A freeze-fracture study of synaptic junction development in the superficial layers of the chick optic tectum.

Synaptogenesis in the superficial layers of the rostral pole of the chick optic tectum has been studied using freeze-fracture techniques. The developmental sequence of intramembrane organization at synaptic junctions involves the accumulation and assembly of intramembrane particles into aggregates characteristic of the mature junctions. By embryonic day seven, areas of loosely-arranged clusters of medium-sized particles are observed on the cytoplasmic membrane leaflets (P-faces) of developing neurites. These clusters are characteristic of the intramembrane organization at presynaptic active zones. At later stages, small pits, characteristic of vesicle fusion sites, are observed interspersed among such P-face particle clusters. Complementary intramembrane specializations are also present on the external leaflets (E-faces) of presynaptic membranes at the active zones. Small solitary aggregates of large-sized particles on the E-faces of neurite plasma membranes are also seen at early embryonic stages. As development progresses, these aggregates increase in size and packing density and occupy large oval domains in postsynaptic membranes. These intramembrane specializations may represent the postsynaptic active zones of asymmetric synapses. Another type of intramembrane specialization, observed during the third week of incubation, is characterized by aggregates of small- and medium-sized particles on the P-face of postsynaptic membranes and is often seen directly apposed to the E-face of a presynaptic terminal. This type of intramembrane specialization may represent the postsynaptic active zone region at symmetrical synaptic contacts.