Parvalbumin immunoreactive Cajal-Retzius and non-Cajal-Retzius neurons in layer I of different cortical regions of human newborn.

Neurons of layer I play an important role in the development of the basic structural and functional organization of the mammalian cerebral cortex. Basic data, however, concerning the spatial and temporal distribution of the neuron populations in layer I are still limited, especially for human material. The present study investigates the distribution of Cajal-Retzius (CR) and non Cajal-Retzius (NCR) neurons in thirteen cortical areas in the newborn human in terms of their relative density and possible subtypes. Neuronal populations were identified by immunohistochemistry for parvalbumin. Three main results are reported. First, parvalbumin-immunoreactive (Parv-ir) CR cells were observed in all of the neocortical areas examined. These areas also had a Parv-ir horizontal fiber plexus in deep layer I, confirming to the horizontal plexus classically associated with CR neurons. Second, many Parv-ir CR cells showed clear signs of degeneration. Third, in addition to the large CR cells, smaller Parv-ir NCR neurons occurred in many of the neocortical areas examined. These were morphologically heterogeneous and may represent several subtypes. By sampling across several areas, we were able to establish that these NCR cells occurred at higher density in primary sensory areas 3, 1, 17, and 41. Because of this variability in density of Parv-ir NCR cells, the ratio of Parv-ir CR to Parv-ir NCR cells is selectively lower in primary sensory areas. Recent investigations in somatosensory cortex of early postnatal rat report complex spatiotemporal patterns of correlated spontaneous activity among neurons in layer I (Schwartz et al. 1998). An interesting possibility is that regional variability in this activity may play a major role in the organization of cortical circuitry in different areas.