Although recent data indicates that the central nervous system is interactive with the peripheral immune system, the site of cell-cell interaction between brain cells and immune cells remains to be determined. Studies using bone marrow chimeras have consistently shown that bone marrow-derived cells under non-inflammatory conditions are located in the leptomeninges, choroid plexuses, perivascular spaces and circumventricular organs (CVOs), suggesting that these structures are the brain-immune interface. However, there has been some controversy over whether bone marrow-derived cells infiltrate the brain parenchyma, which is protected by the blood-brain barrier. Recently we investigated the distribution, density and differentiation of bone marrow-derived cells in the brain parenchyma in a non-inflammatory condition by intra-bone marrow-bone marrow transplantation (IBM-BMT). In addition to the meningeal space and CVOs, bone marrow-derived cells appear in certain specific brain parenchymal regions and exhibit multiple ramified processes with myeloid lineage differentiation. Most of these discrete regions are adjacent to the attachments of choroid plexus that comprise of thinned brain parenchyma consisting of astrocytic processes in the narrow channel between the ependyma and pia. These astrocytic processes express CX3CL1, a possible chemoattractant for myeloid lineage cells. In the choroid plexus stroma, there are clusters of myeloid cells, 80% of which are replaced by newly generated cells originating in the bone marrow during the 8-month period following IBM-BMT. Furthermore, CXCL12-expressing cells and ER-TR7-expressing cells are present in the choroid plexus stroma. Based on this collection of cells in the choroid plexus, we propose that the choroid plexus stroma may function as a niche for bone marrow-derived myeloid cells and as a mechanism for supplying them continuously to the brain parenchyma via the attachments of choroid plexus. We discuss that potential changes in the dynamics of bone marrow-derived myeloid cells and their interactions with brain cells may be involved in neurological and psychiatric disorders.