Midblastula

Last updated

In developmental biology, midblastula or midblastula transition (MBT) occurs during the blastula stage of embryonic development. During this stage, the embryo is referred to as a blastula. The series of changes to the blastula that characterize the midblastula transition include activation of zygotic gene transcription, slowing of the cell cycle, increased asynchrony in cell division, and an increase in cell motility.

Contents

Blastula Before MBT

This is an embryo 4 hours after fertilization before it has undergone MBT. 3 layers are present: yolk syncytial layer (YSL), enveloping layer (EVL), and deep cells (DEL). Zfish midblastula stage embryo.jpg
This is an embryo 4 hours after fertilization before it has undergone MBT. 3 layers are present: yolk syncytial layer (YSL), enveloping layer (EVL), and deep cells (DEL).

Before the embryo undergoes the midblastula transition it is in a state of fast and constant replication of cells. [1] The cell cycle 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 mRNA 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. [2] The zygotic DNA (genetic material) is not being used because it is repressed through a variety of mechanisms such as methylation. [2] This repressed DNA is sometimes referred to as heterochromatin and is tightly packed together inside the cell because it is not being used for transcription.

Characteristics of the MBT

Maternal-zygotic-transition. Changes in RNA over time as the embryo goes through changes in structure from the 1 cell stage to the gastrula stage. The drop in maternal RNA concentration shows the midblastula transition. Blue line represents maternal mRNA and red line represents zygotic mRNA. Maternal-zygotic-transition.png
Maternal-zygotic-transition. Changes in RNA over time as the embryo goes through changes in structure from the 1 cell stage to the gastrula stage. The drop in maternal RNA concentration shows the midblastula transition. Blue line represents maternal mRNA and red line represents zygotic mRNA.

Before the zygote undergoes the midblastula transition it is in a state of fast and constant replication of cells.

Cell Cycle with G1 and G2 phases. Orange circle shows what the cell cycle looks like before the MBT. The multi-colored inside circle shows what the cell cycle looks like after MBT and the addition of G1 and G2 phases. Cell Cycle 3-2.svg
Cell Cycle with G1 and G2 phases. Orange circle shows what the cell cycle looks like before the MBT. The multi-colored inside circle shows what the cell cycle looks like after MBT and the addition of G1 and G2 phases.

Activation of Zygotic Gene Transcription

At this stage, the zygote starts producing its own mRNAs that are made from its own DNA, and no longer uses the maternal mRNA. [3] This can also be called the maternal to zygotic transition. The maternal mRNAs are then degraded. [3] Since the cells are now transcribing their own DNA, this stage is where expression of paternal genes is first observed. [3]

Cell Cycle Changes

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. [1] 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.

Cell Motility

Timing of MBT

The timing of MBT varies between different organisms. Zebrafish MBT occurs at cycle 10, [1] 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 experiments showing that the timing of MBT can be sped up by adding extra DNA [4] 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 proteins 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, [5] the homeodomain protein Pou5f3 (a paralog of mammalian POU5F1 (OCT4) has an analogous role. [6] 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.

Related Research Articles

Developmental biology is the study of the process by which animals and plants grow and develop. Developmental biology also encompasses the biology of regeneration, asexual reproduction, metamorphosis, and the growth and differentiation of stem cells in the adult organism.

<i>Xenopus</i> Genus of amphibians

Xenopus is a genus of highly aquatic frogs native to sub-Saharan Africa. Twenty species are currently described within it. The two best-known species of this genus are Xenopus laevis and Xenopus tropicalis, which are commonly studied as model organisms for developmental biology, cell biology, toxicology, neuroscience and for modelling human disease and birth defects.

Blastulation Sphere of cells formed during early embryonic development in animals

Blastulation is the stage in early animal embryonic development that produces the blastula. The blastula is a hollow sphere of cells known as blastomeres surrounding an inner fluid-filled cavity called the blastocoel. Embryonic development begins with a sperm fertilizing an egg cell to become a zygote, which undergoes many cleavages to develop into a ball of cells called a morula. Only when the blastocoel is formed does the early embryo become a blastula. The blastula precedes the formation of the gastrula in which the germ layers of the embryo form.

