BIOC3800 Exam 2007 John Illingworth’s Marking Scheme
Discuss the roles of primary cilia in sensory transduction.
The lecture course covered cilia in general: centrosomes and basal bodies, 9+2 microtubules
in motile cilia and 9+0 microtubules in non-motile primary cilia, roles of kynesin and dynein
in cilia assembly, bending and recycling. Many eukaryotic cells have a primary cilium. These
were once considered to be vestigial, but are now recognised as important structures. We
distinguished between eukaryotic cilia and bacterial flagellae, noted that mutations in cilia
proteins may produce pleiotropic changes across several sensory modalities (e.g. Bardet-Biedl
syndrome, Usher syndrome) and eventually discussed four sense organs in greater detail:
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Flow sensors in kidneys, various exocrine glands, bones and teeth
Assembly of stereocilia bundles in auditory hair cells.
Vertebrate photoreceptors
Vertebrate smell
Students were expected to read the relevant literature. The course website has extensive links
to recent reviews and original papers. During lectures we used normally used diagrams from
recent papers, accessed on line, rather than preparing new figures for this particular course.
Students were advised to spend 3 hours private study per lecture reading references, but it
seems from their answers that very few of them actually followed this advice.
Flow sensors: (kidney tubules, bile ducts, pancreatic ducts, internal canals in bones and teeth)
Bending of primary cilia is used to monitor fluid flows via changes in the intracellular
calcium concentration. This is used to correctly align subsequent cell divisions in relation to
the tubular lumen, and also allows hard tissues to grow correctly in response to mechanical
stress. Over or under expression of the relevant genes leads to polycystic kidney disease.
Related defects are sometimes seen in other tissues.
Hearing: The primary cilium (kinocilium) is needed for the correct orientation and assembly
of the hair cell bundles – there is no direct role for cilia in hearing transduction, which is
performed by stereocilia (microvilli). Give credit for a reasonable description of the cochlear
amplifier and hearing transduction mechanisms, especially if the student makes it clear that
the cilium is required for assembly rather than transduction.
Vision: The photoreceptor compartments of vertebrate rod and cone cells are hugely modified
primary cilia. [Insects use a different visual transduction system based on microvilli.] Defects
in vertebrate ciliary proteins (e.g. kinesin-2, usherin) lead to defective phototransducer
assembly and maintenance. Material is continuously lost from the distal ends of rod cells and
phagocytosed by cells in the retinal pigment layer. Give credit for a reasonable description of
the visual transduction and adaptation mechanisms, especially where these are linked to
ciliary functions.
Smell: Specific chemoreceptors located on the sensory cilia of olfactory receptor neurons are
linked through G-proteins to adenyl cyclase. Receptor activation raises cAMP which activates
cyclic nucleotide-gated calcium channels, increasing the calcium concentration within the
cilia. This in turn opens calcium-gated chloride channels, leading to chloride efflux and
depolarisation. The distal ends of olfactory cilia lack the two central microtubules and are
non-motile. These are not true primary cilia, but they are closely related to them.
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It was not necessary to write about all four systems to achieve a good mark. Students were
expected to know what primary cilia are, and their relationship with centrosomes and basal
bodies, and how they differ from normal, motile cilia. The ciliary assembly and maintenance
systems are used both for cilia and for the sensors derived from them, so mutations affecting
cilia genes also have widespread effects on sensory transduction.
A discussion of polycystic kidney disease or Usher Syndrome or any of the numerous other
ciliary diseases would have been relevant to this answer.
This question provided an opportunity to write about the transduction process itself in one or
more of these sense organs, describing how the transducers sense, how they are maintained
and how they adapt to changing inputs. There are similarities between the transduction
systems in different sense organs, reflecting their common evolutionary origin, and this would
also have been considered relevant.
The topic was clearly flagged up in the lectures, and students who had read the some of the
recommended papers, as well as the course website, should have found this an easy question
to answer.
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