Developing Ear May Have Tuning Fork
What tunes up an embryo’s ears before it hears its first sound? A new study suggests that support cells in the cochlea, long thought to be inert, have a role in tuning up the hair cells during development. Experiments by Dr. Dwight Bergles and a team at Johns Hopkins suggest that cells in a tissue called Kolliker’s organ produce artificial tones that the developing hair cells use to get ready for a lifetime of hearing. According to Science Daily,
Bergles acknowledges that his experiments beg the question of why a human or any animal would need to “hear” before birth. He speculates that the ability to hear subtle differences, like the inflection in one’s voice, “requires a lot of fine-tuning based on where in the brain the nerves connect. It could be that brief bursts of electrical activity in just a few nerve cells at a time help do that fine-tuning so the system works well.”
The work was reported in Nature Nov. 1.1 Ian D. Forsythe, commenting on the paper in the same issue,2 said, “In the silence that precedes the onset of hearing in the developing auditory system, it seems that the cells of a transient structure known as K�lliker’s organ are capable of generating their own ‘virtual’ music.”
Kolliker’s organ runs alongside the organ of Corti, which contains the hair cells that respond to sound waves in the cochlear fluid. Apparently, the cells of this organ are able to pump ATP into the hair cells which bind to glutamate receptors, – the same receptors used in hearing. This effectively mimics the effects of sound waves with “artificial” chemical tones.
Forsythe explained that this is not random noise. The tones are produced in a coordinated, controlled manner, to help the nerve cells develop a tone map in the brain before birth. A more detailed explanation of this process is provided in footnote 3.3 See also EurekAlert. David Tyler commented on the significance of this paper at Access Research Network.
1. Tritsch, Yi, Gale, Glowatzki and Bergles, “The origin of spontaneous activity in the developing auditory system,” Nature 450, 50-55 (1 November 2007) | doi:10.1038/nature06233.
2. Ian D. Forsythe, “Hearing: A fantasia on K�lliker’s organ,” Nature 450, 43-44 (1 November 2007) | doi:10.1038/450043a.
3. Forsythe, Ibid., “So what do these results mean for our understanding of hearing? A prerequisite for experience-dependent adaptation is that the spontaneous activity should be elicited in a coherent or simultaneous manner, thereby defining a related population of nerve fibres. The observed synchronized activity in IHCs across a distance of around 60 micrometres, or 6�10 IHCs [inner hair cells] (and desynchronization between more distant IHCs), supports the idea that this activity may have a signalling function in defining the association between adjacent regions of the organ of Corti (tonotopy). Inevitably, this activity would cascade onto each subsequent higher level of auditory processing, moulding the development of the central auditory pathways and refining connectivity between the nerve-cell junctions, or synapses.
This process is important because in sensory regions of the brain, the afferent nerve fibres and their contacts with their target neurons maintain a topographic relationship with the peripheral sense organ through chemoaffinity mechanisms, which involve guidance molecules, and experience-dependent refinement.
How would this be explained in Darwinian terms? The reproducing adults would have only carried on this trait had it been a lucky mutation in the embryo. But the embryo had no knowledge of the outside world yet, nor the type of sounds that would be necessary to detect for survival. Evolution cannot look forward nor backward. It can only respond to immediate stimuli. To the embryo, an ATP leak into a hair cell would seem a mistake, like an alarm instead of a tuning fork. And the adult of reproductive age, years later, could not have selected for this trait – it had to already be functional to provide a survival advantage.
Coming up with Darwinian stories after the fact is forced and superfluous. Here is another observation of fine-tuning that rings out “design!”