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Until now, cells dividing by mitosis were thought to grow round and then split into two identical, spherical daughter cells.
New research has found that some cells are isomorphic, meaning they retain their original shape during mitosis and do not turn round.
Isomorphic cells usually divide into asymmetrical daughter cells that are meant for different layers of tissue.
Maybe you remember high school biology. It was probably then that you learned about cells dividing through mitosis, and mentally repeated the terms interphase, prophase, metaphase, anaphase, telophase ad nauseam until they were memorized. What you didn’t know was that something was missing from your sophomore year textbook.
Even scientists were unaware that mitosis is not always symmetrical—until recently. Led by biologists Shane Herbert and Holly Lovegrove of the University of Manchester, a team of researchers found that cells do not always divide into two identical, spherical shapes. The shape of the cell before the phenomenon of mitosis begins can influence whether the cell will become rounded as it divides. If the cell rounds or not will determine if the daughter cells are symmetrical or asymmetrical, not just in terms of size and shape but also in function. Asymmetrical daughters may be headed for different layers of tissue post-mitosis.
“During tissue formation, dynamic cell shape changes drive morphogenesis while asymmetric divisions create cellular diversity,” Herbert and Lovegrove said in a study recently published in Science.
“We found that the shifts in cell morphology that shape tissues could…trigger asymmetric division and direct core identity decisions underpinning tissue building.”
The researchers observed this in the stem cells of zebrafish embryos that were undergoing angiogenesis, or blood vessel formation. Tissues in embryonic organisms are created by the constant reorganization of cells. Both changes in cell shape and diversity among cells affect morphogenesis, or the development of tissues and organs. Since there are multiple layers to tissues, there also need to be multiple shapes of cells, such as epithelial cells that form the outer layer of a tissue, endothelial cells that create a layer which lines blood vessels, or mesenchymal cells which differentiate into connective tissues.
If the asymmetry leads to cell diversity, then why do most metazoan cells (those of all multicellular animals) tend to grow round before mitosis? Mitotic rounding makes sure that genetic material is evenly distributed and that the daughter cells are the same size. However, endothelial cells going through angiogenesis need to reshape themselves from spherical to elongated. Herbert, Lovegrove, and their team discovered that these cells remained isomorphic throughout mitosis, holding onto their morphology from interphase, the period when they are not dividing, without rounding. They also divided asymmetrically to create daughter cells with a longer shape. Asymmetric division was found to be more likely in isomorphic cells.
Would isomorphism in zebrafish translate to human cells? To find out, the researchers used micropatterning, which involves placing cells on patches of proteins in different shapes. Shapes were burned onto a nonstick surface with a UV laser, and cells were only be able to cling to the patches that had been burned onto that surface, each taking on the shape of its patch. This makes it possible to manipulate their shape in vitro and see what happens when the reshaped cells divide. It turned out to be more likely for dividing cells that were already more elongated to stay isomorphic, while shorter cells were more stubborn and grew round.
“Maintenance of interphase morphology in isomorphic division enables extensive information on pre-mitotic cell state to be passed on to daughters, most of which is lost and reset in classical mitotic rounding,” the researchers said in the study. “Indeed, isomorphic division is a much simpler means to retain information than the [more complex process of rounding].”
This revelation about mitosis can go even further. Because asymmetric cell division can cause cells to behave in ways cause them to metastasize, it change how we understand the division of cancerous cells and the spread of cancer, which could lead to next-gen treatments. For now, the discovery is making millions of high school textbooks obsolete.
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