The hyphal growth form of filamentous fungi is an adaptation to the active colonisation of solid substrata. By hyphal extension and regular branching the fungal mycelium can increase in size without disturbing the cell volume/surface area ratio so that metabolite and end-product exchange with the environment can involve translocation over very short distances. Fungal hyphae differ between species, but generally speaking the hyphal filament, when separated into compartments by cross-walls, has an apical compartment which is perhaps up to ten times the length of the intercalary compartments.
The septa which divide hyphae into cells may be:
- complete (imperforate),
- penetrated by cytoplasmic strands,
- perforated by a large central pore.
The pore may be open, and offer little physical hindrance to the passage of cytoplasmic organelles and nuclei, or may be protected by a complex cap structure, called the parenthesome, derived from the endoplasmic reticulum (the dolipore septum of many Basidiomycota). In Ascomycota, which characteristically lack the parenthesome apparatus, the pore may be associated with cytoplasmic organelles known as Woronin bodies.
Over the years mycologists have been very sensitive to the question of whether fungi have cells, and how fungal cells and their interactions compare with those of plant and animal cells. Lower filamentous fungi (e.g. Mucor) have coenocytic hyphae; but they do not form multicellular structures. Hyphae of fungi which do exhibit complex developmental pathways form septa at regular intervals, though the septa usually have a pore. The pore is what worries people about the definition of fungal cells, because the implication carried with the word ‘pore’ is that all of the cytoplasm of a hypha is in continuity even though it might be subdivided by the septa into compartments.
Although movement of cytoplasm and organelles through septa has often been described and is frequently easy to demonstrate, it is also clearly the case that the movement or migration of cytoplasmic components between adjacent cells is under very effective control. There are instances in which nuclei move freely, but mitochondria do not, and others in which rapid migration of vacuoles is not accompanied by migration of any other organelle. Some biochemical experiments have even demonstrated that different sugars can be translocated in opposite directions in a hypha at the same time. There are also numerous examples available where grossly different pathways of differentiation have been followed on the two sides of what appear (to the electron microscope) to be open septal pores (see Chapter 12).
Clearly, whatever the appearance, the hypha can be separated into compartments whose interactions are carefully regulated and which can exhibit contrasting patterns of differentiation. There may still be a semantic argument for preferring ‘compartment’ to ‘cell’, but from this point on we will take the pragmatic view that if it looks like a cell and if it behaves like a cell, then we will call it a cell. But please don’t forget that every fungal cell is just a segment of a tubular hypha!
Cross-walls in fungal hyphae are pretty well always formed at right angles to the long axis of the hypha and this has a major impact on understanding the development of fungal tissues. Except in cases of injury or in hyphal tips already differentiated to form sporing structures, hyphal tip cells are not subdivided by oblique cross-walls, nor by longitudinally oriented ones. Even in fission yeast cells which are forced to produce irregular septation patterns under experimental manipulation, the plane of the septum is always perpendicular to the plane including the longest axis of the cell. In general, then, a fungus converts the one-dimensional hypha into a two-dimensional plate of tissue or three-dimenional block of tissue by controlling the formation of branches. The septum in any branch will be formed at right angles to the long axis of the branch, but its orientation relative to the parent hypha will depend entirely on the positioning of the apex of the new branch.
Primary septa in fungal hyphae are formed by a constriction process in which a belt of microfilaments around the hyphal periphery interacts with microvesicles and other membranous cell organelles (see Chapter 5). Except for the fact that there is no close linkage with mitosis (see above), there is a superficial similarity between fungal septation and animal cell cleavage (cytokinesis); but remember, fungi use organised microvesicles to divide blocks of cytoplasm in the free cell formation process (see Chapter 3; CLICK HERE to view this in a separate window).
Updated December 16, 2016