Question

Trees and most plants simply keep growing. There are no signals for these organisms to stop getting bigger. Many animals, humans included, only grow up to a certain size. Propose a mechanism by which humans only grow to a particular size. I’m not looking for a specific answer you can find somewhere, I simply want you to think about how you stop growing taller after a certain age. You will need to think about how you control growth from when you were one cell big to your current height.

Answer 1

Plants have the ability for unlimited growth because of the presence of meristematic tissue at several locations in their body. The cells of these meristems self-perpetuate and divide. Also, the plants are static, therefore, the big size with sufficient growth support is not harmful whereas animals are mobile in nature, thus, the huge size will result in an increase in energy need and hamper its mobility.

A limit on adult animal's body size is imposed when its body growth progressively slows down with time. This deceleration in mammal's growth is prompted by potent or vigorous suppression of cell proliferation in varied tissues and is driven primarily by local mechanisms instead of systemic.

This progressive decline in proliferation is a result of a genetic program that happens in multiple organs and includes the down-regulation of a huge set of growth-promoting genes. This kind of program depends on growth itself and is not driven simply by time. Thus, a negative feedback loop imposes the limit on adult body size.

Different organs use diverse types of information to accurately and precisely target their adult size. For example:-

• Myostatin (whose concentration depends on muscle mass itself) negatively regulates the cardiac and skeletal muscle growth.
• Bile acid flux (a parameter that reflects the function of the organ) modulates liver growth.
• The initial number of progenitor cells limits the pancreas size, implying a mechanism dependant on cell-cycle counting.