Expression of rbp4, apoeb, and esr2a during the zebrafish larval-juvenile transition. A-C,H,K: rbp4. D,E,I,L: apoeb. F,G,J,M: esr2a. Whole-mount in situ hybridizations (A-G,I) and histological sections after in situ hybridizations (H,J-M) of 35-40 days postfertilization (dpf, around 8.5 mm long) larvae hybridized with digoxigenin-labeled antisense riboprobes. Section planes of H,J-M are indicated by white dotted lines in C,G,I. A-C: rbp4 transcripts were detected in the larval skin (A), and their expression was initially down-regulated in scale epidermal placodes, as shown in enlarged views from the anterior (B) and posterior (C) regions of the trunk. B,C,H,K: rbp4 expression was located in basal epidermal cells (H,K) but not neuromasts (B,C,H). C,K: After this initial down-regulation in the placode field (peripheral epidermal placodes in C), the rbp4 transcript signal was then detected in the central epidermis of scale papillae (C,K; central scale papillae in C). The process was initiated around the initiator squamation locus in epidermal placodes in the midline rows of the caudal peduncle and spread from scale to scale papillae. As a consequence, nascent scale papillae were observed at different stages of development according to their position on each side of the body in relation to the initiator squamation locus. C: Within a scale papilla, the rbp4 hybridization signal gradually invaded the peripheral epidermis of the nonoverlapping scale papillae. D-F: Mirroring the down-regulation of rbp4 expression, apoeb (D,E) and esr2a (F) induction was observed, leading to imbricated rows of expressing epidermal placodes. B,D,F and C,E are gene expression patterns depicted at a similar early and late developmental stage of the scale papilla, respectively. F,G,J: A moderate esr2a hybridization signal was also observed in the epidermis covering the whole larva (F,G) and a very strong signal in the neuromasts (F,G,J). G,I,L,M: apoeb transcripts were then restricted to the anterior and posterior margin epidermis, mostly in their basal cell layer (I,L) and were subsequently restricted to the posterior margin epidermis of the scale papilla (data not shown), a pattern also observed with esr2a (G,M). ame, anterior margin epidermis of the scale papilla; ce, central epidermis of the scale papilla; ds, dermal stroma; ep, epidermal placode; m, muscle cells; my, myoseptum; n, neuromast; pc, pigments cells located within the hypodermis between dermis and muscles; pe, peripheral epidermis of the scale papilla; pme, posterior margin epidermis of the scale papilla; sp, scale papilla. Scale bars = 500 μm in A, 100 μm in B-G,I, and 20 μm in H,J-M.
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