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Midblastula
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<h2 id="blastula-before-mbt">Blastula Before MBT</h2> <p>Before the embryo undergoes the midblastula transition it is in a state of fast and constant replication of cells.<a class="footnote-ref" id="fnref:1" href="#fn:1"><sup>1</sup></a> The <a href="/facts/Cell_cycle/nLMT7K4r">cell cycle</a> is very short. The cells in the zygote are also replicating synchronously, always undergoing cell division at the same time. The zygote is not producing its own <a href="/facts/Messenger_RNA/oUL6qxQn">mRNA</a> but rather it is using mRNAs that were produced in the mother and loaded into the oocyte in order to produce proteins necessary for zygotic growth.<a class="footnote-ref" id="fnref:2" href="#fn:2"><sup>2</sup></a> The zygotic DNA (<a href="/facts/Genome/31Sm08JT">genetic material</a>) is not being used because it is repressed through a variety of mechanisms such as <a href="/facts/Methylation/EvDMWVW5">methylation</a>.<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a> This repressed DNA is sometimes referred to as <a href="/facts/Heterochromatin/frJ8HL41">heterochromatin</a> and is tightly packed together inside the cell because it is not being used for transcription. </p> <h2 id="characteristics-of-the-mbt">Characteristics of the MBT</h2> <p>Before the zygote undergoes the midblastula transition it is in a state of fast and constant replication of cells. </p> <h3>Activation of <a href="/facts/Zygote/mSu2xMWA">Zygotic</a> <a href="/facts/Gene_transcription/bDgp9HDN">Gene Transcription</a></h3> <p>At this stage, the zygote starts producing its own mRNAs that are made from its own DNA, and no longer uses the maternal mRNA.<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a> This can also be called the <a href="/facts/Maternal_to_zygotic_transition/WTzbmltv">maternal to zygotic transition</a>. The maternal mRNAs are then degraded.<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a> Since the cells are now transcribing their own DNA, this stage is where expression of <a href="/facts/Genomic_imprinting/WPD8DaA4">paternal genes</a> is first observed.<a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a> </p> <h3><a href="/facts/Cell_cycle/nLMT7K4r">Cell Cycle</a> Changes</h3> <p>When the zygote begins to produce its own mRNA, the cell cycle begins to slow down and the <a href="/facts/G1_phase/92lq1LdT">G1</a> and <a href="/facts/G2_phase/alNtT0m1">G2 phases</a> are added to the cell cycle.<a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a> The addition of these phases allows the cell to have more time to proofread the new genetic material it is making to ensure there are no mutations. The asynchronous nature of the cell divisions is an important change that occurs during/after the MBT. </p> <h3><a href="/facts/Cell_migration/5ySAMquQ">Cell Motility</a></h3> <h2 id="timing-of-mbt">Timing of MBT</h2> <p>The timing of MBT varies between different organisms. Zebrafish MBT occurs at cycle 10,<a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a> but it occurs at cycle 13 in both <i>Xenopus</i> and <i>Drosophila</i>. Cells are thought to time the MBT by measuring the <a href="/facts/NC_ratio/Ypz2yulp">nucleocytoplasmic</a> ratio, which is the ratio between the volume of the nucleus, which contains DNA, to the volume of cytosol. Evidence for this hypothesis comes from experiments showing that the timing of MBT can be sped up by adding extra DNA<a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a> to make the nucleus larger, or by halving the amount of cytoplasm. The exact methods by which the cell achieves this control is unknown, but it is thought to involve <a href="/facts/Protein/Jxmagu3E">proteins</a> in the <a href="/facts/Cytosol/VyXgygWm">cytosol</a>. </p><p>In <i>Drosophila</i>, the zinc-finger transcription factor Zelda is bound to regulatory regions of genes expressed by the zygote, and in zebrafish,<a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a> the homeodomain protein Pou5f3 (a paralog of mammalian POU5F1 (OCT4) has an analogous role.