Asymmetric Division in Development
Animals are made up of a vast number of distinct cell types. During development, the zygote undergoes many cell divisions that give rise to various cell types, including embryonic stem cells. Asymmetric divisions of these embryonic cells gives rise to one cell of the same potency (self-renewal), and another that maybe of the same potency or stimulated to further differentiate into specialized cell types such as neurons. This stimulated differentiation arises from many factors which can be divided into two broad categories: intrinsic and extrinsic. Intrinsic factors generally involve differing amounts of cell-fate determinants being distributed into each daughter cell. Extrinsic factors involve interactions with neighboring cells and the micro and macro environment of the precursor cell.
In addition to the aforementioned Drosophila neuronal example, it was proposed that the macrosensory organs of the Drosophila, specifically the glial cells, also arise from a similar set of asymmetric division from a single progenitor cell via regulation of the Notch signaling pathway and transcription factors. The precursor cell (pI) divides to produce daughter cells (pII) which further divide to produce further cells (pIII). It was found that there is an inhibition of the Notch signaling pathway in the pIIb (the anterior daughter cell) while there is an upregulation in the pIIa (the posterior daughter cell). The pIIb is then able to asymmetrically divide to further give a subepithelial cell and a daughter cell that differentiates into glial cells.
An example of how extrinsic factors bring about this phenomena is the physical displacement of one of the daughter cells out of the original stem cell niche, exposing it to signalling molecules such as chondroitin sulfate. In this manner, the daughter cell is forced to interact with the heavily sulfated molecules, which stimulate it to differentiate while the other daughter cell remains in the original niche in a quiescent state.
Asymmetric division of stem cells plays a key role in development by allowing for the differentiation of a subset of daughter cells while maintaining stem cell pluripotency. Since it can be controlled by both intrinsic and extrinsic factors, upon delineating these particular factors it may be possible to use this knowledge in applications of tissue and whole organ generation.
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