Axonal growth cones and developing axonal collaterals form synaptic junctions in embryonic mouse spinal cord.

Axonal growth cones and developing axonal collaterals have been studied in electron microscopic and Golgi preparations of embryonic mouse cervical spinal cord. These structures are first observed on embryonic day 11 (E11), and by E12 both axonal growth cones and developing collaterals are observed to be the presynaptic elements of developing synaptic junctions. These relatively undifferentiated portions of axons comprise nearly one-half of the presynaptic components of the synaptic contacts observed on E12, and they continue to constitute about this same proportion during the rest of the embryonic period examined (E13–16). These early synaptic interactions of developing axonal collaterals may be involved in establishing the basic ramification patterns of mature axons by determining which developing collaterals will continue to grow and differentiate. Coated vesicles are observed fused with axonal collateral plasma membranes at the perimeters of developing synaptic contacts. Occasionally thin extensions of postsynaptic surfaces protrude into these coated vesicles, and similar relationships are also observed between presynaptic surfaces and coated vesicles fused with the membranes of postsynaptic elements. Such coated vesicle/ synaptic surface relationships may represent an early stage in the endocytosis of synaptic membranes by both the pre- and postsynaptic elements of developing synaptic contacts. This might represent a mechanism for the exchange of ‘molecular information’ between the components of protosynaptic contacts, and the results of such an exchange could be involved in determining whether protosynaptic contacts will differentiate into synaptic junctions. Developing axonal collaterals exhibit a preferential growth toward the sources of potential postsynaptic elements, and this growth implies that the axons present in the embryonic marginal zone are actively searching for appropriate postsynaptic processes rather than passively awaiting their arrival. Many of the collaterals weave around numerous intervening processes in order to reach their postsynaptic destinations, and unusual subaxolemmal lamellae are commonly observed where collaterals make acute curvatures around neurites which block their direct access to postsynaptic processes. The location of these lamellae suggests that they could be involved in producing changes in the direction of growth as developing axonal collaterals search for synaptic partnerships. Main axonal shafts commonly exhibit synaptic contacts adjacent to where they give rise to developing collaterals, and this observation hints that the formation of synapses by axonal shafts might stimulate collateral sprouting. However, other observations (see text) indicate that the formation of synaptic contacts by axonal shafts is not an obligatory prelude to collateral development. Therefore synaptic interactions of main axonal shafts do not appear to be a requirement for the primary induction of collateral development, but they may facilitate collateral growth by providing a confirmation that axons are located in appropriate synaptogenic fields.