Development of sense organs

Nota:

• Key points  Lineage-dependent vs competitive development   What regulates sensory / accessory cell differentiation in sensory apparatuses?   

1. Senses
   1. General and special senses
   2. Sensory neuron origins
      1. Neural crest e.g. DRG
      2. Neural tube e.g. retina
      3. Placodes, e.g. olfactory epithelium, some cranial nerves (e.g.V)
2. C. elegans
   1. Range of mechanoreceptors
      1. e.g. gentle body touch, harsh body touch
         1. Numerous touch-insensitive mutants screened
      2. Touch cell lineage
         1. Q cell lineage
            1. Q cell – Q1a and Q1p
               1. Q1p gives rise to GBT cell and an interneuron
            2. Control of Q cell division
               1. Loss of MEC-3 transforms touch cell
                  1. UNC-86 induces MEC-3
                     1. UNC-86 and MEC-3 combine to induce touch cell-specific genes
                        1. E.g. involved in
                           1. 15 protofilament microtubules
                           2. specialised ECM
               2. A form of lineage-dependent differentiation – an intrinsic blueprint
         2. UNC-86 turned on in Q1p
         3. Loss UNC-86 gives loss of touch cell
3. Drosophila
   1. Mechanoreceptors
      1. Numerous types of sensory structures
         1. e.g. mechanosensory
         2. Derived from sensory organ precursors (SOP)
            1. SOPs for chordotonal organs express atonal
            2. SOPs for external receptors express ct
      2. Single innervated sensilla
         1. NUMB is asymmetrically distributed between SPIIb and SPIIa
            1. NUMB inhibits NOTCH pathway
               1. Low NOTCH activity required for neural fate
         2. To some extent lineage-dependent development
   2. The drosophila eye
      1. consists of ~800 ommatidia
      2. 8 photoreceptors R1-8
      3. 12 accessory cells
      4. Atonal
         1. During OM development, atonal expression is ↑ in R8 by posterior tip Hh
            1. Atonal and R8 (Hh+) required for OM formation
               1. Activates Notch and Delta producing spaced R8 cells
         2. Interaction cascade
            1. Subsequent OM development depends on cell-cell interaction
               1. e.g. Bride of Sevenless on R8 causes R7 to become a photoreceptor and not a cone cell
            2. Development of stereotypy by competition and rules rather than strict predetermination
4. Vertebrate
   1. vertebrate eye
      1. Retina - laminar structure
      2. Derived from neural and non-neural ectoderm
         1. Requires expression of Pax-6 (eyeless)
      3. Ventricular progenitors produce types in sequence
         1. Not separate progenitors
            1. Neurogenesis of ganglion cells first
      4. Cell production influenced by environment
   2. The ear
      1. develops from the otic placode
         1. Forms into otic pit then otic vesicle (otocyst)
         2. VIII ganglion is placodal
      2. Hair cell development
         1. Hair cell development regulated by cell- cell interactions
            1. High notch activation = supporting cells. Low notch activation = hair cells.
               1. Retinoic acid and thyroid hormone involved later
         2. Hair bundle formation
            1. Poorly understood
            2. In human deaf/blindness Usher Ib myosin VII a altered
               1. Ushers Id and If involve cadherins
   3. Olfactory neurons
      1. Olfactory receptor cells and secretory cells from basal cells
         1. Form throughout life
      2. Olfactory bulb neurons generated (at least in part) from SVZ
         1. Rostral migratory stream
   4. Lateral line
      1. The lateral line detects water motion in fish
         1. Like inner ear sensory systems
            1. Contain sensory neurons -neuromasts
      2. Develops from placodes
         1. Primordial region of placode migrates towards the tail
            1. Deposits neuromasts as it moves