Development of Synaptic Boutons in Layer 4 of the Barrel Field of the Rat Somatosensory Cortex: A Quantitative Analysis

Cereb Cortex. 2016 Feb;26(2):838-854. doi: 10.1093/cercor/bhv270. Epub 2015 Nov 15.


Understanding the structural and functional mechanisms underlying the development of individual brain microcircuits is critical for elucidating their computational properties. As synapses are the key structures defining a given microcircuit, it is imperative to investigate their development and precise structural features. Here, synapses in cortical layer 4 were analyzed throughout the first postnatal month using high-end electron microscopy to generate realistic quantitative 3D models. Besides their overall geometry, the size of active zones and the pools of synaptic vesicles were analyzed. At postnatal day 2 only a few shaft synapses were found, but spine synapses steadily increased with ongoing corticogenesis. From postnatal day 2 to 30 synaptic boutons significantly decreased in size whereas that of active zones remained nearly unchanged despite a reshaping. During the first 2 weeks of postnatal development, a rearrangement of synaptic vesicles from a loose distribution toward a densely packed organization close to the presynaptic density was observed, accompanied by the formation of, first a putative readily releasable pool and later a recycling and reserve pool. The quantitative 3D reconstructions of synapses will enable the comparison of structural and functional aspects of signal transduction thus leading to a better understanding of networks in the developing neocortex.

Keywords: 3D reconstructions; electron microscopy; pool of synaptic vesicles; synapse geometry; synaptogenesis.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Age Factors
  • Animals
  • Animals, Newborn
  • Image Processing, Computer-Assisted
  • Microscopy, Electron
  • Nerve Net / ultrastructure
  • Presynaptic Terminals / physiology*
  • Presynaptic Terminals / ultrastructure
  • Rats
  • Rats, Wistar
  • Somatosensory Cortex / cytology*
  • Somatosensory Cortex / growth & development*
  • Synapses / physiology*
  • Synapses / ultrastructure
  • Synaptic Vesicles / ultrastructure