A maternal effect is a situation where the phenotype of an organism is determined not only by the environment it experiences and its genotype, but also by the environment and genotype of its mother. In genetics, maternal effects occur when an organism shows the phenotype expected from the genotype of the mother, irrespective of its own genotype, often due to the mother supplying messenger RNA or proteins to the egg. Maternal effects can also be caused by the maternal environment independent of genotype, sometimes controlling the size, sex, or behaviour of the offspring. These adaptive maternal effects lead to phenotypes of offspring that increase their fitness. Further, it introduces the concept of phenotypic plasticity, an important evolutionary concept. It has been proposed that maternal effects are important for the evolution of adaptive responses to environmental heterogeneity.

The G1 phase, gap 1 phase, or growth 1 phase, is the first of four phases of the cell cycle that takes place in eukaryotic cell division. In this part of interphase, the cell synthesizes mRNA and proteins in preparation for subsequent steps leading to mitosis. G1 phase ends when the cell moves into the S phase of interphase. Around 30 to 40 percent of cell cycle time is spent in the G1 phase.

An oocyte, oöcyte, or ovocyte is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. An oocyte is produced in a female fetus in the ovary during female gametogenesis. The female germ cells produce a primordial germ cell (PGC), which then undergoes mitosis, forming oogonia. During oogenesis, the oogonia become primary oocytes. An oocyte is a form of genetic material that can be collected for cryoconservation.

<i>Drosophila</i> embryogenesis Embryogenesis of the fruit fly Drosophila, a popular model system

Drosophila embryogenesis, the process by which Drosophila embryos form, is a favorite model system for genetics and developmental biology. The study of its embryogenesis unlocked the century-long puzzle of how development was controlled, creating the field of evolutionary developmental biology. The small size, short generation time, and large brood size make it ideal for genetic studies. Transparent embryos facilitate developmental studies. Drosophila melanogaster was introduced into the field of genetic experiments by Thomas Hunt Morgan in 1909.

Animal embryonic development Process by which the embryo forms and develops

In developmental biology, animal embryonic development, also known as embryogenesis, is the developmental stage of an animal embryo. Embryonic development starts with the fertilization of an egg cell (ovum) by a sperm cell, (spermatozoon). Once fertilized, the ovum becomes a single diploid cell known as a zygote. The zygote undergoes mitotic divisions with no significant growth and cellular differentiation, leading to development of a multicellular embryo after passing through an organizational checkpoint during mid-embryogenesis. In mammals, the term refers chiefly to the early stages of prenatal development, whereas the terms fetus and fetal development describe later stages.

E2F is a group of genes that encodes a family of transcription factors (TF) in higher eukaryotes. Three of them are activators: E2F1, 2 and E2F3a. Six others act as suppressors: E2F3b, E2F4-8. All of them are involved in the cell cycle regulation and synthesis of DNA in mammalian cells. E2Fs as TFs bind to the TTTCCCGC consensus binding site in the target promoter sequence.

Cyclin A is a member of the cyclin family, a group of proteins that function in regulating progression through the cell cycle. The stages that a cell passes through that culminate in its division and replication are collectively known as the cell cycle Since the successful division and replication of a cell is essential for its survival, the cell cycle is tightly regulated by several components to ensure the efficient and error-free progression through the cell cycle. One such regulatory component is cyclin A which plays a role in the regulation of two different cell cycle stages.

<i>Krüppel</i>

Krüppel is a gap gene in Drosophila melanogaster, located on the 2R chromosome, which encodes a zinc finger C2H2 transcription factor. Gap genes work together to establish the anterior-posterior segment patterning of the insect through regulation of the transcription factor encoding pair rule genes. These genes in turn regulate segment polarity genes. Krüppel means "cripple" in German, named for the crippled appearance of mutant larvae, who have failed to develop proper thoracic and anterior segments in the abdominal region. Mutants can also have abdominal mirror duplications.

In the field of developmental biology, regional differentiation is the process by which different areas are identified in the development of the early embryo. The process by which the cells become specified differs between organisms.

The Nodal signaling pathway is a signal transduction pathway important in regional and cellular differentiation during embryonic development.

