Sugawara O, Takagi N, Sasaki M
Chromosome Research Unit, Hokkaido University, Kita-ku, Sapporo, Japan.
Chromosoma. 1983;88(2):133-8. doi: 10.1007/BF00327333.
The allocyclic X chromosome in early female mouse embryos undergoes DNA replication either late or early in the S phase. Earlier studies indicated that the early-replicating X chromosome is restricted to the trophectoderm and primitive endoderm cell lineages in which the allocyclic X is almost exclusively paternal in origin. There has been, however, no compelling evidence for the genetic inactivity of the early-replicating X chromosome and a shift from early to late replication or vice versa. The present study employing a combination of 3H-thymidine autoradiography and BrdU labeling-acridine orange fluorescence staining in day-6 female mouse embryos found that the early-replicating X chromosome can change directly into a late-replicating one. The activity state of the early-replicating X chromosome was examined by electrophoretic determination of the X linked enzyme, phosphoglycerate kinase (PGK-1), in tissues isolated from 6.0-day and day-8.5 Pgk-1a/Pgk-1b embryos. Only the maternally derived Pgk-1 allele was expressed in the proximal endoderm and extraembryonic ectoderm of 6.0-day and the chorion of 8.5-day embryos. Thus, the early-replicating, paternally derived X chromosome found in about 70%-80% of the cells in these tissues seems to be repressed like the late-replicating one.
早期雌性小鼠胚胎中的异固缩X染色体在S期晚期或早期进行DNA复制。早期研究表明，早期复制的X染色体局限于滋养外胚层和原始内胚层细胞谱系，在这些谱系中，异固缩X几乎完全来自父本。然而，目前尚无确凿证据表明早期复制的X染色体具有遗传不活性，以及其从早期复制转变为晚期复制或反之亦然。本研究采用3H-胸腺嘧啶核苷放射自显影和BrdU标记-吖啶橙荧光染色相结合的方法，对6日龄雌性小鼠胚胎进行研究，发现早期复制的X染色体可直接转变为晚期复制的X染色体。通过对从6.0日龄和8.5日龄的Pgk-1a/Pgk-1b胚胎分离的组织中X连锁酶磷酸甘油酸激酶（PGK-1）进行电泳测定，来检测早期复制的X染色体的活性状态。在6.0日龄胚胎的近端内胚层和胚外外胚层以及8.5日龄胚胎的绒毛膜中，只有母源的Pgk-1等位基因表达。因此，在这些组织中约70%-80%的细胞中发现的早期复制、父源的X染色体似乎与晚期复制的X染色体一样受到抑制。
