Burke Ann Campbell
Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts 02138.
J Morphol. 1989 Mar;199(3):363-378. doi: 10.1002/jmor.1051990310.
The chelonian carapace is composed of the endochondral ribs and vertebrae associated with a specialized dermis. The ribs are found in an aberrant position compared to those of all other tetrapods; they are superficial and dorsal to the limb girdles. This morphological arrangement, which constitutes the unique chelonian Bauplan, is examined from a developmental perspective. Embryos of Chelydra serpentina were studied during stages of carapace development. Tissue morphology, autoradiography, and indirect immunofluorescent localization of adhesion molecules indicate that the outgrowth of the embryonic carapace occurs as the result of an epithelial-mesenchymal interaction in the body wall. A carapacial ridge composed of mesenchyme of the dermis and overlying ectoderm is formed dorsal to the ectodermal boundary between somitic and lateral plate mesoderm. It is the anlage of the carapace margin, in which the ribs will eventually terminate. The ectoderm of the carapacial ridge is thickened into a pseudostratified columnar epithelium, which overlies a condensation in the mesenchyme of the dermis. Patterns of cell proliferation and the distribution of N-CAM and fibronectin in the carapacial ridge are consistent with patterns seen in other structures initiated by epithelial-mesenchymal interactions such as feathers and limb buds. Based on an analogy to this developmental mechanism in the development of the limb skeleton, a further analogy with the evolution of the limbs from lateral fin folds is used to form a hypothesis on the evolution of the carapace from elements of the primitive reptilian integument.
龟鳖的背甲由软骨内肋骨和与特殊真皮相关的椎骨组成。与所有其他四足动物的肋骨相比，龟鳖的肋骨位置异常；它们位于肢体带的浅层和背侧。这种构成独特龟鳖体构的形态学排列，从发育的角度进行了研究。在背甲发育阶段对蛇鳄龟胚胎进行了研究。组织形态学、放射自显影以及黏附分子的间接免疫荧光定位表明，胚胎背甲的生长是体壁上皮-间充质相互作用的结果。在体节和侧板中胚层之间的外胚层边界背侧形成了一个由真皮间充质和覆盖其上的外胚层组成的背甲嵴。它是背甲边缘的原基，肋骨最终将在此处终止。背甲嵴的外胚层增厚形成假复层柱状上皮，其覆盖在真皮间充质的一个凝聚物上。背甲嵴中细胞增殖模式以及N-CAM和纤连蛋白的分布与羽毛和肢芽等由上皮-间充质相互作用启动的其他结构中所见模式一致。基于与肢体骨骼发育中这种发育机制的类比，进一步类比肢体从侧鳍褶的进化，形成了一个关于背甲从原始爬行动物体表元素进化而来的假说。
