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Article Evaluation: Midblastula

None of the terms are explained properly and it could use a lot more detail. The original editor used credible sources but they could be expanded upon and some of the reference links do not work. The talk page did not have much activity aside from discussion on improving the references. This page is part of the WikiProjectBiology and was last updated in 2015 so it could use some work. If I were to improve it, I would include detailed definitions and descriptions of scientific terminology. I would also expand on the number of section headings as there are only summary and "timing" sections.

 Draft 

In developmental biology, midblastula or midblastula transition (MBT) occurs during the blastula stage of embryonic development. Several things happen to characterize the midblastula transition. Zygotic gene transcription is activated. The cell cycle slows down and becomes longer and asynchronous. Cells also start to gain motility.

Midblastula Embryo

Zygote Before MBT

Before the zygote undergoes the midblastula transition it is in a state of fast and constant replication of cells. The cell cycle is very short. The zygote is not producing its own mRNA but is using its maternal mRNA that was inherited from the oocyte to produce proteins necessary for zygotic growth. The cells in the zygote are replicating synchronously, always undergoing cell division at the same time. The zygotic genetic material is not being used so it is in a state of hemi-methylation, which is also called heterochromatin which puts the genetic material in a repressed state. This DNA is tightly packed together inside the cell because it is not being used for transcription. The midblastula transition is activated when the nucleocytoplasmic ratio reaches a threshold.

Characteristics of MBT

Activation of Zygotic Gene Transcription

Maternal-zygotic-transition

At this stage, the zygote starts producing RNA that is made from its own DNA, no longer the maternal RNA templates. This can also be called the maternal to zygotic transition. The maternal RNA is degraded and zygotic transcription begins. Since the cells are now transcribing their own DNA, this stage is where differential expression of paternal genes is first observed.

Cell Cycle Changes

Cell Cycle with G1 and G2 phases

When the zygote begins to produce its own mRNA, the cell cycle begins to slow down and the G1 and G2 phases are added to the cell cycle. The addition of these phases slows the cell cycle down but also allows for the cell to proofread the new genetic material it is making to ensure there are no mutations. It is critical that these phases be added so the zygote can accomplish the maternal to zygotic transition. The asynchronous nature of the cell divisions is an important change that occurs during/after the MBT. At this stage the zygote is developing its mesoderm and the timing of the cell divisions is important for this to occu r.

Cell Motility

Cells also start to gain cell motility, and the cells can start moving and organizing themselves within the zygote. The mRNA is localized in different parts of the oocyte, so that as the embryo divides it is segregated into different cells. This segregation is thought to underlie much of the differentiation of cells that occurs after MBT.

Timing of MBT

The timing of MBT varies between different organisms. Zebrafish MBT occurs at cycle 10, but it occurs at cycle 13 in both Xenopus and Drosophila. Cells are thought to time the MBT by measuring the nucleocytoplasmic 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 the observation that the timing of MBT can be sped up by adding extra DNA to make the nucleus larger, or by halving the amount of cytoplasm. The exact method by which the cell achieves this control is unknown, but it is thought to involve a protein that is in the cytosol.

In Drosophila, the zinc-finger transcription factor Zelda is bound to regulatory regions of genes expressed by the zygote, and in zebrafish, the homeodomain protein Pou5f3 (a paralog of mammalian POU5F1 (OCT4) has an analogous role. 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.