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Asynchronous waves key to embryo development
Cell division — known as mitosis — produces two identical daughter cells. It is a part of the cell cycle where chromosomes in a cell nucleus are separated into two identical groups, with each group ending up in its own nucleus. In general, mitosis is the division of the nucleus, and is often followed by cytokinesis, when the cytoplasm divides all it contains including organelles and cell membrane, into two new cells containing roughly equal shares of all of these components.
The study was published in the journal Developmental Cell.
In order to grow, animal embryos must rapidly create more DNA through mitosis to increase their number and diversity of cells. Gap phases occur between cell cycles at the point of transition between phases: Gap 1 (G1) and Synthesis (S), Gap 2 (G2) and Mitosis (M). All phases are controlled by proteins such as cyclins and the enzyme Cdc25.
The simple nervous system of sea squirts (ascidians which are marine chordates, meaning they have a notochord or neural tube) are often used to model embryonic development in complex vertebrates. In ascidians, the embryo's outer layer of cells (its ectoderm) form a neural tube that joins together in a zipper-like motion moving from its "tail" to to its "head". But why and how does this happen?
Researchers fluorescently labeled one DNA replication protein in order to visually follow and analyze the timing of all the mitotic divions involved in creating and closing that neural tube. Using time-lapse pictures, they focused on the differences happening between the 10th and 11th mitotic cell cycles, and saw a unique G2-phase distinction occur.
This change is apparently due to a decline in the amount (expression) of the Cdc25 protein — Cdc stands for "Cell Division Cycle" — an enzyme that removes inhibitory phosphate residues controlling entry into and progression through various phases of the cell cycle.
This reversal in direction causes cell layers to cover over each other, and ultimately "zipper" a furrow of cells developing along the middle of the elongating embryo. This furrow of cells is the neural tube - to become the backbone or notochord of that organism.
The article, "Developmental Control of Cell-Cycle Compensation Provides a Switch for Patterned Mitosis at the Onset of Chordate Neurulation" was published in Developmental Cell at DOI: http://dx.doi.org/10.1016/j.devcel.2016.03.013
A sea squirt embryo has synchronized cell divisions from its tail to its head until it reaches an
11th cell cycle of cell divisions. At that point, expression of protein from the Cdc25 gene drops.
This begins an asynchronous cycle of reversed head to tail cell divisions — overlapping previous
cell layers made tail to head. Eventually, the action of these tail to head, head to tail
directional reverses "zipper" close the neural tube or notochord of that animal.
Image Credit: University of Tsukuba, Japan