Thymic epithelial cells (TECs) are one of the most important components in thymic microenvironment supporting thymocyte development and maturation. TECs, composed of cortical and medullary TECs, are derived from a common bipotent progenitor, mediating thymocyte positive and negative selections. Multiple levels of signals including intracellular signaling networks and cell-cell interaction are required for TEC development and differentiation. Transcription factors Foxn1 and autoimmune regulator (Aire) are powerful regulators promoting TEC development and differentiation. Crosstalks with thymocytes and other stromal cells for extrinsic signals like RANKL, CD40L, lymphotoxin, fibroblast growth factor (FGF) and Wnt are also definitely required to establish a functional thymic microenvironment. In this review, we will summarize our current understanding about TEC development and differentiation, and its underlying multiple signal pathways.
Cell Communication ; genetics ; Cell Differentiation ; Epithelial Cells ; cytology ; metabolism ; Forkhead Transcription Factors ; genetics ; metabolism ; Humans ; Signal Transduction ; genetics ; Thymocytes ; cytology ; metabolism ; Thymus Gland ; cytology ; growth & development ; Transcription Factors ; genetics ; metabolism

Bone mesenchymal stem cells (BMSCs) have been used worldwide to treat spinal cord injury, but their therapeutic mechanism is poorly understood. In this study, BMSCs were transplanted to aneurysm clip-injured rats to demonstrate their protective effect. We observed myelin sheaths through Luxol fast blue (LFB) staining, osmic acid staining, TUNEL and transmission electron microscopy (TEM). We performed Western blotting to analyze the expressions of brain-derived neurotrophic factor (BDNF) and caspase 3. BMSCs were transplanted at 1, 7 and 14 days after spinal cord injury. Hindlimb movement (Basso, Beattie and Bresnahan; BBB) score, CNPase (2', 3'-cyclic-nucleotide 3'-phosphodiesterase), myelin basic protein (MBP) and caspase 3 protein levels were detected. Immunofluorescence was used to test the differentiation of BMSCs after implanted into damaged spinal cord and co-expression of CNPase-caspase 3+. At 7 days after BMSCs transplantation, some injected BMSCs expressed neuronal and oligodendrocyte markers. And both locomotor skills and ultra-structural features of myelin sheaths were significantly improved. The expressions of BDNF were clearly increased by BMSCs transplantation, the expression of caspase 3 was the opposite. Compared with the 1 and 14 days transplantation after spinal cord injury, MBP and CNPase expressions were highest, caspase 3 expression was lowest in 7 days BMSCs transplantation. After BMSCs transplantation, CNPase-caspase 3+ cells scattered in the white matter of the spinal cord. Therefore, BMSCs had a tendency to differentiate into neurons and oligodendrocytes after transplantation, which could promote the secretion of BDNF. BMSCs protected neural myelin sheaths by inhibiting oligodendrocyte apoptosis via increased secretion of BDNF after SCI. The best therapeutic time was 7 days after spinal cord injury.