Human thymic epithelium and T cell development: current issues and future directions.

The human thymus develops early in fetal gestation with morphologic maturity reached by the beginning of the second trimester. TE3+ cortical thymic epithelium is most likely derived from endodermal third pharyngeal pouch, while A2B5/TE4+ medullary and subcapsular cortical thymic epithelium is likely derived from third pharyngeal cleft ectoderm. Fetal liver and yolk sac CD7+, CD4-, CD8-, surface(s) CD3- T cell precursors begin to colonize the thymus between 7 and 8 weeks of fetal gestation, followed by rapid expression of other T lineage surface molecules on developing thymocytes. CD7+, CD4-, CD8-, sCD3- thymocytes give rise to T cells of both the TCR alpha beta and TCR gamma delta lineages. Human thymic epithelial cells produce numerous cytokines including IL1, IL6, TGF alpha, leukemia inhibitory factor (LIF), M-CSF, G-CSF and GM-CSF- molecules that likely play important roles in multiple stages of thymocyte selection, activation and differentiation. Important areas for future research on human thymic epithelium include study of lymphoid and non-lineage differentiation potentials of CD7+, CD4-, CD8-, sCD3- T cell precursors in response to TE-cell produced cytokines, study of the triggering signals of cytokine release within the thymic microenvironment, and study of TCR-MHC mediated TE-thymocyte interactions.

人胸腺在胎儿妊娠早期发育，在妊娠中期开始时达到形态学成熟。TE3 +皮质胸腺上皮很可能源自内胚层第三咽囊，而A2B5/TE4 +髓质和被膜下皮质胸腺上皮可能源自第三咽裂外胚层。胎儿肝脏和卵黄囊的CD7 +、CD4 -、CD8 -、表面CD3 - T细胞前体在胎儿妊娠7至8周之间开始定植于胸腺，随后在发育中的胸腺细胞上快速表达其他T谱系表面分子。CD7 +、CD4 -、CD8 -、表面CD3 -胸腺细胞产生TCRαβ和TCRγδ谱系的T细胞。人胸腺上皮细胞产生多种细胞因子，包括IL1、IL6、TGFα、白血病抑制因子（LIF）、M - CSF、G - CSF和GM - CSF等分子，这些分子可能在胸腺细胞选择、激活和分化的多个阶段发挥重要作用。未来人胸腺上皮研究的重要领域包括研究CD7 +、CD4 -、CD8 -、表面CD3 - T细胞前体在应对胸腺上皮细胞产生的细胞因子时的淋巴细胞和非谱系分化潜能，研究胸腺微环境内细胞因子释放的触发信号，以及研究TCR - MHC介导的胸腺上皮 - 胸腺细胞相互作用。