Background
The exact biological function of CD30 in the thymus during development has been only partially elucidated, although data indicate it may be involved in negative selection. This study was prompted by the observation of a positive reaction of thymic epithelial cells (TECs), Hassall's corpuscles, and thymocytes with the monoclonal antibody CD30 during the late first and second trimester.
Material and Method
Twenty paraffin-embedded fetal thymus specimens at the late first and second trimester were investigated by conventional histology and immunohistology for CD30 expression. To provide additional information on the nature and localization of CD30+ thymocytes and CD30+ TECs, in situ hybridization (ISH) was performed on the specimens.
Results
1) In the medulla, a statistically significant difference between CD30+ thymocytes from the late first trimester and those from the second trimester (p<0.0001, t-test) was demonstrated. No significant difference was found concerning CD30+ thymocytes in the cortex. 2) Many medullary TECs and Hassall's corpuscles showed high expression of CD30 during the second trimester, whereas small numbers of CD30+ TECs were found during the late first trimester. No statistically significant difference was found concerning CD30+ TECs in the cortex. CD30 was expressed by ISH in many cells in the medulla and along the septa, whereas the cortex showed little if any expression. Accordingly, a higher CD30 expression was found in medullary than in cortical thymocytes.
Conclusions
Comparison of CD30 expression by TECs and thymocytes during the late first trimester and second trimester suggests an important role for CD30 in thymic selection.

Keywords: T-Lymphocytes - metabolism, Pregnancy Trimester, First, Pregnancy Trimester, Second, Pregnancy, In Situ Hybridization, Fetus, Thymus Gland - metabolism, Epithelial Cells - metabolism, Antigens, CD30 - metabolism