Growth, Differentiation and Sexuality

sis occurs, coinciding with a gradient of plasticity.
The rigidification process occurs in the wall
area, outside the cytoplasmic membrane. In this
process, chitin and (1-3)-β-glucan become covalently
linked and the glucan part becomes modified,
branched and (1-6)-β-linkages are inserted.
This model explains not only the generation of the
hyphal form but also the fact that the secretion
of large molecules occurs exclusively at growing
hyphal apices. The looser and highly hydrated construction
of the wall at the very apex allows proteins
and other large molecules to diffuse across the wall,
whereas in subapical regions the wall is rigid and
forms a barrier for large molecules. Components of
the wall-synthesising apparatus and proteins which
need to be secreted are probably all synthesised
at the ER, packed in vesicles and guided by the
cytoskeleton to the apex. Therefore, at the apex
all vesicles are collected in a microscopically visible
structure which has been referred to as the
Spitzenkörperorvesiclesupplycentre.Thefunctionandroleofthisstructureisnotwellunderstood
yet.
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