The timetable of innervation and its control in the chick wing bud.

How is the timing of nerve outgrowth controlled during development? This question has been examined by grafting early limb buds between chick embryos of different ages, before innervation, and assessing the morphological pattern of nerves at later stages. Grafted limbs continued to develop according to their own timetable and were invaded by nerves from the host. Irrespective of the age of the host, the development of the pattern of innervation followed the time course appropriate to the age of the grafted limb. Young nerves in an old limb showed accelerated development; old nerves in a young limb showed retarded development. The process of innervation is apparently governed not by the intrinsic developmental timetable of the neurons, but rather by the rate of construction of pathways for them in the peripheral tissue, and by the times at which their specific targets, such as muscles, differentiate.