Subplate neurons: A missing link among neurotrophins, activity, and ocular dominance plasticity?

The subplate is a transient structure comprised of a subset of the earliest neurons produced in the cerebral cortex (1). Although it has now been almost 30 years since the subplate was first described (2), a definitive function for the subplate remains unproven. In general, the subplate is believed to be important for the formation of connections between thalamus and cortex. Subplate neurons have been hypothesized to pioneer both the feedforward thalamocortical and the feedback corticothalamic pathways, guide selection of the correct cortical area by thalamic axons, participate in specification of cortical areas, and provide a transient target for thalamic axons until their target neurons in the cortical plate are born (3). In addition to these early developmental functions, the subplate may influence later events in the development of functional organization within the cortical plate. Specifically, subplate neurons may play a role in the formation of ocular dominance (OD) columns in primary visual cortex (4, 5). Despite the importance of this function, the cellular mechanisms that mediate subplate influences on thalamocortical connectivity remain a mystery. The paper by Lein et al. in a recent issue of PNAS (6) uncovers a molecular link between subplate neurons and cortical activity that helps to explain the role of the subplate in formation of OD columns.

Subplate neurons are not only the first neurons generated in the cortex, they are also the earliest to mature, differentiate, and make functional connections. Subplate neurons are the first cortical neurons to receive synaptic inputs from thalamic axons (refs. 4, 7, and 8; Fig. 1). During fetal development, thalamic axons invade the subplate and wait there for a significant period of time, ranging from 2 weeks in the cat to a month in the primate, before invading the cortical plate and forming synaptic connections …