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Chapter 11 : Growth Polarity
Highly polar growth allows Aspergillus fumigatus to explore and invade blood vessels and tissues, resulting in the necrosis characteristic of invasive aspergillosis. A. fumigatus conidia that survive to germinate eventually elaborate hyphae that invade blood vessels and from there disseminate to other sites in the host and continue to extend by polar growth, forming the filamentous, branching network common in histological sections from invasive aspergillosis patients. Most of the more detailed knowledge about polar growth in A. fumigatus has come from in vitro characterization of early development in both A. fumigatus and A. nidulans. Saccharomyces cerevisiae undergoes polar growth, although the growth is less dramatic than that is seen in filamentous fungi. Unbudded yeast cells expand isotropically, switching to polar growth with bud emergence. A recent review of polar growth in filamentous fungi highlighted several of the unique features, most notably a role for microtubules in polar growth of filamentous fungi and the importance of the Spitzenkorpefer, a collection of vesicles at the tip that appears to be analogous to the polarisome. In most cases the deletion of polarity-related cell wall genes leads to early polarization as measured by germ tube emergence. A better understanding of the mechanisms underlying polar growth of A. fumigatus in vitro and in vivo might allow the contributions of polarity and of robust growth to the organism's pathogenesis to be separated.