A survey of morphogenesis during the early postnatal period in PMBSF barrels of mouse SmI cortex with emphasis on barrel D4.

The long term goal of this work is to understand synaptogenesis in homologous regions of the cerebral cortex, i.e. a whisker barrel. Hemispheres of aldehyde perfused mice, at various ages from P6 to P65 (DOB = P0; three each), were osmicated and sectioned at 40 microns parallel to the pia. Barrels were identified, mapped and measured in sections through mid-level layer IV, and then embedded for electron microscopy. The main findings were: (1) Cell bodies and large diameter dendrites thin out in barrel hollows from P6 to P8. (2) Degeneration occurs primarily from P6 to P11, peaking on P8. (3) Single synapses from narrow, tubular axons predominate before P14; afterwards, multiple synapses from bag-like terminals increase in number. (4) The number of spines increases dramatically between P9 and P12. (5) Asymmetrical and symmetrical synapses occur at all ages studied; their junction lengths are not significantly different at any age. (6) Asymmetrical synapse density increases rapidly from P6 to P8, slowly from P9 to P.12, sharply between P13 and P14 along with patterned whisking, slowly to P20 and drops in adults. (7) Synapses onto spiny and non-spiny stellate cell bodies increase markedly from P10 to P20. (8) Changes in density of asymmetrical synapses in neuropil and of symmetrical synapses on spiny stellate cell bodies follow similar sequences but the sequence in neuropil is 72 h earlier. (9) When barrel size is taken into account, synaptogenesis is monotonic, increasing sharply in the second postnatal week followed by a slower increase into adulthood.