Ectoderm specification Stage in embryonic development

In Xenopus laevis, the specification of the three germ layers occurs at the blastula stage. Great efforts have been made to determine the factors that specify the endoderm and mesoderm. On the other hand, only a few examples of genes that are required for ectoderm specification have been described in the last decade. The first molecule identified to be required for the specification of ectoderm was the ubiquitin ligase Ectodermin ; later, it was found that the deubiquitinating enzyme, FAM/USP9x, is able to overcome the effects of ubiquitination made by Ectodermin in Smad4. Two transcription factors have been proposed to control gene expression of ectodermal specific genes: POU91/Oct3/4 and FoxIe1/Xema. A new factor specific for the ectoderm, XFDL156, has shown to be essential for suppression of mesoderm differentiation from pluripotent cells.

This article is about the role of Fibroblast Growth Factor Signaling in Mesoderm Formation.

Maternal to zygotic transition is the stage in embryonic development during which development comes under the exclusive control of the zygotic genome rather than the maternal (egg) genome. The egg contains stored maternal genetic material mRNA which controls embryo development until the onset of MZT. After MZT the diploid embryo takes over genetic control. This requires both zygotic genome activation (ZGA) and degradation of maternal products. This process is important because it is the first time that the new embryonic genome is utilized and the paternal and maternal genomes are used in combination. The zygotic genome now drives embryo development.

In molecular biology mir-430 microRNA is a short RNA molecule. MicroRNAs function to regulate the expression levels of other genes by several mechanisms.

<i>Homeotic protein bicoid</i> Protein-coding gene in the species Drosophila melanogaster

Homeotic protein bicoid is encoded by the bcd maternal effect gene in Drosophilia. Homeotic protein bicoid concentration gradient patterns the anterior-posterior (A-P) axis during Drosophila embryogenesis. Bicoid was the first protein demonstrated to act as a morphogen. Although bicoid is important for the development of Drosophila and other higher dipterans, it is absent from most other insects, where its role is accomplished by other genes.

Smaug (protein) RNA-binding protein in Drosophila

Smaug is a RNA-binding protein in Drosophila that helps in maternal to zygotic transition (MZT). The protein is named after the fictional character Smaug, the dragon in J.R.R. Tolkien's 1937 novel The Hobbit. The MZT ends with the midblastula transition (MBT), which is defined as the first developmental event in Drosophila that depends on zygotic mRNA. In Drosophila, the initial developmental events are controlled by maternal mRNAs like Hsp83, nanos, string, Pgc, and cyclin B mRNA. Degradation of these mRNAs, which is expected to terminate maternal control and enable zygotic control of embryogenesis, happens at interphase of nuclear division cycle 14. During this transition smaug protein targets the maternal mRNA for destruction using miRs. Thus activating the zygotic genes. Smaug is expected to play a role in expression of three miRNAs – miR-3, miR-6, miR-309 and miR-286 during MZT in Drosophila. Among them smaug dependent expression of miR-309 is needed for destabilization of 410 maternal mRNAs. In smaug mutants almost 85% of maternal mRNA is found to be stable. Smaug also binds to 3′ untranslated region (UTR) elements known as SMG response elements (SREs) on nanos mRNA and represses its expression by recruiting a protein called Cup(an eIF4E-binding protein that blocks the binding of eIF4G to eIF4E). There after it recruits deadenylation complex CCR4-Not on to the nanos mRNA which leads to deadenylation and subsequent decay of the mRNA. It is also found to be involved in degradation and repression of maternal Hsp83 mRNA by recruiting CCR4/POP2/NOT deadenylase to the mRNA. The human Smaug protein homologs are SAMD4A and SAMD4B.

Antonio Jesus Giraldez is a Spanish developmental biologist and RNA researcher at Yale University School of Medicine, where he serves as chair of the department of genetics and Fergus F. Wallace Professor of Genetics. He is also affiliated with the Yale Cancer Center and the Yale Stem Cell Center.

References

  1. 1 2 3 Kane D, Kimmel C (1993). "The zebrafish midblastula transition". Development. 119 (2): 447–456. doi:10.1242/dev.119.2.447. PMID   8287796.
  2. 1 2 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.
  3. 1 2 3 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 ]
  4. ↑ 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.
  5. ↑ 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 ]
  6. ↑ 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.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.