Huerta Zepeda Alejandra, Torres Padilla María Elena, Guerra López Rodrigo
Departamento de Biología Molecular y Biotecnología, Instituto de Investigaciones Biomédicas, UNAM, Ciudad Universitaria, México, DF.
Ginecol Obstet Mex. 2008 Jan;76(1):52-64.
The development of the mammalian embryo begins with the fertilization of the mature oocyte by the sperm. However, many processes that lead to the production of functional gametes precede this event. First of all, both male and female germ cells form during gametogenesis. The gametogenesis comprises four different steps: a) the specification and migration of primordial germ cells, b) the increase in the number of germ cells through mitotic divisions, c) the reduction in chromosomal number through meiosis, and d) a final structural and functional maturation of the oocyte and the sperm. Once the oocyte and the sperm have matured, the newly formed gametes are released from the gonads upon the appropriate hormonal stimulus and are subsequently transported to the oviduct, where the oocyte awaits to be fertilized by the sperm. The fertilized oocyte, now called zygote, undergoes the maternal-to-zygotic transition, characterized by the degradation of maternal transcripts and the concomitant synthesis of transcripts by the newly formed zygote. The production of these new transcripts is the result of the genome activation of the zygote. At the same time, the sperm and egg's chromatin experience a series of changes that will result in the formation of the male and female pronuclei. In the male pronucleus an exchange of protamines for histones takes place. Furthermore, the parental genomes are subject to modification through DNA demethylation, and the proteins, around which the DNA is 'packed', the histones, are also subject to covalent modifications. These modifications constitute some of the most prominent changes involved in the epigenetic reprogramming of the two gametes. Finally, the animal-vegetal poles that will begin the first divisions or 'cleavage' to give rise to the blastocyst, where we can already distinguish an embryonic-abembryonic axis. The blastocyst will then implant in the uterus previously prepared for implantation.
哺乳动物胚胎的发育始于精子使成熟卵母细胞受精。然而，在这一事件之前，还有许多导致功能性配子产生的过程。首先，雄性和雌性生殖细胞在配子发生过程中形成。配子发生包括四个不同的步骤：a）原始生殖细胞的特化和迁移；b）通过有丝分裂增加生殖细胞数量；c）通过减数分裂减少染色体数量；d）卵母细胞和精子的最终结构和功能成熟。一旦卵母细胞和精子成熟，新形成的配子在适当的激素刺激下从性腺释放出来，随后被运输到输卵管，卵母细胞在那里等待精子受精。受精后的卵母细胞，现在称为合子，经历母源-合子转变，其特征是母源转录本的降解以及新形成的合子同时合成转录本。这些新转录本的产生是合子基因组激活的结果。与此同时，精子和卵子的染色质经历一系列变化，最终形成雄原核和雌原核。在雄原核中，鱼精蛋白被组蛋白取代。此外，亲代基因组通过DNA去甲基化进行修饰，围绕DNA“包装”的蛋白质——组蛋白，也会发生共价修饰。这些修饰是两个配子表观遗传重编程中一些最显著的变化。最后，动物极和植物极将开始第一次分裂或“卵裂”，形成囊胚，在囊胚中我们已经可以区分胚胎-反胚胎轴。然后，囊胚将植入先前准备好植入的子宫中。
