Introduction: Potential stem cell niches (SNs) were recently reported in intervertebral discs (IVDs) and knee joints\r\n(KJs) in different mammals (located adjacent to the epiphyseal plate; EP). The aim here was to examine further\r\npossible cellular migration and migration directions of cells originating from niches possibly involved in\r\nregeneration of cartilaginous tissues in the IVD and in the KJ regions in adult mammals.\r\nMethods: In total, 33 rabbits were used in studies A through C.\r\nA. IVD cells were sorted; fluorescence-activated cell sorting (FACS) by size (forward scatter; =10 �µm or >10 �µm or\r\nGDF5 cells (anti-GDF5 antibody). Sorted cells, labeled with cell tracer (carboxyfluorescein-diacetatesuccinimidyl\r\nester; CDFA-SE) were applied on IVD explants in vitro. Migrating cells/distance was evaluated by\r\nfluorescence- and confocal-microscopy (FC).\r\nB. DNA labeling was performed with BrdU (oral administration). Animals were killed (14 to 56 days), KJs collected,\r\nand BrdU cells visualized with immunohistochemistry (IHC)/anti-BrdU antibody in SN and articular cartilage (AC).\r\nC. Cell tracer: (Fe-nanoparticles: Endorem) were injected into SNs of IVDs (LI-LV) and KJs (tibia). Animals were\r\nkilled after 2 to 6 weeks. Fe-labeled cells were traced by ferric-iron staining (Prussian blue reaction; Mallory\r\nmethod).\r\nResults: A. GDF5 cells and =10-�µm cells displayed the best migration capability in IVD explants. GDF5 cells were\r\ndetected at a tissue depth of 1,300 �µm (16 days). B. BrdU cells were observed in early time points in niches of KJs,\r\nand at later time points in AC, indicating a gradual migration of cells. C. Fe cells were detected in IVDs; in annulus\r\nfibrosus (AF) in 11 of 12 animals and in nucleus pulposus (NP) in two of 12 animals. In AC (tibia), Fe cells were\r\ndetected in six of 12 animals. In the potential migration route (PMR), from niches toward the IVD, Fe cells (three of\r\n12 animals) and in PMR toward AC (KJs) (six of 12 animals) were detected.\r\nConclusions: Results indicate similar cellular migration patterns in cartilage regions (IVD and KJs) with migration\r\nfrom stem cell niche areas into the mature cartilaginous tissues of both the KJs and the IVD. These findings of a\r\ncellular migration pattern in mature cartilage are of interest from tissue-repair and engineering perspectives.
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