Cooper Rory L, Lloyd Victoria J, Di-Poï Nicolas, Fletcher Alexander G, Barrett Paul M, Fraser Gareth J
1Department of Animal and Plant Sciences, University of Sheffield, Sheffield, UK.
2Program in Developmental Biology, Institute of Biotechnology, University of Helsinki, Helsinki, Finland.
Evodevo. 2019 Aug 13;10:19. doi: 10.1186/s13227-019-0130-9. eCollection 2019.
Vertebrates possess a diverse range of integumentary epithelial appendages, including scales, feathers and hair. These structures share extensive early developmental homology, as they mostly originate from a conserved anatomical placode. In the context of avian epithelial appendages, feathers and scutate scales are known to develop from an anatomical placode. However, our understanding of avian reticulate (footpad) scale development remains unclear.
Here, we demonstrate that reticulate scales develop from restricted circular domains of thickened epithelium, with localised conserved gene expression in both the epithelium and underlying mesenchyme. These domains constitute either anatomical placodes, or circular initiatory fields (comparable to the avian feather tract). Subsequent patterning of reticulate scales is consistent with reaction-diffusion (RD) simulation, whereby this primary domain subdivides into smaller secondary units, which produce individual scales. In contrast, the footpad scales of a squamate model (the bearded dragon, ) develop synchronously across the ventral footpad surface.
Widely conserved gene signalling underlies the initial development of avian reticulate scales. However, their subsequent patterning is distinct from the footpad scale patterning of a squamate model, and the feather and scutate scale patterning of birds. Therefore, we suggest reticulate scales are a comparatively derived epithelial appendage, patterned through a modified RD system.
脊椎动物拥有多种皮肤上皮附属器，包括鳞片、羽毛和毛发。这些结构在早期发育中具有广泛的同源性，因为它们大多起源于一个保守的解剖基板。在鸟类上皮附属器的背景下，已知羽毛和盾鳞从解剖基板发育而来。然而，我们对鸟类网状（脚垫）鳞片发育的了解仍然不清楚。
在这里，我们证明网状鳞片从增厚上皮的受限圆形区域发育而来，在上皮和下方间充质中都有局部保守的基因表达。这些区域构成解剖基板或圆形起始场（类似于鸟类的羽区）。网状鳞片随后的图案形成与反应扩散（RD）模拟一致，即这个主要区域细分为较小的次级单元，产生单个鳞片。相比之下，一种有鳞目模型（鬃狮蜥）的脚垫鳞片在腹侧脚垫表面同步发育。
广泛保守的基因信号是鸟类网状鳞片初始发育的基础。然而，它们随后的图案形成与有鳞目模型的脚垫鳞片图案形成以及鸟类的羽毛和盾鳞图案形成不同。因此，我们认为网状鳞片是一种相对衍生的上皮附属器，通过改良的RD系统形成图案。
