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Fungal ultrastructure

Transmission electron microscopy of thin sections of
fungal hyphae has been one of the most important tools
for understanding the behavior of fungal hyphae, and
for elucidating the mechanisms of apical growth. The
initial studies on fungal ultrastructure relied on chem-
ical fixationmethods: the hyphae were “fixed” by
immersion in aldehydes such as glutaraldehyde, then
post-treated with electrondense substances (osmium
tetroxide and uranyl acetate) to give maximum contrast.
The superb images of Oomycota shown in Figs 3.2
and 3.3 are widely acknowledged to be among the
best that have ever been produced by this technique.
But a newer technique termed freeze substitutionwas
developed in the late 1970s, and this provides even
greater resolution (Howard & Aist 1979) so it has now
become the standard (Fig. 3.4). In this technique,
the living hyphae are plunged into liquid propane
(−190°C) causing almost instantaneous preservation of
the hyphae, then transferred to cold (−80°C) acetone
with osmium tetroxide and uranyl acetate, and slowly
brought back to room temperature.
Comparison of the electron micrographs obtained by
these two methods should be made with caution, for
at least two reasons:

1 The chemically fixed hyphae belong to a species of
Pythium(Oomycota), which is not a true fungus, and

the specimen was stained to give maximum resolu-

tion of the internal organelles, so the wall is poorly

stained.

2 The hyphae prepared by freeze substitution represent

a true fungus, Athelia(Sclerotium) rolfsii, and some of

the organelles of fungi are different from those of

the Oomycota, as discussed later in this chapter.

However, it is notable that the degree of preservation

of the hyphae is much better in the freeze-substituted

material. In particular, the plasma membrane has a

smooth profile compared with the many indentations

seen with the chemical fixation method, and the sub-

cellular organelles are more clearly defined.

The zonation of organelles in the apical

compartment

All actively growing fungal hyphae – including

hyphae of the fungus-like Oomycota – show a clearly

defined polarity in the arrangement of organelles

from the hyphal tip back towards the base of the api-

cal compartment. The extreme hyphal tip (Figs 3.2, 3.4)

contains a large accumulation of membrane-bound

vesicles, but no other major organelles. These vesicles

show differences in electron density, suggesting that

they have different contents. Most of the vesicles are

thought to be derived from golgi bodies(or the func-

FUNGAL STRUCTURE AND ULTRASTRUCTURE 49

Fig. 3.1Diagrammatic representation of a fungal hypha, showing progressive aging and vacuolation behind the hyphal
tip. In the oldest regions, the walls may break down by autolysis or the mycelial nutrients may accumulate in chlamy-
dospores (thick-walled resting spores that serve in dormant survival). Aut =autolysis; AVC =apical vesicle cluster; Chlam
=chlamydospore; ER =endoplasmic reticulum; G =Golgi/Golgi equivalent; Gl =glycogen; L =lipid; M =mitochon-
dria; MT =microtubules; MW =melanized wall; N =nucleus; P =plasmalemma; R =ribosomes; S =septum; SP =
septal plug; V =vacuole; W =wall; Wo =Woronin body.