Bio 580, Developmental Biology.  Friday, May 2, 1997
The "meaning" of ionic currents during development

• Polarity of the simplest gametes of plants:  Fucus
  
• Polarity of animal oocytes:
  • Sea urchin
  • Amphibian
  • Fish
  • Insect
    

Hypotheses about Ionic currents and development:

A general null hypothesis:
• H_o: There are no measureable currents associated with oocyte development.
  - all measured currents are artifacts of improper Ringer's media
  - Sun, Y. A., and R. J. Wyman. 1987. Lack of an oocyte to nurse cell voltage difference in Drosophila. Neurosci. 13: 1139.
  - Bohrmann, J. 1991. In vitro culture of Drosophila ovarian follicles: the influence of different media on development, RNA synthesis, protein synthesis and potassium uptake. R. Arch. Dev. Biol. 199:315-326.
  - Bohrmann, J., and H. Gutzeit. 1987. Evidence against electrophoresis as the principal mode of protein transport in vitellogenic ovarian follicles of Drosophila. Development (Camb) 101: 279-288.
  

  
  

Some alternate hypotheses:
• H_A1: An activation process in plant, insect and vertebrate eggs is a unifying similar phenomenon
  - the currents have the same developmental basis:
  • direct electrophoresis of cytoplasmic constituents
    - unlikely due to forces involved
  • lateral electrophoresis of membrane constituents
    - maximal force is experienced at the membrane level
  • orientation of cytoskeleton for polarization processes
    - weak forces may be enough for orientation of cytoplasmic processes
  • pacemaker site for initiation of depolarization waves
    - direct involvement in the processes of fertilization
  • something else ...
    

  
  

• H_A2: The phenomenon is real but is unrelated to development.
  i.e. expulsion of calcium in mammalian eggs is a measure of its health.
  • transient inward Ca⁺² currents are not measureable by the ion probe
    - but -
  • slow expulsion of Ca⁺² by an active process is measured
    

    
    

• H_A3: ionic currents are generated at many developing structures.
  
  • root hairs, pollen tubes, developing embryos, regenerating limbs
    - all exhibit ion currents
    - we are observing a multiplicity of causes (and effects), each unique to the physiology of the individual processes