Meckel's cartilage breakdown offers clues to mammalian middle ear evolution.
Meckel's cartilage breakdown offers clues to mammalian middle ear evolution.
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DOI：
10.1038/s41559-017-0093
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发表时间：
2017-03-06
影响因子：
16.8
通讯作者：
Tucker AS
中科院分区：
文献类型：
--
作者：
Anthwal N;Urban DJ;Luo ZX;Sears KE;Tucker AS
哺乳动物耳朵进化的一个关键转变是将前哺乳动物突触类的初级颌关节结合到现存哺乳动物的最终哺乳动物中耳中。这种进化的转变分两步进行，从部分或“过渡”的哺乳动物中耳开始，外鼓室和鼓槌仍然通过骨化的梅克尔软骨（MC）与下颌骨相连，正如在许多中生代哺乳动物中观察到的那样。接着是MC分解，将外鼓室和锤骨从下颌骨上解放出来，形成最终的哺乳动物中耳。在这里，我们报告了破软骨细胞在MC分解中的作用的新发现，揭示了兽类哺乳动物如何失去连接耳朵和下巴的MC。小鼠和负鼠的破胚细胞的遗传或药理学损失导致胚胎MC在幼年期后持续存在，突变小鼠的MC骨化。持续性MC在小鼠牙髓上形成独特的出生后凹槽。这种形态与中生代哺乳动物的骨化MC和麦克尔沟相似。在爬行动物中没有观察到破胚细胞向mcc募集，在爬行动物中，mcc以软骨结构存在。我们假设中耳膜的骨化是哺乳动物祖先的特征，并且在中耳膜骨化之前，细胞聚集到中耳膜的时间发生了变化，这是现存哺乳动物中耳进化的关键发育机制。
A key transformation in mammalian ear evolution was incorporation of the primary jaw joint of premammalian synapsids into the definitive mammalian middle ear of living mammals. This evolutionary transition occurred in two-steps, starting with a partial or “transitional” mammalian middle ear in which the ectotympanic and malleus were still connected to the mandible by an ossified Meckel’s Cartilage (MC), as observed in many Mesozoic mammals. This was followed by MC breakdown, freeing the ectotympanic and the malleus from the mandible and creating the definitive mammalian middle ear. Here we report novel findings on the role of chondroclasts in MC breakdown, shedding light on how therian mammals lost MC connecting the ear to the jaw. Genetic or pharmacological loss of clast cells in mice and opossums leads to persistence of embryonic MC beyond juvenile stages, with MC ossification in mutant mice. The persistent MC causes a distinctive postnatal groove on the mouse dentary. This morphology phenocopies the ossified MC and Meckelian groove observed in Mesozoic mammals. Clast cell recruitment to MC is not observed in reptiles, where MC persists as a cartilaginous structure. We hypothesize that ossification of MC is an ancestral feature of mammaliaforms, and that a shift in the timing of clast cell recruitment to MC prior to its ossification is a key developmental mechanism for the evolution of the definitive mammalian middle ear in extant therians.