Page 371 - AWSAR 2.0

is “No”. This implies that elephants have more cells than mice have. Does it mean that more cells make bigger bodies? If it is so, then to ask how proportionality is maintained might seem trivial. In biology, however, things are seldom this plain. Cells, in fact, grow in size under a lot of circumstances. For example, if one kidney fails, to take up the extra work load, the other kidney grows by increasing cell size rather than adding new cells. Similarly in partial liver damage, where a substantial part of the liver is rendered useless, the rest of the liver tissue first grows by increasing cell size and subsequently, by addition of new cells. How do cells change their size? Do cells regulate their size dynamically like organs do? How do they do so?
When it comes to changes in size, nothing beats skin. Although it is the largest organ in our body and composed of multiple layers, it starts as a thin two-layered blanket cum raincoat cum armour
that mammalian embryos are
swaddled in, even before they
are born. Impressively, skin
manages to maintain a decent
fit throughout the life no matter
how fat, thin or pregnant we get.
Hence, it is not only interesting, but also essential to understand how skin regulates its size during embryo development as well as during adaptive growth. Due to its great experimental accessibility, skin is an
attractive system to understand the detailed molecular mechanisms of cell and tissue scaling to biologists.
In our lab at the Department of biological sciences,TIFR,westudyzebrafishepidermis (outer layer of skin). Zebrafish are small fresh water fish indigenous to India and have become a popular model system among
Ms. Kirti Gupta || 347
scientists studying vertebrate biology, because of their small size (4-5 cm), rapid embryonic growth, ease of rearing, and large clutch size. Zebrafish embryos are particularly useful to study developmental processes, as they are nearly transparent, enabling biologists to study embryo development by simply using just a bench-top microscope.
Our group at TIFR started by asking the simple question: How are cell sizes and cell numbers regulated in the epidermis? To answer this, we characterized cell size and cell proliferation in the embryonic epidermis under various conditions affecting either of the two. Using genetic tools and drug treatments, we either stopped cells from growing big or stopped them from dividing. We found that although skin cells maintain a uniform size under normal conditions, they show a remarkable ability to change both their size and number when put under a stress condition.
Also, we observed that skin cell population can maintain a dynamic balance between cell size and cell number, where cell size or cell number compensate for each other to keep the tissue growing and maintaining its function as a barrier. This paradigm of inhibiting the cell division to force the cells to grow bigger hence allowed us to study processes operating at the molecular level enabling a cell to grow in size.
Although microscopic, cells comprise enormous internal complexity. One can imagine the cell to be like a city, having boundaries (cell membrane), transport system (cytoskeleton and motor proteins), storage units (Golgi), dumping yards (Lysosomes) and infrastructure to produce material goods (various metabolic processes) for its sustenance. If such a system has to
Life comes in all shapes and sizes as minute as the single- celled yeast and as gigantic as whales and trees. Unlike most plants, which continue to grow throughout their life, animals have a strict life-span and body plan.