Midblastula transition

Hlfoo Oocytes In the zygote up to midblastula transition Replaces somatic HI during somatic cell nuclear transfer RI ... 

In contrast to Xenopus laevis, the maternal histone pool in the mouse one-cell embryo (based on synthetic rates for histones) is probably sufficient for only one to two rounds of DNA replication (Wassarman and Mrozak, 1981). Consistent with such a small histone pool is the observation that poly spermic eggs have the capacity to transform up to three to four sperm nuclei into metaphase chromosomes (Clarke and Masui, 1986) a similar capacity was also determined from experiments that manipulated the cytoplasmic volume by either bisection or cell fusion (Clarke and Masui, 1987). This small pool of maternal histones may hence be insufficient to prevent effectively the assembly of stable basal transcription complexes. Thus, titration of the maternal histone pool by an increase in the mass of DNA due to blasto-mere proliferation may not be a critical factor in regulating the onset of transcription in the mouse embryo and other mammalian eggs this is because the maternal transcription factors may be able to outcompete successfully maternal histones for the newly replicated chromatin. This could, at least in part, account for the early onset of transcription in mammalian embryos ranging from rodents to humans (Telford et al., 1990). Moreover, the lack of arapid S phase in the mouse embryo and other mammalian embryos would permit sufficient time for productively assembled transcription complexes to generate full-length transcripts. In contrast to mammalian embryos, S phase is very short prior to the midblastula transition in Xenopus laevis (Newport and Kirschner, 1982) and hence these rapid rounds of DNA replication could prematurely terminate the transcription of genes for which transcription had initiated. 

It is important to note that early stages of Xenopus development rely completely on maternal stores of RNA and protein transcription does not begin until the so-called midblastula transition, about 7h after fertilization and when there are 4096 cells (that is, after 12 cleavages) (7). 

This pattern of development—one cleavage every 30 min—lasts until cycle 13, when division becomes asynchronous, and it slows down significantly (7). This point, the midblastula transition (MBT), is also marked by the onset of cell motility and zygotic transcription. It corresponds to stage 8 of (6) and presages the next important stage in Xenopus development gastrulation. 

Understanding the molecular basis of pattern formation and differentiation in frog embryos was previously hindered by the lack of a system for temporal and tissue-specific expression of wild-type and mutant forms of developmentally important genes. RNA injection, the most common transient expression method in Xenopus, has been effectively used to study maternally expressed genes. However, since RNAs are translated immediately after injection, this method is unfavorable for the study of zygotic gene products that are expressed only after the midblastula transition. Direct injection of DNA can be used to express genes behind temporal and tissue-specific promoters after the midblastula transition. 

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