These are filamentous structures made up of fungal cells. They are branched, tubular structures, 2–10 μm in width. Hyphae either have septa (partitions or cross-walls) or do not (coenocytic). The cells of most fungi grow as tubular, elongated, and thread-like structures called hyphae. Hyphae elongate through apical growth at their tips.
- An example mentioned is Aspergillus, recognised by its filamentous, septate hyphae. Zygomycetes (like Rhizopus and Absidia) produce broad, non-septate hyphae. Actinomycetes are Gram-positive bacteria, but are mentioned as tending to grow in branched filaments resembling mycelial masses in older cultures, though they are distinct from fungi.
- Mycelium: This is a mass of hyphae. It is a web or mat like structure of intertwined branching hyphae that form multicellular colonies. Moulds are made up of mycelia. Substrate mycelia penetrate the nutrient substrate, while aerial mycelia develop above the medium. The entirety of the mycelia is called the fungal thallus, body, or colony.
- Pseudohyphae: Several elongated yeast cells chained together that resemble true hyphae are called pseudohyphae.
- Candida albicans can form Gram-positive pseudohyphae.