<a class="footnote-ref" id="fnref:11" href="#fn:11"><sup>11</sup></a> Without the function of these proteins MBT gene expression synchrony is disrupted, but particular mechanisms of coordinating the timing of gene expression are still unknown but being studied. </p> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>Kane D, Kimmel C (1993). "The zebrafish midblastula transition". Development. 119 (2): 447–456. doi:10.1242/dev.119.2.447. PMID 8287796. <a href="http://dev.biologists.org/cgi/content/abstract/119/2/447" target="_blank">http://dev.biologists.org/cgi/content/abstract/119/2/447</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>Meehana R, Dunicana D, Ruzova A, Pennings S (2005). "Epigenetic silencing in embryogenesis". Exp. Cell Biol. 309 (2): 241–249. doi:10.1016/j.yexcr.2005.06.023. PMID 16112110. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>Meehana R, Dunicana D, Ruzova A, Pennings S (2005). "Epigenetic silencing in embryogenesis". Exp. Cell Biol. 309 (2): 241–249. doi:10.1016/j.yexcr.2005.06.023. PMID 16112110. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>Masui Y, Wang P (1998). "Cell cycle transition in early embryonic development of Xenopus laevis" (PDF). Biol. Cell. 90 (8): 537–548. doi:10.1016/S0248-4900(99)80011-2. PMID 10068998.[permanent dead link] <a href="http://submit.biolcell.org/boc/090/0537/boc0900537.pdf" target="_blank">http://submit.biolcell.org/boc/090/0537/boc0900537.pdf</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>Masui Y, Wang P (1998). "Cell cycle transition in early embryonic development of Xenopus laevis" (PDF). Biol. Cell. 90 (8): 537–548. doi:10.1016/S0248-4900(99)80011-2. PMID 10068998.[permanent dead link] <a href="http://submit.biolcell.org/boc/090/0537/boc0900537.pdf" target="_blank">http://submit.biolcell.org/boc/090/0537/boc0900537.pdf</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>Masui Y, Wang P (1998). "Cell cycle transition in early embryonic development of Xenopus laevis" (PDF). Biol. Cell. 90 (8): 537–548. doi:10.1016/S0248-4900(99)80011-2. PMID 10068998.[permanent dead link] <a href="http://submit.biolcell.org/boc/090/0537/boc0900537.pdf" target="_blank">http://submit.biolcell.org/boc/090/0537/boc0900537.pdf</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> <li id="fn:7"><p>Kane D, Kimmel C (1993). "The zebrafish midblastula transition". Development. 119 (2): 447–456. doi:10.1242/dev.119.2.447. PMID 8287796. <a href="http://dev.biologists.org/cgi/content/abstract/119/2/447" target="_blank">http://dev.biologists.org/cgi/content/abstract/119/2/447</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li> <li id="fn:8"><p>Kane D, Kimmel C (1993). "The zebrafish midblastula transition". Development. 119 (2): 447–456. doi:10.1242/dev.119.2.447. PMID 8287796. <a href="http://dev.biologists.org/cgi/content/abstract/119/2/447" target="_blank">http://dev.biologists.org/cgi/content/abstract/119/2/447</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></p></li> <li id="fn:9"><p>Mita I, Obata C (1984). "Timing of early morphogenetic events in tetraploid starfish embryos". J. Exp. Zool. 229 (2): 215–222. doi:10.1002/jez.1402290206. Archived from the original on 2013-01-05. <a href="https://archive.today/20130105084212/http://www3.interscience.wiley.com/journal/110494611/abstract" target="_blank">https://archive.today/20130105084212/http://www3.interscience.wiley.com/journal/110494611/abstract</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></p></li> <li id="fn:10"><p>Mita I (1983). "Studies on factors affecting the timing of early morphogenetic events during starfish embryogenesis". J. Exp. Zool. 225 (2): 293–9. doi:10.1002/jez.1402250212.[dead link] <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></p></li> <li id="fn:11"><p>Harrison, MM; Li, XY; Kaplan, T; Botchan, MR; Eisen, MB (Oct 2011). "Zelda binding in the early Drosophila melanogaster embryo marks regions subsequently activated at the maternal-to-zygotic transition". PLOS Genet. 7 (10): e1002266. doi:10.1371/journal.pgen.1002266. PMC 3197655. PMID 22028662. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3197655" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3197655</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></p></li> </ol>