Mesodermal determinants in the Tubifex embryo

在Tubifex胚胎中D象限的第二（2d）和第四（4d）微节分别被指定为外胚层和中胚层胚胎干细胞的前体。为了深入了解这种干细胞特化的机制，我们使用2d和4d的cDNA之间的减法杂交进行了谱系特异性母体mRNA的系统筛选。我们初步鉴定了16个基因，包括Impact、HMGB、Nucleolin、TERE1、ELAV和RNA结合蛋白。从这些基因合成的核糖核酸探针的整体安装原位杂交显示，母系提供的成绩单“表现”在一个类似的方式，以极的蛋黄（蛋黄缺陷的细胞质）。也就是说，这些转录本首先在第一次有丝分裂结束时分离到动物极和植物极，划分到D象限，最后定位到2d和4d细胞。在这项研究的过程中，我们没有遇到这样的基因，将被隔离到2d或4d。除上述消减实验外，我们还分离并鉴定了几个发育基因（背、毛、尾和vasa）。除了尾部的合子表达，这些基因表达的母系和成绩单存储在卵母细胞中被发现表现在一个非常相似的方式在减法实验中发现的基因转录本。

The second (2d) and fourth (4d) micromeres of the D quadrant in the Tubifex embryo are specified as precursors of ectodermal and mesodermal embryonic stem cells, respectively. To gain an insight into the mechanisms of this stem cell specification, we performed a systematic screen of lineage specific maternal mRNAs, using subtraction hybridization between cDNAs from 2d and 4d. We initially identified 16 genes, which include Impact, HMGB, Nucleolin, TERE1, ELAV and RNA binding proteins. Whole-mount in situ hybridization with riboprobes synthesized from these genes revealed that maternally supplied transcripts "behave" in a similar way to the pole plasms (yolk-deficient cytoplasm). That is, these transcripts are first segregated to the animal pole and the vegetal pole at the end of the first mitosis, partitioned to D quadrants, and finally localized to 2d and 4d cells. During the course of this study, we did not come across such genes as would be segregated exclusively to either 2d or 4d. Besides the above-mentioned subtraction experiments, we isolated and characterized several development genes (dorsal, hairy, decapentaplegic caudal and vasa) in Tubifex. Except caudal that is expressed zygotically, these genes are expressed maternally and transcripts stored in oocytes are found to behave in a very similar fashion to the gene transcripts discovered in the subtraction experiments.