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Microglia
Microglia inherently play a role in synaptic pruning during adolescence. As resident immune cells of the central nervous system, microglia’s main role is phagocytosis and engulfment. Studies have found that during critical periods in the visual cortex, neural synapses become the target of microglial phagocytosis. Neurons who received less frequent input from retinal ganglion cells during early postnatal periods were more prone to be engulfed and pruned by microglia, as per monocular deprivation experiments. Similar results were found when manipulating G-coupled purinergic receptors on microglial processes. Blocking these receptors or performing a knockout experiment significantly lowered microglial interactions and synaptic pruning during the early visual cortex critical period. More recently, the expression of the complement component 4 gene has been found to significantly contribute to abnormally high levels of microglial synaptic pruning during early stages of development in schizophrenic neurons and microglia, suggesting a genomic connection between the immune system and critical periods.

Spine Motility
Dendritic spine motility is the altering of the dendritic morphology of a neuron, specifically the appearing and disappearing of the small protrusions known as spines. In early postnatal development, spine motility has been found to be at very high levels. Due to its most pronounced occurrence during postnatal days 11 through 15, spine motility is thought to have a role in neurogenesis. Motility levels significantly decrease before the start of the visual cortex critical period and monocular deprivation experiments show that motility levels steadily decrease until the critical period is over, hinting that motility might not be explicitly involved in this process. However, binocular deprivation before eye-opening resulted in a significant up-regulation of spine motility until the peak of the critical period, resulting in controversial findings regarding the role of dendritic spine motility.