The organization of paramembranous densities during postnatal maturation of synaptic junctions in the cerebral cortex

Abstract

The postnatal maturation of synaptic junctions in layers I and II of the somatic sensory cortex has been studied by selectively staining with ethanolic phosphotungstic acid (E-PTA) the paramembranous synaptic densities in unosmicated tissue taken from newborn through 21-day-old kittens. The presynaptic component of an immature synapse consists of a narrow band or plaque of moderately dense material. This gradually develops into a series of interconnected triangular projections during the first postnatal week. Later, the presynaptic dense projections increase in height but remain joined at their bases by thin filamentous densities. Cleft material is present and restricted to the contact region at all stages of maturation of the synaptic junction. The postsynaptic component is visualized initially as a band or plaque of dense material slightly thicker than the presynaptic density. By the tenth day, the postsynaptic density has thickened and developed focal densities. At this stage, two types of synaptic junctions are distinguished according to the degree of focalization or separation into discrete units of the paramembranous densities. During the third week, the organization of the presynaptic dense projections and the cleft densities is unchanged. However, a thin unstained band subdivides the postsynaptic component of most junctions into inner and outer dense bands. Very few definitive examples of PTA-stained components of the dendritic spine apparatus are found during the third week, whereas they are found readily in adult preparations. This suggests that certain components of the spine apparatus develop later than 21 days.