Surface specializations of neurites in embryonic mouse spinal cord

Abstract

Focal surface specializations occur upon axons, axon growth cone filopodia and, somewhat less frequently, upon dendrites as well as dendritic growth cones in embroyonic 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 (∼ 5 nm) less dense layer between the outer surface of the plasma membrane and the thicker (∼ 11.5–15 nm) electron-dense, distal layer of the specializations. The dense layer contains spherical profiles (∼ 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 membrane 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 concanavalian 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 cell located in the intermediate and ventricular zones. No selective enhancement of neurite surfaces occurred which might have correlated with the surface specializations describes above. Finally, the possibility is discussed that the neurite surface specialization might play some role in intercellular recognition phenomena leading to the formation of synaptic juctions and puncta adhaerentia.