Cajal-Retzius cells: update on structural and functional properties of these mystic neurons that bridged the 20th century.

Cajal-Retzius cells (CRc) represent a mostly transient neuronal cell type localized in the uppermost layer of the developing neocortex. The observation that CRc are a major source of the extracellular matrix protein reelin, which is essential for the laminar development of the cerebral cortex, attracted the interest in this unique cell type. In this review we will (i) describe the morphological and molecular properties of neocortical CRc, with a special emphasize on the question which markers can be used to identify CRc, (ii) summarize reports that identified the different developmental origins of CRc, (iii) discuss the fate of CRc, including recent evidence for apoptotic cell death and a possible persistence of some CRc, (iv) provide a detailed description of the electrical membrane properties and transmitter receptors of CRc, and (v) address the role of CRc in early neuronal circuits and cortical development. Finally, we speculate whether CRc may provide a link between early network activity and the structural maturation of neocortical circuits.