Assessment of first-trimester thymus size and correlation with maternal diseases and fetal outcome.

INTRODUCTION: We investigated the reliability of fetal thymus measurement during first-trimester screening, and associated fetal thymus size with crown-rump length, maternal diseases and fetal outcome. MATERIAL AND METHODS: In a retrospective cohort of 971 normal singleton first-trimester fetuses, we measured the anterior-posterior diameter of the thymus in a midsagittal plane in 767 fetuses. The intra-observer and inter-observer reliabilities were tested by intra-class correlation coefficient. We correlated thymus size with fetal crown-rump length, and investigated its association with maternal diseases (diabetes mellitus, rheumatic disorders, hypertension and coagulation disorders) and fetal outcome (small for gestational age, preterm birth and umbilical artery pH) using regression analyses. RESULTS: The intra-observer and inter-observer reliabilities of fetal thymus measurement were excellent (intra-class correlation coefficient 0.926, 95% CI 0.745-0.981 and 0.945, 95% CI 0.886-0.993, respectively). A linear relationship was found between crown-rump length and thymus size (ß = 0.023, p = 0.001). Pregnancies affected by maternal diabetes had a decreased fetal thymus size (ß = -0.209, p = 0.001), whereas in pregnancies affected by maternal rheumatic disease the thymus size was increased (ß = 0.285, p < 0.001). Fetal thymus size was not associated with maternal hypertension or maternal coagulation disorders. There was a positive association between preterm birth and fetal thymus size (p < 0.001). CONCLUSION: Measurement of first-trimester thymus size is reliable. Fetal thymus size has a linear correlation with crown-rump length. Maternal diabetes, rheumatic disease and preterm birth appear to have an association with fetal thymus size.

The thy-box for sonographic assessment of the fetal thymus: nomogram and review of the literature.

OBJECTIVES: To assess the feasibility and reproducibility of fetal thy-box visualization and to set normative data for the fetal thymus using this technique. METHODS: We performed a cross-sectional observational study in pregnant women in their second trimester of pregnancy who attended the fetal medicine unit of the Hospital Gregorio Marañón from March 2011 to March 2013. Using thy-box sonography, which sets boundaries within the thymus, we assessed the feasibility of the thy-box and analyzed the factors related to its visualization. Measurements of the thy-box anteroposterior and transverse diameters were performed in healthy fetuses. Interobserver agreement was studied for both items. We set normative data for the thy-box diameters in singleton and twin pregnancies. RESULTS: A total of 337 patients from 15 to 37 weeks were recruited after exclusion criteria. Thy-box feasibility was 74.2% (250 of 337). Nonfeasible cases were related to the fetal anterior spine and advanced gestational age. Assessment of the thy-box anteroposterior and transverse diameters was achievable in 250 fetuses. Interobserver agreement was good for feasibility and measurements (κ = 0.80). Linear growth of the thy-box in both diameters was observed in relation to gestational age during the second half of pregnancy. Thy-box diameters did not show significant differences according to the type of gestation, fetal sex, or chorionicity. CONCLUSIONS: Thy-box diameters are achievable and reproducible by sonography in normal singleton and twin pregnancies during the second trimester; however, although feasible, the thy-box transverse diameter measurement is not reliable beyond 28 weeks. Further studies are needed to evaluate thy-box diameters in pathologic cases, especially conotruncal anomalies.

Assessment of the fetal thymus by two- and three-dimensional ultrasound during normal human gestation and in fetuses with congenital heart defects.

