Institute of Zoology and Limnology, University of Innsbruck, Technikerstraße 25, A-6020 Innsbruck, Austria.
Integr Comp Biol. 2003 Feb;43(1):47-54. doi: 10.1093/icb/43.1.47.
Muscle tissue may have played a central role in the early evolution of mesoderm. The first function of myocytes could have been to control swimming and gliding motion in ciliated vermiform organisms, as it still is in such present-day basal Bilateria as the Nemertodermatida. The only mesodermal cells between epidermis and gastrodermis in Nemertodermatida are myocytes, and conceivably the myocyte was, in fact, the original mesodermal cell type. In Nemertodermatida as well as the Acoela, myocytes are subepithelial fiber-type muscle cells and appear to originate from the gastrodermal epithelium by emigration of single cells. Other mesodermal cells in the acoels are the peripheral parenchyma (connective tissue) and tunica cells of the gonads, and these also arise from the gastrodermis. Musculature in many of the coelomate protostomes and deuterostomes, on the other hand, is in the form of epitheliomuscular (myoepithelial) cells, and this cell type may also have been an early form of the mesodermal myocyte. The mesodermal bands in the small annelid Polygordius and in juvenile enteropneusts have cells intermediate between mesenchymal and epithelial in their histological organization as they develop into myoepithelia. If acoelomates were derived from coelomates by progenesis, then the fiber-type muscles of acoelomates could be products of foreshortened differentiation of such tissue. The precise serial patterning of circular muscle cells along the anterior-posterior axis during embryonic development in the acoel Convoluta pulchra provides a model for early steps in the gradual evolution of segmentation from iterated organ systems.
肌肉组织可能在中胚层的早期进化中发挥了核心作用。肌细胞的第一个功能可能是控制纤毛蠕虫状生物的游泳和滑行运动，就像在当今的基础两侧对称动物门Nemertodermatida 中仍然存在的那样。在 Nemertodermatida 中，表皮和腹皮之间唯一的中胚层细胞是肌细胞，而且肌细胞实际上可能是原始的中胚层细胞类型。在 Nemertodermatida 以及 Acoela 中，肌细胞是上皮下纤维型肌肉细胞，似乎是通过单个细胞的迁移从腹皮上皮起源的。Acoela 中的其他中胚层细胞是外周实质（结缔组织）和性腺的被膜细胞，这些细胞也起源于腹皮。另一方面，许多体腔原口动物和后口动物的肌肉组织是上皮肌肉（肌上皮）细胞的形式，这种细胞类型也可能是早期的中胚层肌细胞形式。在小环节动物 Polygordius 中的中胚层带和幼体肠鳃类动物中，在它们发育成肌上皮的过程中，其组织学结构介于间充质和上皮之间的细胞。如果无体腔动物是由体腔动物通过幼体发生衍生而来的，那么无体腔动物的纤维型肌肉可能是这种组织缩短分化的产物。在无体腔动物 Convoluta pulchra 的胚胎发育过程中，沿着前后轴排列的环形肌肉细胞的精确序列模式为从重复器官系统逐渐进化为分节的早期步骤提供了模型。
