From the smooth blood vessels of our arteries and veins, to the unevenly textured sacs of our internal organs, our bodies are made up of various tissues that are arranged in complex shapes that help perform specific functions.
But how do cells fold themselves precisely into such complex structures during development? What are the fundamental forces driving this process?
Recently, a team of scientists discovered a mechanical process through which an otherwise simple piece of cell can transform into a delicate ring-shaped semicircular canal in the inner ear. It involves hyaluronic acid, which swells up with water produced by cells, and the forces that guide this expansion to shape the thin cell-to-cell connections of tissues. The study was recently published in the journal Cell.
Time-lapse photograph of the formation of a semicircular canal in the inner ear of a two-day-old zebrafish embryo. | Image credit: Akankshi Munjal/Acquired using Zeiss LSM 710 and 3D rendered with FlouRender
A transparent model
The team has been studying how cells develop into complex three-dimensional structures. To solve this problem, they found a classic and ideal model creature, which is the zebrafish.
Zebrafish. | Image credit: Oregon State University under CC BY-SA
Zebrafish are characterized by the fact that they are transparent and, as long as placed under a microscope, the entire process of development from a single cell to a larvae that can swim and have intact body parts can be observed.
Among the many body parts, the semicircular tube has piqued the interest of researchers. These are the three liquid-filled tubules in the inner ear that are essential parts of balance and spatial localization. Little is known about how the semicircular canal is formed, in part because in many species it is blocked by the middle and outer ears. But in zebrafish, these tubules are very close to the surface, allowing researchers to observe their development under a microscope.
Schematic diagram of the semicircular canal. | Image credit: Akankshi Munjal/Cell2021
Thus, the inner ear of the zebrafish is an ideal model for illustrating how cells work together to form the complex structures required for biological function.
A whole new mechanism
The traditional idea about the emergence of tissue shape is that two common proteins, actin and myosin, act as micro-motors within the cell, push and pull in different directions, folding the tissue into a specific shape.
But the researchers found that the semicircular canals of zebrafish are formed through a distinctly different process. During development, cells, under the control of genes, produce hyaluronic acid, a polysaccharide substance that is very common in living organisms. (Because it also has a strong water retention effect, it has also become one of the most famous anti-wrinkle agents in beauty products.) ）
Once it enters the extracellular matrix, the microenvironment outside the cell, the acid swells up, which is no different from dropping a piece of diaper into a swimming pool.
This swelling creates enough force to move nearby cells. But since the pressure is the same in all directions, the researchers also need to solve the problem of how the tissue eventually stretches in one direction, rather than the other, forming an elongated shape.
They found that this was done by a thin link between cells, also known as cytocinches, which limits the direction of this force. For example, it's like putting a tight jacket over a water balloon and making it take shape into an ellipsoid. This perfect combination of swelling and bondage gradually shaped the initially flat pieces of cells into tubes.
Schematic diagram of the semicircular canal formation mechanism of zebrafish. | Image credit: Cell
Broader meaning
The team believes that although the study was conducted in zebrafish, the findings could have broader implications.
Genes that control hyaluronic acid production in zebrafish semicircular canals are also found in mammalian semicircular canals. In addition, hyaluronic acid is also present in many parts of the body, including the skin and joints. This all suggests that they may play an important role in shaping many tissues and organs. There is reason to speculate that this may be a broad and conservative cross-species mechanism.
Cells must use many different forces to complete the structure they need. Scientists also hope the discovery will encourage researchers to think about other possibilities for participating in shaping organizations.
#创作团队:
Compile: M ka
Typography: Wenwen
#参考来源:
https://hms.harvard.edu/news/shape-things
https://www.cell.com/cell/fulltext/S0092-8674(21)01373-8
#图片来源:
Cover image: darksouls1/pixabay
First image: pixabay