Emerging Complexity of Layer I in Human Cerebral Cortex

0 Oxford University Press 2003. All rights reserved 1 Present address: Department of Anatomy and Developmental Biology, University College London , UK 2 Department of Neuroscience, University of Connecticut School of Medicine , Farmington, CT 06030, USA In this study we examine possible origins and migratory routes of human cortical neurons, with special emphasis on the preplate and layer I. In embryonic stages, two main cell types, Cajal-Retzius cells, and cells labeled with interneuron markers (calretinin, calbindin and GABA), were present in the preplate layer. In addition, a number of preplate GABAergic cells co-expressed either Nkx2.1 or Dlx transcription factors, findings consistent with their origin in the ganglionic eminence and subsequent tangential migration to the layer I. The orientation of the leading process indicates that some of these cells descend to the cortical plate. However, the finding of radially oriented GABAergic, NKX2.1+ and DLX+ cells in the cortical ventricular zone, argues that, unlike in rodents, a significant subpopulation of these cells originates in the cortical ventricular zone. In embryonic stages, expression of Reelin in Cajal-Retzius cells as well as Reelin/DLX2+ cells in the embryonic ganglionic eminence and the olfactory region, suggest that these cells in human may have diverse origins. In later fetal stages in human (17-22 gestational weeks) layer I and the newly formed subpial granular layer, contained a population of small interneurons that originated mainly in the lateral ganglionic eminence, since the majority of these cells were double-labeled with DLX/GABA, and rarely with NKX2.1/GABA. Therefore, neurons in the human cortical layer I are heterogeneous, with more complex origin and migratory routes than in rodents. In addition to the ganglionic eminence, both the expended subventricular zone and subpial granular layer, contribute to the neuronal population of the developing layer I and underlining cortical plate. Introduction Because of the recent advances made in understanding the development of cortical interneurons in rodents (Parnavelas, 2000; Marin and Rubenstein, 2001), non-human primates (Zecevic and Rakic, 2001) and humans (Letinic et al., 2002), it is important to re-examine the development of human layer I. There is good evidence that layer I in primates contains a greater variety of neurons, including interneurons, in comparison to subprimate species (Zecevic and Milosevic, 1997; Meyer and Goffinet, 1998; Zecevic et al., 1999; Zecevic and Rakic, 2001). The available literature indicates several possible sources of cortical cells, some of which require migration of precursor cells through divisional boundaries. One such source is the ganglionic eminence (GE) of the ventral telencephalon, which has two parts, medial (MGE) and lateral (LGE), that are distinguishable in humans from Carnegie stage 18 (ORahilly and Muller, 1994; Grasbon-Frodl et al., 1996). In primates, the GE was considered to be the main source of basal ganglia neurons (Brand and Rakic, 1979; Sidman and Rakic, 1982), as well as the subclass of interneurons for the thalamus (Rakic and Sidman, 1969; Letinic and Rakic, 2001) and in rodents, cerebral cortex interneurons (De Carlos et al., 1996; Anderson et al., 1997a, 1999, 2001; Tamamaki et al., 1997; Chapouton et al., 1999; Lavdas et al., 1999; Wichterle et al., 1999). The GE is also a major source of forebrain oligodendrocytes (He et al., 2001; Marshall and Goldman, 2002; Ulfig et al., 2002; Rakic and Zecevic, 2003). In contrast to pyramidal cells that migrate radially from the cortical ventricular zone (Sidman and Rakic, 1973; Rakic, 1974, 1988), cortical interneurons use tangential migration to reach the developing cortex. Dlx 1 and 2 (Anderson et al., 1997a,b, 2001; Eisenstat et al., 1999; Corbin et al., 2001), Nkx 2.1 (Sussel et al., 1999; Marin et al., 2000) and Lhx 6.1 (Lavdas et al., 1999) homeobox genes, which in rodents are expressed in the GE, are thought to play a role in the differentiation and tangential migration of cortical interneurons from the ventral forebrain to the neocortex. These transcription factors have the advantage of being region specific in early development, and thus can provide the information about the site of origin for different cell types later on, after the cell migrated to different, often distant, regions. Although much has been learned in the past few years about regulation of genesis and migration of cortical interneurons in rodents, information in primates is still scarce. In the monkey, diffuse expression of Dlx1 mRNA in layer I indicated that in primates, too, some cortical neurons have a subcortical origin (Zecevic and Rakic, 2001). However, in contrast to rodents, where the majority if not all cortical interneurons originate in the GE, in primates a large number of cortical interneurons seem to originate in the cortical ventricular (VZ) and subventricular (SVZ) zones (Rakic and Zecevic, 2001; Zecevic and Rakic, 2001; Letinic et al., 2002). Therefore, in human brains, several proliferative zones, the GE, cortical VZ and SVZ, as well as the subpial granular layer (SGL), are potential sources of layer I neurons in later fetal development. The migratory routes of neurons from these different sources are more complex than has been recognized. For example, the SGL that is much more prominent and last longer in primates, has been suggested to serve as a conduit for late generated neurons coming from the olfactory region (Meyer and Goffinet, 1998; Meyer and Wahle, 1999; Zecevic and Rakic, 2001). Moreover, here we observed a dynamic trafficking in both directions, as judged by the orientations of the leading processes of migratory interneurons, at the subcorticalcortical junction, and in the subventricular and intermediate zones of human fetal brains. Primate CajalRetzius cells are among the first cells to appear in the primordial plexiform layer (PPL) (Meyer and Goffinet, 1998; Zecevic et al., 1999; Meyer et al., 2000; Zecevic and Rakic, 2001) but their exact origin is still under investigation. Furthermore, genesis of layer I neurons in primates last throughout the entire course of cortical neurogenesis, which complicates interpretation of their pedigree (Zecevic and Rakic, 2001). The role of CajalRetzius cells in secreting the glycoprotein Reelin, which is necessary for normal migration and regular layering of cortical plate neurons, is well described (DArcangelo et al., 1995, 1997; Ogawa et al., 1995; Del Rio et al., 1997). It appears likely that several classes of CajalRetzius neurons exist, and that they have different origins (Zecevic and Rakic, 2001; Meyer et al., 2002). The normal development of layer I in humans may have considerable clinical importance, since interneurons play a substantial role in the function of the cerebral cortex, and their impairment has long been suspected in some psychiatric disorders. CajalRetzius cells also have an import (...truncated)