Department of Biology and Chemistry, Université Grenoble-Alpes, La Tronche, France.
Directorate "Earth and History of Life, " Royal Belgian Institute of Natural Sciences, Bruxelles, Belgium.
Exp Dermatol. 2019 Apr;28(4):503-508. doi: 10.1111/exd.13391. Epub 2017 Aug 25.
While every jawed vertebrate, or its recent ancestor, possesses teeth, skin appendages are characteristic of the living clades: skin denticles (odontodes) in chondrichthyans, dermal scales in teleosts, ducted multicellular glands in amphibians, epidermal scales in squamates, feathers in birds and hair-gland complexes in mammals, all of them showing a dense periodic patterning. While the odontode origin of teleost scales is generally accepted, the origin of both feather and hair is still debated. They appear long before mammals and birds, at least in the Jurassic in mammaliaforms and in ornithodires (pterosaurs and dinosaurs), and are contemporary to scales of early squamates. Epidermal scales might have appeared several times in evolution, and basal amniotes could not have developed a scaled dry integument, as the function of hair follicle requires its association with glands. In areas such as amnion, cornea or plantar pads, the formation of feather and hair is prevented early in embryogenesis, but can be easily reverted by playing with the Wnt/BMP/Shh pathways, which both imply the plasticity and the default competence of ectoderm. Conserved ectodermal/mesenchymal signalling pathways lead to placode formation, while later the crosstalk differs, as well as the final performing tissue(s): both epidermis and dermis for teeth and odontodes, mostly dermis for teleosts scales and only epidermis for squamate scale, feather and hair. We therefore suggest that tooth, dermal scale, epidermal scale, feather and hair evolved in parallel from a shared placode/dermal cell unit, which was present in a common ancestor, an early vertebrate gnathostome with odontodes, ca. 420 million years ago.
虽然每一种有颚脊椎动物或其最近的祖先都有牙齿，但皮肤附属物是现存进化枝的特征：软骨鱼类的皮肤齿（牙质）、硬骨鱼类的真皮鳞、两栖类的有导管的多细胞腺、爬行动物的表皮鳞、鸟类的羽毛和哺乳动物的毛腺复合体，所有这些都表现出密集的周期性模式。虽然硬骨鱼鳞片的牙质起源被普遍接受，但羽毛和毛发的起源仍存在争议。它们早在哺乳动物和鸟类出现之前就出现了，至少在侏罗纪的哺乳形类和鸟臀目恐龙（翼龙和恐龙）中出现，与早期爬行动物的鳞片同时出现。表皮鳞可能在进化中多次出现，而基干羊膜动物不可能发展出有鳞的干燥表皮，因为毛囊的功能要求其与腺体相关联。在羊膜、角膜或跖垫等区域，在胚胎发生的早期就阻止了羽毛和毛发的形成，但通过玩弄 Wnt/BMP/Shh 通路很容易逆转，这都意味着外胚层的可塑性和默认能力。保守的外胚层/间充质信号通路导致了颅顶的形成，而后来的串扰则不同，以及最终的执行组织也不同：牙齿和牙质的表皮和真皮，硬骨鱼鳞片的主要是真皮，而爬行动物鳞片、羽毛和毛发的只有表皮。因此，我们认为牙齿、真皮鳞、表皮鳞、羽毛和毛发是从一个共同的颅顶/真皮细胞单元进化而来的，这个单元存在于一个共同的祖先中，一个大约 4.2 亿年前的有颚脊椎动物，有牙质。
