Clarke H J
Department of Obstetrics and Gynecology, McGill University, Montreal, Que., Canada.
Biochem Cell Biol. 1992 Oct-Nov;70(10-11):856-66. doi: 10.1139/o92-134.
Changes in nuclear structure and chromatin composition regulate gene activity in many cell types and could play a similar role during early mammalian embryogenesis. Oocytes of the mouse contain the three major lamin species present in somatic cells, although lamin A synthesized by oocytes has a higher molecular mass than the somatic species. Oocyte chromatin contains core histones similar to those of somatic cells, as well as elements that are immunologically related to protamines. In contrast, somatic-type histone H1 is not present. DNA topoisomerase II has not yet been identified in mammalian oocytes, but is abundant in frog oocytes. In contrast to oocytes, sperm do not contain a typical nuclear lamina. DNA topoisomerase II is detectable until late spermiogenesis. Although the DNA of sperm is associated mainly with protamines, some histone may be retained. There is also evidence that the arrangement of the DNA in the nucleus is nonrandom. These results demonstrate differences in nuclear and chromatin composition between oocytes and sperm. After fertilization, the nuclei of cleavage-stage blastomeres undergo programmed modifications. Lamin B is synthesized, whereas lamin A is not. In addition, a set of nuclear proteins is transiently synthesized in mice at the two-cell stage. Changes in embryonic chromatin composition also occur. The relative abundance of transcripts from different core histone genes differs between mouse oocytes and blastocysts. Furthermore, somatic histone H1 becomes detectable beginning at the mid-four-cell stage. As well, during early cleavage stages, expression of plasmid-borne genes becomes dependent on enhancers. Thus, developmentally regulated changes in nuclear and chromatin composition occur during early mammalian embryogenesis, and these may be important for the initiation and regulation of embryonic gene activity.
核结构和染色质组成的变化在许多细胞类型中调节基因活性，并且在早期哺乳动物胚胎发生过程中可能发挥类似作用。小鼠卵母细胞含有体细胞中存在的三种主要核纤层蛋白，尽管卵母细胞合成的核纤层蛋白A的分子量比体细胞中的大。卵母细胞染色质含有与体细胞相似的核心组蛋白，以及与鱼精蛋白免疫相关的成分。相比之下，体细胞型组蛋白H1不存在。在哺乳动物卵母细胞中尚未鉴定出DNA拓扑异构酶II，但在蛙卵母细胞中含量丰富。与卵母细胞不同，精子不含有典型的核纤层。直到精子发生后期才能检测到DNA拓扑异构酶II。虽然精子的DNA主要与鱼精蛋白结合，但可能会保留一些组蛋白。也有证据表明细胞核中DNA的排列是非随机的。这些结果表明卵母细胞和精子在核和染色质组成上存在差异。受精后，卵裂期卵裂球的细胞核经历程序性修饰。合成了核纤层蛋白B，而核纤层蛋白A没有。此外，在小鼠的二细胞期会短暂合成一组核蛋白。胚胎染色质组成也会发生变化。小鼠卵母细胞和囊胚中不同核心组蛋白基因转录本的相对丰度不同。此外，从四细胞中期开始可以检测到体细胞组蛋白H1。同样，在早期卵裂阶段，质粒携带基因的表达开始依赖于增强子。因此，在早期哺乳动物胚胎发生过程中会发生核和染色质组成的发育调控变化，这些变化可能对胚胎基因活性的启动和调节很重要。
