Developmental changes of Reelin-expressing cell populations in the marginal zone of the neocortex of the European wild boar, Sus scrofa

Cortical Cajal-Retzius cells (CRc) are the main source of Reelin and essential for radial migration. We studied the development of cells expressing Reelin, Tbr1, p73 and calretinin in pig neocortex from E35 to P30. CRc originated around E35 from the cortical hem and filled the marginal zone (MZ). At E45, in an expanding cortex, CRc were more loosely arranged and more mature. At midgestation E60-E70, CRc with elongated horizontally oriented somata and long dendrites formed a dense network of processes in the upper MZ. Their axons were characterized by the presence of long axon initial segments and densely ramified in the lower MZ. Concurrently, the first GAD-positive/Reelin-positive as well as GAD-positive/Reelin-negative interneurons appeared in the MZ and in cortical layers. At E85, in an already highly gyrated cortex, the switch in Reelin-positive neuronal populations was largely completed. At E100 and P30, CRc were rare. A subpial granular layer was not detectable in pig. In sum, by form, distribution and the presence of a calretinin-positive axon plexus in the lower MZ, the pig CRc closely resembled the lagomorph and carnivore rather than the human morphotypes, and they become complemented by interneurons already after midgestation.