Madical brain Notes 10

  Morphology Fungi range in form and size from unicellular yeasts to large mushrooms and puffballs.  Yeasts  are unicellular, do not have flagella and reproduce asexually by budding or trans-verse fission, or sexually by spore formation. Multicellular forms such as moulds have long, branched, threadlike filaments called  hyphae , which aggregate together to form a tangled  mycelium  (Figure 8.1a). In some fungi the hyphae have crosswalls or septa (sing: septum) separating cells, which may nevertheless be joined by one or more pores, which permit cytoplasmic streaming, a form of internal transport. Such hyphae are said to be  septate ; others have no crosswalls and are therefore  coenocytic (i.e. many nuclei within a single plasma membrane, Figure 8.1b). Many fungi are  dimorphic , that is, they exist in two distinct forms. Some fungi that cause human infections can change from the yeast form in the human to a mycelial form in the environment ...

  Classification of the Fungi The Fungi are arranged into four major phyla on the basis of differences in their sexual reproduction. These are: The designation ‘higher’ and ‘lower’ Fungi, is an unof-ficial distinction based on the fact that the latter two (much larger) groups possess septate hyphae. In some books, you may come across references to a group called the Deuteromycota or Fungi Imperfecti. This is not a taxonomic grouping, but a ‘holding area’ for species in which only an asexual stage has been recognised, and which cannot therefore be as-signed to any of the above groups. It is now possible, however, by means of DNA analysis, to place such species with their nearest relatives, mostly in the Ascomycota or Basidiomycota. In the following pages, we shall look at each of the phyla in turn, concentrating particularly on their life cycles. Zygomycota The Zygomycota is a relatively small phylum, comprising less than a thousand species. Its members are typically found in soil, ...

  Fungi and disease A limited number of fungi are pathogenic to humans (Table 8.1).  Mycoses  (sing: mycosis) in humans may be cutaneous, or systemic; in the latter, spores generally enter the body by inhalation, but subsequently spread to other organ systems via the blood, causing serious, even fatal disease. Cutaneous  mycoses are the most common fungal infections found in humans, andare caused by fungi known as  dermatophytes , which are able to utilise the keratin of skin, hair or nails by secreting the enzyme  keratinase . Popular names for such infections include ringworm and athletes’ foot. They are highly contagious, but not usually serious conditions. Systemic  mycoses can be much more serious, and include conditions such as histo-plasmosis and blastomycosis. The former is caused by  Histoplasma capsulatum , and is associated with areas where there is contamination by bat or bird excrement. It is thought that the number of people displayi...