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Molecular identification of the skin transformation center of anuran larval skin using genes of Rana adult keratin (RAK) and SPARC as probes

Anuran larval skin undergoes a process of metamorphosis into preadult and adult skin. Basal skein, larval basal and adult basal cells are basement membrane‐attaching cells in the larval, preadult and adult epidermis, respectively, and are identified as cells expressing genes of RLK (Rana larval keratin), both RLK and RAK (Rana adult keratin), and RAK. Larval to preadult skin conversion takes place in the histological entity called the skin transformation center (STC). The present study performed a cDNA subtractive gene screening on cDNA of the larval and the preadult skin, and cloned the secreted protein acidic and rich in cysteine (SPARC) gene as an upregulated gene in the larva to preadult skin conversion. RAK gene‐positive basal skein cells and fibroblasts in and around the STC were weakly and strongly sparc‐positive, respectively. Using sparc and rak, we redefined the STC and visualized it on a histological section as an approximately 150 µm‐long region that contained about 20 rak‐negative and weakly sparc‐positive basal cells. Intense sparc expression was observed in basal skein cells, but not in larval basal cells, suggesting that SPARC acts as a suppressor of rak during epidermal differentiation. This suggestion was tested by investigating the effect of SPARC on cultured larval basal cells. We observed that SPARC suppressed the expression of rak, but not rlk.