Feature Article: What is a Cajal-Retzius cell? A Reassessment of a Classical Cell Type Based on Recent Observations in the Developing Neocortex

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Abstract
The term ‘Cajal–Retzius cell' is applied to neurons of the human embryonic marginal zone which display, as a salient feature, radial ascending processes that contact the pial surface, and a horizontal axon plexus located in the deep marginal zone. These cells were first described by Retzius (Retzius, 1893, 1894) (see the cell labelled ‘Retzius, 1893' in Fig. 1 ). There is consensus that homologous elements are present in the non-primate neocortex, where their morphology is much simpler, as initially described in 1891 by Cajal (Fig. 2 ) (Cajal, 1891). In spite of several reviews (Duckett and Pease, 1968; König, 1978; Huntley and Jones, 1990; Marín-Padilla, 1978, 1990; Supèr et al., 1998), the definition of these cells has remained somewhat confusing, in part because Cajal and Retzius studied different species and different developmental stages, and also because their original publications have not been generally available. In fact, the cells drawn by Cajal (Cajal, 1899a,b, 1911) from his human material (see the cell labelled A, ‘Cajal, 1899' in Fig. 1 ) appear morphologically so different from those described by Retzius (Retzius, 1893, 1894) that they must belong to different, although possibly related, cell classes. The discrepancies in the observations by Retzius and Cajal could reflect vagaries of the Golgi method, or different magnifications or drawing techniques, rather than actual morphological differences in the cell types represented. However, recent data have confirmed the accuracy of the original observations. The morphological definition of Cajal–Retzius cells is thus based on observations of rather disparate cells that may not be homologous in all mammalian species.

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