7.2 Selected to survive: studies of the PNS
Viktor Hamburger carried out a series of classic embryologieal experiments over a period of about 30 years. He investigated the relationship between the size of target tissue in chick embryos and the size of the pool of neurons that innervated it. His technique was to remove or add target tissue to the tissue which would eventually form a limb, usually the hind limb, and is called the limb bud. A few days later he observed the effect of the tissue addition or removal on the pool of neurons destined to innervate the limb bud. The surgical manipulation (addition or removal of tissue) was carried out on embryonic day 2.5 (E2.5 day) chick embryos and the neural pool examined at E9.5 days (Hollyday and Hamburger, 1976).
Where would he look for changes in the size of the neural pool? (Hint: where do the neurons that innervate the limb have their cell bodies?)
There are three possibilities: the ventral horn for motor neurons, the dorsal root ganglion for sensory neurons and the sympathetic ganglion for neurons of the sympathetic nervous system.
Hamburger looked at all of these and his results for the motor and sensory neurons are summarised in Figure 19.
The results of these and many similar studies led to two hypotheses that have made a significant contribution to the way we think about development. The first hypothesis is that it is the size of the target that ultimately determines the size of the neuron pool that innervates it. In other words, the number of neurons in the neuron pool is reduced or increased to match the size of the target; the target does not increase or decrease in size to match the number of neurons in the neuron pool.
The second hypothesis is about the mechanism by which the matching of neuron pool size to target size is achieved. The quaint hypothesis is that neurons must obtain an elixir from their targets. Neurons that obtain sufficient elixir stay alive. The elixir is secreted by targets, and received by innervating neurons at synapses. This is the neurotrophin hypothesis and it is, by and large, correct.