OBJECTIVES: Our objectives were to compare the size and volume of the developing fetal thymus obtained by two-dimensional ultrasound (2D-US) and three-dimensional ultrasound (3D-US), develop normative data for thymus volume (TV), and investigate TV in fetuses with congenital heart disease (CHD) and normal twin gestations. METHODS: We studied 321 fetuses (gestational age (GA): 17-39 weeks) including 238 normal singletons, 64 normal twins and 19 singleton fetuses with CHD. We used 2D-US to assess fetal thymus maximum transverse diameter (MTD), maximum transverse area (MTA), anteroposterior diameter (APD) and superoinferior diameter (SID). TV was obtained by 3D-US using virtual organ computer-aided analysis. Measurements were adjusted for estimated fetal weight where appropriate. Linear regression analysis, general linear models and Fisher's Z-transformation were used where appropriate. A nomogram of fetal TV based on singleton gestations was produced according to previously published methods. RESULTS: Ultrasound assessment of the fetal thymus was possible in 95.3% (306/321) of cases. Both 3D-US and 2D-US measurements were significantly correlated with GA (TV r = 0.989; MTA r = 0.918; MTD r = 0.884; APD r = 0.849; and SID r = 0.816; all P < 0.05). After Fisher's Z-transformation, the correlation between the TV and GA was significantly stronger than that between any individual 2D-US measurement and GA (P < 0.05). Normal twin fetuses had TVs similar to those of singletons adjusted for estimated fetal weight and GA (P = 0.85). TV adjusted for estimated fetal weight and GA was significantly lower in fetuses with CHD than in normal singletons (P < 0.05). CONCLUSION: 2D-US and 3D-US are useful tools for evaluation of the size and volume of the human fetal thymus through gestation. Fetal TV by 3D-US seems to reflect normal development of the thymus in utero better than do 2D-US measurements. Lower TV should be expected in association with CHDs.

Flow cytometric assessment of human T-cell differentiation in thymus and bone marrow.

Using multidimensional flow cytometry we have defined and quantified the human T-cell differentiation pathway, focusing on those events occurring among the most immature thymocytes and putative bone marrow (BM) T-precursors. Early thymocytes were found to express the CD34 antigen and consisted of a mean 1.2% of cells within human pediatric (n = 9) and 2.0% in fetal thymi (n = 4). All CD34+ thymocytes were atypical blast by morphology, expressed intracytoplasmatic, but not cell surface, CD3, and were cell surface CD2+, CD5+, CD7+, CD38+, CD45+, CD45RA+, CD49d+, and LECAM-1(Leu8)high. CD34high thymocytes lacked surface expression of CD4 and CD8, but as CD34 expression diminished there was a coordinate increase in CD4 levels, followed by the appearance of CD8. The expression of CD1 and CD10 also increased concomitant with the loss of CD34, whereas expression of LECAM-1 diminished with CD34 downregulation. The differential expression of these antigens on early thymocytes (as well as the number of thymocytes displaying these patterns) was highly reproducible among the nine pediatric and four fetal specimens examined, suggesting a precise, stereotyped regulation of early differentiation events. Cell populations with antigen expression patterns suggestive of pluripotent stem cell (CD34high, CD38-), or non-T-lineage committed stem cells (CD34+, CD33+ or CD34+, CD19+) were not identified in either fetal or pediatric thymi (sensitivity = 1/10(4)). The presence of cells with the antigenic profile of the earliest CD34+ thymocytes was explored in human BM. Putative BM T-cell precursors with the appropriate phenotype (CD34+, CD7+, CD5+, CD2+, LECAM-1high) were readily identified in fetal specimens (constituting +/- 2% of the CD34+ population), but could not be reliably detected in adults. In contrast with thymi, only 13% of these cells expressed cytoplasmatic CD3, suggesting the presence of the immediate precursor of the putative prothymocyte population. This was further supported by the detection of CD34bright, CD7+, CD2-, CD5-, LECAM-1moderate cells in fetal specimens. Our results document the flow of cell surface differentiation during T-lymphopoiesis and suggest that T-lineage features are first acquired in the BM. The ability to reproducibly identify and isolate T-cell precursor populations of precisely defined maturational stage in marrow and thymus by multiparameter flow cytometry will facilitate characterization of the molecular events controlling T-lineage differentiation.