Neural crest – a population of cells along the neural tube that later will form the peripheral nervous system

Growth cone - first described by Ramon y Cajal – ameboid specialization at the tip of a growing axon

Axon migration – the navigation of axons through the embryonic environment to reach a target (such as a muscle fiber)

Guidepost cells - mark off the end of one pathway and the beginning of the next that developing axons use to migrate to their target

Pioneer axons – the first axons to develop and migrate in the embryo, whose tracts are used by later developing axons to migrate to their target (although not all late-growing axons project along such a pre-established pathway)

Growth cues - the axon migrates in a step-wise, segmental pattern along their trajectory through the coordinate action of growth cues, which are actually extracellular molecules located in the extracellular matrix. Cues can be short-range (local) or long-range, and each can be positive (and attracting) or negative (and repelling). The cues are recognized by specific receptors on the growth cones.

Chemotropic guidance mechanisms - cell migration directed by diffusible chemoattractants and repellants

Chemoattraction - factors are secreted by the targets of developing axons; the factors are diffusible and can attract the axons from a distance to help guide them to the correct site where synaptic contact should be made.

Chemorepulsion – the opposite of chemoattraction; cells located near neurons may secrete a repellant to make sure the axons are “pushed” away in the right direction of their target organ. Another type of chemorepulsion occurs when factors deflect axons away from a particular target; or, factors may assist in keeping the axon in the vicinity of the target organ and not grow beyond (or “overshoot”) the correct destination.

Neurotrophic theory – More neurons than are needed for innervation of a target are formed during the developmental program. Many of these neurons (estimated to be as much as 50% of each class of neurons) will be lost through a process of “programmed cell death”, or apoptosis. Target organs provide limited survival factors for neurons; when synaptic contact is made between a developing axon and its target organ (such as a muscle fiber cell) the ability of the neuron to compete for the survival factor will determine whether it survives, or undergoes programmed cell death. The theory that neuron survival is dependant on their successful competition for neurotrophic factors (proteins secreted by the target organ (or, neighboring cells or from the neuron itself) is known as the neurotrophic theory.