Chapter 3
Fungal structure and
ultrastructure
This chapter is divided into the following major sections:- the structure of a fungal hypha
- fungal ultrastructure
- the hypha as part of a colony
- the structure of yeasts
- fungal walls and wall components
- septa
- the fungal nucleus
- cytoplasmic organelles
- the fungal cytoskeleton and molecular motors
Fungi have many unique features in terms of their struc-
ture, cellular components, and cellular organization.
These features are intimately linked to the mechanisms
of fungal growth and therefore to the many activities
of fungi as decomposer organisms, plant pathogens,
and pathogens of humans. In this chapter we consider
the main structural and ultrastructural features of
fungi, and some of the powerful techniques that
enable us to view the dynamics of subcellular com-
ponents in living fungal hyphae, giving an insight into
the way that fungi grow.Overview: the structure of a fungal hypha
The hypha is essentially a tube with a rigid wall, con-
taining a moving slug of protoplasm (Fig. 3.1). It is of
indeterminate length but often has a fairly constant
diameter, ranging from 2μm to 30μm or more (usu-
ally 5–10μm), depending on the species and growth
conditions. Hyphae grow only at their tips, where
there is a tapered region termed the extension zone;
this can be up to 30μm long in the fastest-growing
hyphae such as Neurospora crassawhich can extend at
up to 40μm per minute. Behind the growing tip, the
hypha ages progressively and in the oldest regions it
may break down by autolysis or be broken down by
the enzymes of other organisms (heterolysis). While the
tip is growing, the protoplasm moves continuously from
the older regions of the hypha towards the tip. So, a
fungal hypha continuously extends at one end and con-
tinuously ages at the other end, drawing the protoplasm
forward as it grows.
The hyphae of most fungi have cross walls (septa;
singular septum) at fairly regular intervals, but septa
are absent from hyphae of most Oomycota and Zygo-
mycota, except where they occur as complete walls
to isolate old or reproductive regions. Nevertheless, the
functional distinction between septate and aseptate
fungi is not as great as might be thought, because septa
have pores through which the cytoplasm and even the
nuclei can migrate. Strictly speaking, therefore, septate
hyphae do not consist of cells but of interconnected
compartments, like the compartments of a train. As
we will see later, this enables cellular components to
move either forwards or backwards along dedicated
pathways, so that a hypha can function as an integrated
unit.
All fungal hyphae are surrounded by a wall of
complex organization, described later. It is thin at the
apex (about 50 nm in Neurospora crassa) but thickens
to about 125 nm at 250μm behind the tip. The
plasma membrane lies close to the wall and seems to
be firmly attached to it because hyphae are difficult to
plasmolyse.