Clarke H J, McLay D W, Mohamed O A
Department of Obstetrics and Gynecology, McGill University, Montreal, Canada.
Dev Genet. 1998;22(1):17-30. doi: 10.1002/(SICI)1520-6408(1998)22:1<17::AID-DVG3>3.0.CO;2-A.
A unique characteristic of the oocyte is that, although it is a differentiated cell, it can to give rise to a population of undifferentiated embryonic cells. This transition from a differentiated to a totipotential condition is thought to be mediated in part by changes in chromatin composition or configuration. In many non-mammalian organisms, oocytes contain unique subtypes of the linker histone H1, which are replaced in early embryos by the so-called somatic histone H1 subtypes. We review evidence that such histone H1 subtype switches also occur in mammals. Immunologically detectable somatic H1 is present in mitotically proliferating oogonia but gradually becomes undetectable after the oocytes enter meiosis. Immunoreactive somatic H1 remains undetectable throughout oogenesis and the early cell cycles after fertilization. Following activation of the embryonic genome, it is assembled onto chromatin. In contrast to the absence of immunoreactive protein, mRNAs encoding each of the five mammalian somatic H1 subtypes are present in growing oocytes and newly fertilized embryos, indicating that post-transcriptional mechanisms regulate expression of these genes. This maternal mRNA is degraded at the late 2-cell stage, and embryonically encoded mRNAs accumulate after embryos reach the 4-cell stage. During the period when somatic H1 is not detectable, oocytes and embryos contain mRNA encoding a sixth subtype, histone H1(0) which accumulates in differentiated somatic cells, and the nuclei can be stained with an H1(0)-specific antibody. We propose that the linker histone composition of the oocyte lineage resembles that of other mammalian cells, namely, that the somatic H1 subtypes predominate in mitotically active oogonia, that histone H1(0) becomes prominent in differentiated oocytes, and that following fertilization and transcriptional activation of the embryonic somatic H1 genes, the somatic H1 subtypes are reassembled onto chromatin of the embryonic cells. Potential functions of these linker histone subtype switches are discussed, including stabilization by H1(0) of the differentiated state of the oocytes, protection of the oocyte chromatin from factors that remodel sperm chromatin after fertilization, and restoration by the incorporation of the somatic H1 subtypes of the totipotential state of embryonic nuclei.
卵母细胞的一个独特特征是，尽管它是一种分化细胞，但它能够产生一群未分化的胚胎细胞。这种从分化状态到全能状态的转变被认为部分是由染色质组成或构型的变化介导的。在许多非哺乳动物生物中，卵母细胞含有连接组蛋白H1的独特亚型，在早期胚胎中被所谓的体细胞组蛋白H1亚型所取代。我们综述了证据表明这种组蛋白H1亚型转换在哺乳动物中也会发生。免疫可检测的体细胞H1存在于有丝分裂增殖的卵原细胞中，但在卵母细胞进入减数分裂后逐渐变得不可检测。免疫反应性体细胞H1在整个卵子发生过程以及受精后的早期细胞周期中仍然不可检测。在胚胎基因组激活后，它被组装到染色质上。与缺乏免疫反应性蛋白相反，编码五种哺乳动物体细胞H1亚型的mRNA存在于生长中的卵母细胞和新受精的胚胎中，这表明转录后机制调节这些基因的表达。这种母体mRNA在2细胞晚期降解，胚胎编码的mRNA在胚胎达到4细胞阶段后积累。在体细胞H1不可检测的时期，卵母细胞和胚胎含有编码第六种亚型组蛋白H1(0)的mRNA，H1(0)在分化的体细胞中积累，并且细胞核可以用H1(0)特异性抗体染色。我们提出卵母细胞谱系的连接组蛋白组成类似于其他哺乳动物细胞，即体细胞H1亚型在有丝分裂活跃的卵原细胞中占主导地位，组蛋白H1(0)在分化的卵母细胞中变得突出，并且在受精和胚胎体细胞H1基因转录激活后，体细胞H1亚型被重新组装到胚胎细胞的染色质上。讨论了这些连接组蛋白亚型转换的潜在功能，包括H1(0)对卵母细胞分化状态的稳定作用、保护卵母细胞染色质免受受精后重塑精子染色质因子的影响，以及通过体细胞H1亚型的掺入恢复胚胎细胞核的全能状态。
