The changing roles of neurons in the cortical subplate

Michael J. Friedlander

doi:10.3389/neuro.05.015.2009

Abstract

Neurons may serve different functions over the course of an organism's life. Recent evidence suggests that cortical subplate (SP) neurons including those that reside in the white matter may perform longitudinal multi-tasking at different stages of development. These cells play a key role in early cortical development in coordinating thalamocortical reciprocal innervation. At later stages of development, they become integrated within the cortical microcircuitry. This type of longitudinal multi-tasking can enhance the capacity for information processing by populations of cells serving different functions over the lifespan. Subplate cells are initially derived when cells from the ventricular zone underlying the cortex migrate to the cortical preplate that is subsequently split by the differentiating neurons of the cortical plate with some neurons locating in the marginal zone and others settling below in the SP. While the cortical plate neurons form most of the cortical layers (layers 2–6), the marginal zone neurons form layer 1 and the SP neurons become interstitial cells of the white matter as well as forming a compact sublayer along the bottom of layer 6. After serving as transient innervation targets for thalamocortical axons, most of these cells die and layer 4 neurons become innervated by thalamic axons. However, 10–20% survives, remaining into adulthood along the bottom of layer 6 and as a scattered population of interstitial neurons in the white matter. Surviving SP cells' axons project throughout the overlying laminae, reaching layer 1 and issuing axon collaterals within white matter and in lower layer 6. This suggests that they participate in local synaptic networks, as well. Moreover, they receive excitatory and inhibitory synaptic inputs, potentially monitoring outputs from axon collaterals of cortical efferents, from cortical afferents and/or from each other. We explore our understanding of the functional connectivity of these cells at different stages of development.

Highlights

Most of these cells die soon after the innervation of the cortical plate by thalamic axons and the retraction of the SP neurons’ axons that innervate layer 4, many of them (10–20%) survive (Chun and Shatz, 1989; Torres-Reveron and Friedlander, 2007)
SUMMARY There still remain considerable issues to be resolved regarding the role of these intriguing SP neurons within the mature neocortex
How do the surviving cells avoid elimination during development? Which cells provide the synaptic inputs to these neurons? What are the functional properties of these neurons in vivo? What role do the many neuromodulators released by these cells play in information processing? Do these surviving white matter and SP neurons retain the capacity to re-enable early developmental processes in the adult cortex after injury or disease? The answers to many of these questions must await experiments where these cells are studied in the adult brain in vivo with selective targeting techniques

Summary

Introduction

Most of these cells die soon after the innervation of the cortical plate by thalamic axons and the retraction of the SP neurons’ axons that innervate layer 4, many of them (10–20%) survive (Chun and Shatz, 1989; Torres-Reveron and Friedlander, 2007).

Results

Conclusion