Tokuyasu K T, Maher P A
Department of Biology, University of California at San Diego, La Jolla 92093.
J Cell Biol. 1987 Dec;105(6 Pt 1):2795-801. doi: 10.1083/jcb.105.6.2795.
In whole mount preparations of the 9 somite stage chick embryonic hearts that were immunofluorescently double labeled for titin and alpha-actinin, presumptive myofibrils were recognized as rows of several periodically aligned titin spots. Within these titin spots, smaller alpha-actinin dots were observed. These periodical arrangements of titin spots and alpha-actinin dots were not found in the 7 somite stage hearts. In wide myofibrils in the 10 somite stage hearts, the alpha-actinin dots and titin spots simultaneously became 'lines.' To study the ultrastructural features of the titin-positive regions in the 6-9 somite stage hearts, the thoracic portions of the embryos were immunofluorescently labeled for titin and embedded in resin. Ultrathin sections were mounted on electron microscopic grids and examined in immunofluorescence optics. The titin-positive regions thus identified were then examined in the electron microscope. No readily discernable specific ultrastructural features were found in titin-positive regions of the 6 somite stage cardiac primodia. Examination of the sections of the 9 somite stage hearts, on the other hand, revealed the occasional presence of small dense bodies, Z bodies, in the titin-positive regions. These observations strongly suggest that these Z bodies are the ultrastructural counterparts of the alpha-actinin dots seen by immunofluorescence optics and that they are formed nearly at the time of the formation of the first myofibrils. In some of the nascent myofibrils the Z bodies were found to be considerably narrower than the myofibrils, implying that the Z bodies are required not for the assembly of myofibrils per se but for their stabilization. Immunofluorescent labeling for titin and alpha-actinin revealed that the length of the shortest sarcomeres in the first myofibrils is approximately 1.5 micron, approximately the width of the A bands of mature myofibrils. The possibility that the A bands might define the initial length of nascent sarcomeres was indicated.
在对9体节期鸡胚心脏进行的整装标本制备中，对肌联蛋白和α - 辅肌动蛋白进行免疫荧光双标记，推测的肌原纤维被识别为几排周期性排列的肌联蛋白斑点。在这些肌联蛋白斑点内，观察到较小的α - 辅肌动蛋白点。在7体节期心脏中未发现肌联蛋白斑点和α - 辅肌动蛋白点的这种周期性排列。在10体节期心脏的宽肌原纤维中，α - 辅肌动蛋白点和肌联蛋白斑点同时变成了“线”。为了研究6 - 9体节期心脏中肌联蛋白阳性区域的超微结构特征，将胚胎的胸部进行肌联蛋白免疫荧光标记并包埋在树脂中。超薄切片安装在电子显微镜网格上，并在免疫荧光光学显微镜下检查。然后在电子显微镜下检查如此鉴定出的肌联蛋白阳性区域。在6体节期心脏原基的肌联蛋白阳性区域未发现易于辨别的特定超微结构特征。另一方面，对9体节期心脏切片的检查显示，在肌联蛋白阳性区域偶尔存在小的致密体，即Z体。这些观察结果强烈表明，这些Z体是免疫荧光光学显微镜下看到的α - 辅肌动蛋白点的超微结构对应物，并且它们几乎在第一批肌原纤维形成时形成。在一些新生肌原纤维中，发现Z体比肌原纤维窄得多，这意味着Z体本身并非肌原纤维组装所必需，而是用于其稳定。对肌联蛋白和α - 辅肌动蛋白的免疫荧光标记显示，第一批肌原纤维中最短肌节的长度约为1.5微米，大约是成熟肌原纤维A带的宽度。这表明A带可能决定新生肌节的初始长度。
