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Midblastula transitionComments

Midblastula transition

The midblastula transition (MBT) refers to a number of dramatic changes that occur at the midblastula stage in some embryos and that were first observed in Xenopus.

The MBT is initiated when nuclear/cytoplasmic ratio reaches a threshold. As the cytoplasm is divided into more and more cells, this ratio increases.

Before MBT
All embryonic cells undergo cell division at the same time.
Zygotic heterochromatin (condensed, hypo-acetylated, H3 methylated).
Only maternal mRNAs are translated.

MBT Events
Activation of zygotic genes.
Cell cycle lengthened (G1 & G2 added).
Asynchronous cycles of different cells.
Development of cellular motility.

After MBT
Cell division becomes asynchronous
Embryo begins transcribing its own DNA.
Differential expression of paternal genes begins.

MBT occurs when a factor in cytoplasm has been titrated by an increased number of nuclei (high level of DNA). Increasing DNA levels (via DNA injection or extra sperm) makes the MBT occur earlier; decreasing DNA levels (via a haploid Drosophila embryo) makes the MBT occur later.

There is activation of transcription when nuclear DNA (which has increased exponentially during cleavage) reaches a certain level that titrates out some general repressor stored in egg cytoplasm. These events are:

DivisionSynchronous Rapid Divisions in Drosophila = 10 min. Asynchronous, slow divisions; G1, G2 added to cell cycle in Xenopus = 30 min
TranscriptionBegin transition from maternal to zygotic control of embryogenesis.
MotilityAmphibians = prepare for gastrulation

The first two events are also seen in the Drosophila embryo at the blastoderm stage. Not all embryos have a midblastula transition, however; the sea urchin embryo, for example, has RNA transcription starting at fertilization, so there is no dramatic increase in transcription at the blastula stage.