The synaptic structure in the CA1 region of rat hippocampus at postnatal day 90 was observed under transmission electron microscopy (37,000x).

The synaptic structural parameters were measured as described in the section of Materials and Methods. Also, the synaptic structural parameters of control (A), Pb (pre-weaning) (B), and Pb (post-weaning) group (C) were compared in Table 5. The ultrastructural features of synapses (insets a–c) in three groups (A, B, C) and the synaptic curvature measurement (D) were illustrated. The length of synaptic active zone was indicated by two arrowheads. The thickness of postsynaptic density (PSD) was at the thickest part of PSD and limited by two long arrowheads; Po and Pn showed the synaptic curvature measurement points; SC represents the synaptic cleft; SV represents synaptic vesicles.