The growth rate of sensory nerve fibres in the mammalian embryo.

I have used a novel quantitative electron microscopic method to determine the rate at which nerve fibres grow towards their targets during development. The rate of recruitment of nerve fibres to the maxillary nerve of the mouse embryo was determined by counting the number of axon profiles in the nerve sectioned close to its emergence from the trigeminal ganglion at closely staged intervals throughout its early development. The rate of change of fibre number with distance along this nerve was determined by counting the number of axon profiles at intervals along the nerve at stages during the period that fibres are growing to their targets. From these two parameters, both of which were linear functions during the midperiod of fibre recruitment to the nerve, it has been possible to calculate that embryonic sensory nerve fibres grow to their peripheral targets at the surprisingly slow rate of just over 20 micron h-1.