Collagen IV staining on tissue sections reveals invasion of keratinocytes through the basal membrane (BM).

<p>The BM was stained against collagen IV (green). Nuclei were counterstained with DAPI (blue). Consecutive sections stained for pan-cytokeratin are shown in comparison. <b>A)</b> In normal skin, the BM (white arrows) marks the barrier between epidermis and dermis. <b>B)</b> Early stage carcinoma formation in UV-irradiated skin. A lack of collagen IV expression indicates the disruption of the BM (orange arrows) accompanied by downward migrating cells (black arrow). <b>C)</b> In the edge region of a UV-induced KSCC, the BM is lost and invading altered keratinocytes are detectable. <b>D)</b> In nKSCC, invasion of neoplastic cells is advanced as indicated by pan-cytokeratin staining. The discontinuous staining of the BM marks transition zones where invading neoplastic squamous cells acquire a spindle cell phenotype (Scale bars: 100 μm).