Fig. 3. Mesenchymal-epidermal interactions assessed by heterologous organotypic co-cultures (OTC). Heterologous OTC consisting of ctsl^+/+ fibroblasts or ctsl^-/- fibroblasts in collagen type I gels topped by normal human primary keratinocytes were grown air-exposed for 7 days. Paraffin sections were stained in haematoxylin and eosin (HE) or by immunohistochemistry for the proliferation marker Ki67 (Ki67, brown nuclear staining) and the differentiation markers cytokeratin 10 (K10, brown staining) and transglutaminase (TG, brown staining). The experiment is representative for a total of 16 OTC grown with primary dermal fibroblasts from ctsl^+/+ or ctsl^-/- skin (two independent fibroblast preparations per genotype). Bars, 100 µm.