Cortical Presynaptic Boutons Progressively Engulf Spinules as They Mature.

Despite decades of discussion in the neuroanatomical literature, the role of the synaptic "spinule" in synaptic development and function remains elusive. Canonically, spinules are finger-like projections that emerge from postsynaptic spines and can become enveloped by presynaptic boutons. When a presynaptic bouton encapsulates a spinule in this manner, the membrane apposition between the spinule and surrounding bouton can be significantly larger than the membrane interface at the synaptic active zone.