Sarah Kucenas1, Pernelle Pulh2, Piotr Topilko2 and Cody J. Smith3 1Department of Biology, University of Virginia, Charlottesville, Virginia 22904, USA 2Institut Mondor de Recherche Biomédicale, Inserm U955-Team 9, 94010 Créteil, France 3Department of Biological Sciences, University of Notre Dame, Notre Dame, Indiana 46556, USA Correspondence: csmith67nd.edu

Neural cells are segregated into their distinct central nervous system (CNS) and peripheral nervous system (PNS) domains. However, at specialized regions of the nervous system known as transition zones (TZs), glial cells from both the CNS and PNS are uniquely present with other specialized TZ cells. Herein we review the current understanding of vertebrate TZ cells. The article discusses the distinct cells at vertebrate TZs with a focus on cells that are located on the peripheral side of the spinal cord TZs. In addition to the developmental origin and differentiation of these TZ cells, the functional importance and the role of TZ cells in disease are highlighted. This article also reviews the common and unique features of vertebrate TZs from zebrafish to mice. We propose challenges and open questions in the field that could lead to exciting insights in the field of glial biology.