Originally identified by S. Ramon y Cajal (1832-1934) and Gustaf Retzius (1842-1919), they are neurons situated in the marginal zone of the human embryo that have radial ascending connections with pia mater (the innermost membrane enveloping the brain and spinal cord), and a horizontal axon plexus located in the deep marginal zone. Evident in mid-gestation in the human fetes, there is some controversy about their functions and developmental course. One reason is that Retzius cells appear to be distinct from those identified by Cajal and that they correspond more closely to what was previously taken to be human Cajal-Retzius cells. It is now thought that Retzius cells play a role in the migration, placement and inside-outside organisation of the cortical plate neurons. Once they have served these functions, it is believed they disappear, perhaps through apoptosis. However, recent research has established that Retzius cells are transient, and restricted to the period of cortical migration. Cajal cells, in contrast, mature later in development, and may even persist into adult life. Whatever the case, Cajal-Retzius cells (or perhaps just Retzius cells) represent a possible example of an ontogenetic adaptation and a qualitative regression in development.
See Apoptosis (or cell death), Brain (or encephalon), Cell migration, Cortical plate, Neuronal migration disorders, Ontogenetic adaptation, Qualitative and quantitative regressions