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. 2020 Jan 29;16(2):239–250. doi: 10.1007/s12015-020-09955-y

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© The Author(s) 2020

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

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Fig. 1. — Human thymus cell architecture. The human thymus is located in the upper anterior part of the chest behind the sternum between lungs and lies on top of the heart along the trachea. The thymus reaches its maximum weight (about 28 gram) during puberty. This pinkish-gray organ consists of two lobes parted into lobules by connective tissue strands (trabeculae). Each thymic lobule has a cortex and medulla. Hematopoietic precursor cells (HPC) enters the thymus through postcapillary venules located at the corticomedullary junction (CMJ) and migrate to the capsule, committed CD4-CD8- T precursor cells (TPC) located in the subcapsular region, and immature CD4+CD8+ cortical thymocytes migrate through the cortex and CMJ to the medullar zone. The medulla contains CD4+ and CD8+ naïve thymocytes that will migrate to the periphery. The stromal-epithelial compartment of the thymus is represented by minor populations of EpCam+(CD326+)Foxn1+ bipotent thymic epithelial precursor cells/thymic epithelial stem cells (TEPC/TESC) and mesenchymal stem cells (MSC) located probably in the thymic parenchyma close to the CMJ region, as well as EpCam+CD205+ cortical thymic epithelial cells (cTEC) located in the cortex and EpCam+Air+ medullary thymic epithelial cells (mTEC) located in the medulla. Moreover, the cortex and the medulla contain also macrophages, fibroblasts and dendritic cells (DC) that together with cTEC and mTEC participate in the differentiation, maturation, positive and negative selection of thymocytes. HPC generate all thymocyte populations and alternatively may generate macrophages and DC; TEPC/TESC generate cTEC and mTEC lineages depending on local microenvironment and cross-talk with cortical or medullary thymocytes; MSC generate thymic fibroblasts and adipocytes. BV: Blood vessel; DT: Dead thymocyte; HC: Hassall’s corpuscle.