Most animals produce egg cells, which are the largest cells they produce. In humans, egg cells are 10,000 times larger than sperm cells and several times larger than regular body cells. The egg (or oocyte) is large for a reason: it must store enough resources to support the embryo's continued growth after fertilisation, as well as the creation of mitochondria to provide power for all of this, but the biological family has yet to be fully investigated.
For what purpose are egg cells 10,000 times larger than sperm cells? Scientists have found the perfect combination of biology and physics!
A team of biologists and mathematicians from MIT have discovered that oocytes expand incredibly quickly before fertilisation in Drosophila. These cells work on the same concept as gas-exchanging bubbles of varying sizes. The researchers discovered that the "nurture cells" that surround the larger oocytes import their contents into the larger oocytes in the same way as air enters a larger oocyte from a smaller balloon. The bigger balloons are connected by little tubes.
Jörn Dunkel, an associate professor of physics and applied mathematics at MIT, said: "This research shows how physics and biology work together to create such powerful systems, as well as how nature uses physical processes to achieve this. Consider what might happen if living creatures existed. One of the goals of embryonic development is to make the mechanism repeatable, and physics provides a powerful tool for doing so."
The oocyte will create one egg cell and 15 nursery cells
In female fruit flies, an immature oocyte goes through four cell division cycles before producing one egg cell and 15 nursery cells. On the other hand, cell separation is insufficient. Each cell communicates with the others through a network of microscopic channels. In the same way that valves allow chemicals to pass between cells, channels do the same.
The contractile force of the cells did not appear to increase at all in the early stages, according to the researchers. This indicates that the transfer mechanism is not a "squeeze" mechanism. Although it may appear that huge balloons leak air to small balloons until they reach the same size, air really flows from tiny to giant balloons. Because small balloons with a larger curvature can withstand more surface tension than large balloons, the pressure is higher, forcing air out of the tiny balloons and into the large ones. "This is a tremendously powerful mechanism," the scientist said of one of them, despite the fact that it is counter-intuitive.
The term "double balloon effect" refers to a circumstance in which two balloons are inflated simultaneously.
Based on the mathematical formula created to describe the "double balloon effect," the researchers built a model that shows how cell contents are carried from 15 small nursing cells to gigantic oocytes. It has been revealed that the nursing cells closest to the oocyte layer will transmit their contents first, followed by the feeding cells of the next layer, due to their size and interrelation. Throughout this step, there will be little squeezing and uniform shrinkage, and no positive squeezing phenomena or cell peristaltic deformation will occur until the second phase begins.
According to the researchers, these findings show how cells coordinate their behaviours via biological and physical principles, and that the development of oocytes and early embryos in Drosophila and other invertebrates is comparable to that of mammals. However, it's unclear whether humans and other mammals go through the similar egg cell growth process.