Objective Layer 1 is the top layer of the cerebral cortex, which originates from the neocortical marginal zone (MZ).Although recently studies showed that layer 1 mainly populated Cajal-Retzius (C-R) Cells and some types of interneuron, the developing morphological properties of layer 1 neurons remain incompletely quantified.Using 3-D model neurons reconstructed from neurobiotin-labeled neurons in the mouse neocortical layer 1, this study provides a detailed morphological analysis at early postnatal days (P) from P1 to P14.This quantification provides new insight into the potential value of the layer 1 neuron morphology for cortical information processing.Methods We micro-injected 4% neurobiotin into layer 1 neurons from P1 to P15 under the neocortical horizontal slices.After revealed neurobiotin with Cy3-streptavidin, we reconstructed the neurons and analyzed their morphological data.Results (1) For the interneuron, three developmental periods were revealed, which were characterized by distinct growing rates and properties of alterations in different compartments.From P 1 to P5, the interneurons had a few short dendrites with a low density of spines.From P6 to 8, the extending of dendritic branches and the densities of spines were significantly increased.After P9, almost all compartments of interneuron grew slow.(2) Typical C-R cells showed an ovoid soma, a thick stem dendrite originating from one pole of the soma and a long main axon originating always from the opposite pole of the soma.Two types of C-R ceils according the dendritic branches, bipolar and multi-polar, were observed during P1 to P7.After P8, the number of C-R cells was dramatically decreased.(3) Dye coupling is a frequent observation in the immature neocortex.In about 30% of the neurobiotin-labeled C-R cells dye coupling to a group of 2-20 neurons in layer 1 and 2-5 pyramidal neurons in layers 2-3 were observed, (4) At P 15, we found two types of interneuron in layer 1.Approximately two third of the neurons look like neurogliaform neurons.This kind of neuron had a few dendritic branches and a very dense local axonal field, which was largely contained within layer Ⅰ.Another third were multi-polar neurons with axons descending out of layer Ⅰ.Conclusion Correlation of dendritic branching characteristics with the distribution of two kinds of neurons in layer 1 suggested distinct roles in cortical information processing.