[Demonstration of Cajal-Retzius cells and their processes in the neocortex of newborn mice using horseradish peroxidase].

In newborn mouse neocortex, the so-called Cajal-Retzius cells (CRc), which are usually considered as neurons due to their polarity, located exclusively in the first cortical layer, were visualized using local application of horseradish peroxidase (HRP) on the neocortex followed by its tangential sectioning. Close to the application site, the exogenous enzyme, taken up by some of the CRc, was revealed with 3,3' diaminobenzidin (DAB) according to the Graham and Karnovsky technique. The brown reaction product was seen to fill almost completely some of these cells giving a "Golgi like" picture. They had a fusiform bipolar pericaryon and processes extending only into the first cortical layer. A single thick process whose length reached 300 to 400 microns, almost rectilinear and tapering progressively at its end was a dendrite which bore thin expansions reaching the cortical surface where they sometimes ramified between the endfeet of the radial glia. The dendrite sometimes showed symmetrical synapses with an afferent axon of unknown origin. The CRc axon was very thin (0.5 microns in diameter) and gave off at random numerous collaterals whose number and trajectories varied greatly from one cell to the other. The axonal processes could often be followed over a millimeter. They ended either abruptly because HRP had not diffused far enough into the process or terminated with large growth cones bearing numerous digitiform filopodia. The presence of growth cones thus suggested that the processes were exploring the cellular environment of the first cortical layer. In the newborn, CRcs appeared as still immature neurons.