Surface specializations of neurites in embryonic mouse spinal cord

Focal surface specializations occur upon axons, axon growth cone filopodia and, somewhat less frequently, upon dendrites as well as dendritic growth cones in embryonic mouse spinal cord. These surface specializations are observed during the early synaptogenic period (embryonic days 12-16) when the axons of the marginal zone are forming synaptic junctions with motor neuron dendrites growing into their domain. At relatively low electron microscopic magnifications, the specializations appear as discrete patches of electron-dense material located just outside the plasma membrane. Higher resolution and an optimum section plane reveal the following ultrastructural characteristics of these specializations. There is a narrow (approximately 5 nm) less dense layer between the outer surface of the plasms membrane and the thicker (approximately 11.5-15 nm) electron-dense, distal layer of the specializations. The dense layer contains spherical profiles (approximately 10-15 nm in diameter) which have relatively clear centers and non-laminar, opaque perimeters. The surface specializations are commonly associated with an accumulation of dense, fibrillar cytoplasmic material that undercoats the cytoplasmic surface of the plasma membrance directly opposite to the external specializations. Furthermore, some of these surface specializations appear to be forming or merging with the cleft material of protosynaptic junctions and primitive puncta adhaerentia-like contacts. We have also examined the pattern of concanavalin A-peroxidase staining on the cell surfaces in embryonic spinal cord. At embryonic day 13, the earliest stage examined in the concanavalin A-peroxidase experiments, there is already an ubiquitous staining around all the cellular processes in the marginal zone as well as around the perikarya and processes of cells located in the intermediate and ventricular zones. No selective enhancement of neurite surfaces occurred which might have correlated with the surface specializations described above. Finally, the possibility is discussed that the neurite surface specializations might play some role in intercellular recognition phenomena leading to the formation of synaptic junctions and puncta adhaerentia.