Fig. 2.

Ultrastructural analysis of adult-born mossy fiber synaptic outputs in the CA3 subfield. (A–D) 3D reconstructions (Left) of serial sections from mossy fiber boutons in the adult mouse with accompanying high-magnification electron micrographs (Right) at 2 wpi (A), 4 wpi (B), 8 wpi (C), and 16 wpi (D). At 2 wpi, synaptic contacts are visible only onto the dendritic shaft (A; arrowheads). Over time, boutons became more mature with synaptic contacts onto dendritic spines (B–D; arrowheads). (E) 3D reconstruction of a mature, unlabeled mossy fiber bouton in the CA3 subfield and accompanying low-magnification (Lower Left) and high-magnification (Lower Right) electron micrographs. The bouton has been pseudocolored in green in the low-magnification image, and the high-magnification image shows synaptic contacts onto an invading dendritic spine (arrowheads). (Scale bars in 3D reconstructions and low-magnification micrograph, 1 μm; in high-magnification micrographs, 0.5 μm.) Green, axonal bouton; purple, dendrite; yellow, synapse. D, dendrite; M, mitochondria; S, dendritic spine; T, axon terminal. (F) Mossy fiber bouton staging paradigm to describe the overall maturation state of boutons across development (see Methods). Shown is the percentage of boutons categorized in each stage for all developmental time points. This suggests that boutons born in the adult brain are qualitatively similar to mature, unlabeled boutons by 8 weeks.