Bellomo D, Lander A, Harragan I, Brown N A
Department of Child Health, St. George's Hospital Medical School, London, United Kingdom.
Dev Dyn. 1996 Apr;205(4):471-85. doi: 10.1002/(SICI)1097-0177(199604)205:4<471::AID-AJA10>3.0.CO;2-4.
During gastrulation, the node of the mammalian embryo appears to be an organising centre, homologous to Hensen's node in the chick and the dorsal lip of the amphibian blastopore. In addition, the node serves as a precursor population for the head process, notochord and foregut endoderm. We have studied node architecture and cell morphology by electron microscopy, and cell proliferation using bromodeoxyuridine incorporation and mitotic counts. The dorsal (ectodermal) and ventral (endodermal) components of the node are two distinct populations, separated by a basement membrane. The ventral node, contiguous with the head process, is characterised by a relatively low proliferation rate, with only approximately 10% of cells incorporating BrdU over 4 hr, compared to > 95% in surrounding mesodermal and ectodermal tissues. This is the case from the beginning of node formation, at the no-allantoic-bud stage, until the 7 somite stage, and is not compatible with the idea that the ventral node is a stem cell population. The dorsal node is highly proliferative, its rate of division being indistinguishable from the neurectoderm, with which it is contiguous. In the ventral node, two regions can be recognised: cells in the "pit" are columnar and all monociliated; around them lies a "crown" of cells arranged radially in a horseshoe shape and less often ciliated. Node derivatives share common features with the ventral node; the head process and the notochord are relatively quiescent; and some head process cells are also monociliated. Node and head process monocilia are immotile and appear to be associated with non-proliferation. We suggest that the ventral node contains all the properties of the organiser, while the dorsal node is indistinct from the surrounding epiblast. The cranial end of the foregut pouch, the thyroid diverticulum, and the promyocardium of early somite stage embryos are also areas of low cell division. All the described regions of relative quiescence are sites of expression of members of the TGF beta family, which may be involved in maintaining non-proliferation.
在原肠胚形成过程中，哺乳动物胚胎的节点似乎是一个组织中心，与鸡胚中的亨氏节以及两栖类胚孔的背唇同源。此外，该节点是头突、脊索和前肠内胚层的前体细胞群。我们通过电子显微镜研究了节点结构和细胞形态，并使用溴脱氧尿苷掺入法和有丝分裂计数来研究细胞增殖。节点的背侧（外胚层）和腹侧（内胚层）成分是两个不同的细胞群，由基膜分隔。与头突相邻的腹侧节点的特征是增殖率相对较低，在4小时内只有约10%的细胞掺入溴脱氧尿苷，而周围中胚层和外胚层组织中的掺入率超过95%。从节点形成开始，即无尿囊芽阶段，到7体节阶段都是如此，这与腹侧节点是干细胞群的观点不一致。背侧节点高度增殖，其分裂速率与相邻的神经外胚层无法区分。在腹侧节点中，可以识别出两个区域：“凹陷”中的细胞呈柱状且均为单纤毛；围绕它们的是呈马蹄形径向排列且较少有纤毛的“冠状”细胞。节点衍生物与腹侧节点具有共同特征；头突和脊索相对静止；并且一些头突细胞也是单纤毛的。节点和头突的单纤毛是不动的，似乎与非增殖有关。我们认为腹侧节点具备组织者的所有特性，而背侧节点与周围的上胚层并无明显差异。早期体节阶段胚胎的前肠囊的头端、甲状腺憩室和原心肌也是细胞分裂率较低的区域。所有描述的相对静止区域都是TGF-β家族成员的表达位点，这可能参与维持非增殖状态。
