Donner J, Kliesch S, Brehm R, Bergmann M. From carcinoma in situ to testicular germ cell tumour. 2004;112:79–88. The mechanisms of invasive tumour development from pre-invasive CIS are unknown. We examined changes in functional parameters of the tubular wall according to the increase in CIS cells and tubular size. Immunohistochemistry was performed on 37 testicular specimens from 25 patients with carcinoma in situ and/or malignant germ cell tumour for the detection of actin/myosin in myocytes, and laminin/integrin α6 in the basement membrane of seminiferous tubules. Tumour cells were detected by PlAP, Sertoli cells by inhibin alpha and vimentin and by cytokeratin 18/connexin 26 immunoreactivity, which is selectively expressed together with CIS. Areas showing clusters of tumour cells surrounded by a fibrous sheet could be identified as enlarged tubules because of focal Sertoli cell-specific co-expression of inhibin alpha, vimentin, cytokeratin 18, and connexin 26 immunoreaction. These clusters exhibited an intact basement membrane shown by a persistent laminin/integrin α6 immunoreactivity, but myocytes had lost their contractility indicated by the loss of myosin/actin immunoreactivity. They often showed septa originating from the fibrous sheet containing numerous capillaries. Focal areas of syncytiotrophoblastic cells within classical seminoma also expressing inhibin alpha, cytokeratin 18, and connexin 26 could be differentiated from single Sertoli cells within tumor cell clusters by typical hCG but absence of vimentin immunoreactivity. In contrast to the current concept of CIS cells passing the tubular wall, these data provide evidence for an additional theory, i.e. that the switch from pre-invasive CIS to invasive tumour takes place in situ by tubular enlargement due to tumour cell proliferation followed by Sertoli cell degeneration and conversion of the tubular wall into connective tissue.