The neocortex, the complex strata of neurons in the mammalian cerebrum, comprises the ridges and grooves seen on the top layer of any 3-D depiction of the human brain. It is involved in processes such as sensory perception, language, and conscious thought. 

In the 1890s, Cajal and another scientist, Gustaf Retzius, independently identified bipolar neurons with horizontal nuclei in the developing neocortex of different types of mammals.  Shown in layer d of the drawing above, these cells – now known as Cajal-Retzius cells – are not only important for the transmission of information through the neocortex, but they also play a role in brain development. They secrete a protein called Reelin, which is a critical component of a signaling pathway for neuronal migration, ensuring that new cortical cells migrating along the long radial glia (a) and into the upper layers of the neocortex will stop when they reach the correct position.

Diseases associated with a lack of Reelin expression include schizophrenia, bipolar disorder, and autism. Although Cajal was not aware of the existence of Reelin, or the developmental importance of the Cajal-Retzius cells, he accurately captured many of the major features of neocortical development using only his microscope.

This plate was created by Leo Guzman and Jeremy Swan for an exhibit in the Porter Neuroscience Building at the NIH and is being provided here, courtesy of the Cajal Institute (Neuroscience Research Center of the Spanish Main Research Council) Producido con el permiso de El Instituto Cajal (IC), Consejo Superior de Investigaciones Científicas (CSIC).  As such, it's being shared under a creative commons license.  Reuse of these materials should be attributed to the Cajal Institute.