Why are animals 3D?
By Citlali Helenes Gonzalez, on 23 November 2017
Have you heard that our body is mainly composed of 70% water? Although true, the percentage varies from 55% of water in adult women, all the way up to 78% in babies, with the percentage for adult men somewhere in between. This is also true for animals, where some — like the jellyfish — have even 90% of their body composed of water. With this in mind, why don’t animals, including us, look like a soup? How can animals have a defined 3D structure?
Animals are made out of cells, the building blocks of our organs and tissues. But cells are basically a bag of water and chemicals; so again, why don’t animals look like giant bags of chemicals? The most obvious reason is that animals have bones that give structure to the rest of the body. But even bones are 31% water, and organs with no bones, such as hearts, still have a unique 3D form. Hearts have defined chambers (see the elephant heart below); they’re not just a mush of cells. The answer lies not in the cells themselves but in what surrounds them.
Cells are engulfed by the extracellular matrix (ECM) which is mainly composed of proteins. This matrix encompasses the space in-between cells, gives them structural support and acts like a scaffold. It can also act as a pathway for cells to migrate along and it gives out chemical and physical cues that cells respond to. The ECM varies from organ to organ. The brain, for example, is mainly composed of cells with an ECM of only 20% of the total mass. In contrast, cartilage has fewer cells and around 70% of its mass is ECM. Every cell type is surrounded by a specific matrix that will affect its function. Studying this extracellular environment is important to understand how cells develop, how they interact with each other, and how they react to disease.
At the same time, by studying the ECM, researchers can get an idea of how an organ or tissue is structured and how to replicate its intricate architecture. Scientists that work in tissue engineering use a technique which consists of washing away the cells of an organ, literally. By using detergents, the cells are washed away in cycles until just the extracellular matrix is left. In this manner, they can analyse its composition and experiment with the matrix with the end goal of growing an organ in the lab. Therefore, one day we could replace diseased or aged organs with new ones without the need for transplantation. The unique composition of the ECM provides cells with the support they need to survive, and at the same time, gives animals and their organs a defined 3D structure.