Transcriptional and Histological Analyses of the Thymic Developmental Process in the Fetal Pig

The humanized pig model, in which human cells or tissues can be functionally maintainedin pigs, can be an invaluable tool for human medical research. Although the recentdevelopment of immunodeficient pigs has opened the door for the development of such amodel, the efficient engraftment and differentiation of human cells may be difficult toachieve. The transplantation of human cells into fetal pigs, whose immune system isimmature, will ameliorate this problem. Therefore, we examined the development of porcinefetal thymus, which is critical for the establishment of the immune system. We firstanalyzed the levels of mRNA expression of genes that are relevant to the function ofthymic epithelial cells or thymocytes in whole thymi from 35 to 85 days of gestation (DG)and at 2 days postpartum (DP) by quantitative RT-PCR. In addition, immunohistochemicalanalyses of thymic epithelial cells from DG35 to DG55 and DP2 were performed. Theseanalyses showed that the thymic cortex was formed as early as DG35, and thymic medullagradually developed from DG45 to DG55. These findings suggested that, at least beforeDG45, the thymus do not differentiate to form fully functional T cells.