- Septa (cross-walls) can be seen by light microscopy, as illustrated in
of images and in this movie
clip from the Fungal Cell Biology Group,
University of Edinburgh. But electron microscopy has revealed that
several different types of septa exist among the major taxonomic groups
- Oomycota and Zygomycota:
- In general, the hyphae of
fungi belonging to these groups are not regularly septate (although
there are some exceptions).
- But septa in the form of COMPLETE CROSS-WALLS are formed to isolate
old or damaged regions of the mycelium or
to separate reproductive structures from somatic hyphae.
- Ascomycota and some mitosporic fungi:
- Hyphae of fungi belonging to these groups (and the Basidiomycota) possess
perforated septa at regular intervals along their length.
- The septum consists of a simple plate with a relatively LARGE CENTRAL
PORE (50-500 nm diameter) - this allows cytoplasmic streaming
(the movement of organelles, incl. nuclei) between adjacent
- Cytoplasmic streaming enables sub-apical and intercalary (central)
compartments of young hyphae to contribute towards growth
of the hyphal tip - transporting nutrients and essential
enzymes to the apex - so maximizing the capacity for somatic
- Associated with each septum are spherical, membrane-bound organelles
called WORONIN BODIES that ........
- are composed of protein;
- remain close to the septal pore and tend not to be disturbed by the
cytoplasmic streaming taking place;
- tend to be of the same or larger diameter than the septal pore and
are, therefore, capable of blocking the pore;
- will block the septal pore if the adjacent hyphal compartment is damaged
or ageing and becoming highly vacuolated.
- Not all fungi belonging to the Acomycota possess Woronin bodies - those
that don't often possess LARGE HEXAGONAL CRYSTALS OF PROTEIN
in the cytoplasm that are capable of serving the same function,
i.e. they can seal the septal pores of damaged or ageing hyphae.
- Some other mitosporic fungi:
- A number of mitosporic fungi possess septa with a single central pore,
similar to that observed in the Ascomycota.
- But other mitosporic fungi may possess MULTIPERFORATE SEPTA.
- E.g. the septa of Geotrichum candidum (illustrated
above) possess characteristic MICROPORES (approx. 9 nm diameter).
- The number of pores in each septum can vary up to a maximum of approx.
- These micropores allow cytoplasmic continuity between adjacent hyphal
compartments, but are too small to allow cytoplasmic streaming
to occur to the extent observed in fungi possessing larger
- The most complex type of septum is found in fungi
belonging to the Basidiomycota.
- Each septum is characterized by a swelling around the central pore (DOLIPORE)
and a hemispherical perforated cap (PARENTHOSOME) on either
side of the pore - illustrated above.
- The perforated parenthosome allows cytoplasmic continuity but prevents
the movement of major organelles.
- The plasma membrane lines both sides of the septum and the dolipore
swelling, but the membrane of the parenthosome is derived
from endoplasmic reticulum.
- Functions of septa:
- Act as STRUCTURAL SUPPORTS
- The addition of plate-like cross-walls to what is essentially a long
tube-like structure (hypha) will help stabilize it.
- Act as the FIRST LINE OF DEFENCE when part of a hypha is damaged
- Large-pored septa that have Woronin bodies or large proteinaceous
crystals associated with them have the advantage that
cytoplasmic streaming can occur between adjacent compartments.
- But at the same time a mechanism exists for rapidly sealing the septal
pore under conditions of stress (e.g. if the hypha
is damaged) thereby helping protect the mycelium.
- Facilitate DIFFERENTIATION in fungi
- Septa can isolate adjacent compartments from one another so that different
biochemical and physiological processes can occur
within them - these may result
in differentiation of the hyphae into specialized structures,
such as those associated with sporulation.
- It's unlikely to be coincidental that the most complex and highly
differentiated sporulating structures we see are those
produced by fungi possessing the most complex types
of septa, i.e. fungi belonging to the Basidiomycota.