Novel calretinin and reelin expressing neuronal population includes Cajal-Retzius-type cells in the neocortex of adult pigs.

Cajal-Retzius cells and their secreted product reelin are essential for the lamination of the cerebral cortex. In all species studied to date Cajal-Retzius cells form a transient neuronal population that almost completely disappears from the neocortex postnatally. Recently, in the hippocampal formation of adult domestic pig, we have found a large calretinin- and reelin-immunoreactive cell population that morphologically corresponded to Cajal-Retzius cells. In the present study, we examined calretinin- and reelin-immunoreactive neurons in layer I of the prefrontal, temporal, parietal and occipital neocortical areas of newborn, young adult and adult domestic pigs. Large numbers of bipolar or fusiform calretinin-positive cells were found in the upper half of layer I in all examined age groups. The morphology of these neurons resembled that of the Cajal-Retzius cells. Layer I was occupied by a dense calretinin-positive axonal plexus that was similar to the previously described axons of Cajal-Retzius cells in other species. In a similar location, where calretinin-positive cells occurred in layer I, large numbers of reelin-immunoreactive cells were found in all examined age groups. In addition, reelin colocalized with calretinin in layer I neurons. The number of calretinin and reelin-positive neurons decreased from 1 day to one year, but calretinin-positive Cajal-Retzius-type cells still comprised a remarkable large population in 12-month-old animals. Correlated light and electron microscopic examination of calretinin-labeled Cajal-Retzius-type cells indicated that these cells are integrated in the synaptic circuitry of the neocortex. Our results suggest that Cajal-Retzius cells do not disappear inevitably from the mature neocortex in all mammalian species. The function of this cell type is not known, but late persisting Cajal-Retzius-type cells in the domestic pig provide an opportunity to study their neuronal connections and the possible role of reelin in plasticity and regeneration of neocortex.