## WT1 Immunohistochemical Expression in Human Embryonal/Fetal and Neoplastic Tissues  
### WT1 Immunohistochemical Expression in Human Embryonal/Fetal Tissues  
From gestational weeks 7 to 24, WT1 expression has been found in several tissues, in a nuclear or a cytoplasmic localization. WT1 nuclear staining can be observed in different structures of metanephros and mesonephros [37; 51; 52], including glomeruli and sub-capsular blastema of nephrogenic zone (Figure 1). WT1 expression in round, undifferentiated mesenchymal cells (blastematous component) undergoing epithelial differentiation is a model of controlled epithelial-mesenchymal transition resulting in nephrogenesis [53; 54; 55; 56]. WT1 is expressed in mesothelial cells (excoelomic epithelium) that are found above both ovaries and testes, in the developing sex cords and in the gonadal mesenchyme (Figure 1). In the later phases of development, nuclear staining is manteined in the secondary sex cords of both testes and ovaries, while it gradually disappears from their surrounding mesenchyme, to be restricted to epithelial cells surrounding the oocytes [51]. Between the seventh and the tenth week of gestational age, WT1 nuclear expression can be found in the mesothelial cells of serosal surfaces covering the abdominal and pelvic visceral organs (uterus and ovaries, bladder, stomach, small and large intestine, pancreas) pleura and peritoneum. Notably, a conspicuous amount of mesenchymal sub-mesothelial cells along mesothelial cells displays WT1 nuclear expression [37]. During human developmental phases (from 8 to 28 wGA) small clusters of neuroblasts are located in the paravertebral regions, in the cortex of adrenal glands and within the muscle wall of the developing stomach and small/large intestine. WT1 cytoplasmic expression in neuroblasts is strong and diffuse during the different phases of development, while no nuclear immunoreactivity can be demonstrated. These cells gradually differentiate into ganglion cells, at first as immature ganglion cells and finally as mature ganglion cells. In immature ganglion cells, unlike neuroblasts, WT1 cytoplasmic immunoexpression is weak/focal and sometimes absent, while ganglion cells of adult sympathetic ganglia, adrenal glands and myoenteric nervous plexuses, are completely negative. Similar to the adult adrenal medulla and paraganglia, extra- and intra-adrenal differentiating chromaffin cells are not WT1 stained. The expression of WT1 is strongly localized in the cytoplasm of radial glia cells of both the developing spinal cord and cerebral cortex along the entire thickness of the neural tube. WT1 expression in the undifferentiated radial glial cells seems to support the theory that it is necessary to maintain cells in an undifferentiated state [30; 57; 58; 59]. During the early phases of development (from 6 wGA) embryonic myoblasts forming myotomes exhibit strong cytoplasmic staining for WT1. Moreover, in both primary and secondary myotubes of the developing muscles, WT1 is strongly and diffusely expressed in the cytoplasm of these cells. From 20 wGA, WT1 cytoplasmic expression becomes more heterogeneous, ranging from focally strong to weak or absent within the same muscle fiber (mosaic-type expression) (Figure 1). WT1 nuclear immunoreactivity is lacking in skeletal muscle during all phases of myogenesis [41]. Several studies have shown that WT1 is crucial for heart development [60; 61], and it may also be involved in the proliferation of cardiac myocytes. In addition, WT1 plays a key role in the conversion of epicardial cells to mesenchymal cells. This is demonstrated by strong WT1 cytoplasmic expression in cardiomyocytes of both atria and ventricles during the different phases of heart development, as in somatic skeletal muscle cells. In addition, WT1 is expressed in the cytoplasm of the endothelial cells of blood vessels (aorta, arteries, veins, capillaries) in all developing tissues (Figure 1) [62]. These findings are also maintained in the endothelial cells of both adult tissues and benign and malignant tumors.