Histology and in situ hybridization

Histological sections were obtained after fixation in PBS 4% paraformaldehyde pH 7.4 (12 h at 4 °C), paraffin or ‘CryoWax’ (Durán et al. 2011) embedding and staining with haematoxylin‐eosin‐picrosirius (Becerra et al. 1983) or Mallory's trichrome (Pearse, 1985; Kiernan, 2015). Sections were photographed in a Zeiss Axioskop (Zeiss, Germany) or a Multizoom Nikon AZ‐100 microscope under Nomarski optics. RNA antisense probes against msxa, msxc, msxd (Akimenko et al. 1995), shh (Laforest et al. 1998), bmp4 (Murciano et al. 2002) or dlx3 (Akimenko et al. 1994) genes were obtained after Quint et al. (2002). Fins after ray cuts were fixed with 4% paraformaldehyde stained with probes and whole‐mounted (Akimenko et al. 1995; Laforest et al. 1998). Photographs were taken in a Nikon Eclipse E 800 (Nikon, Japan) microscope.