Many months ago I wrote only a short post looking at the phenomena of indeterminate growth and wondered whether it was possible to trigger that type of evolutionary or ecological step in humans entitled “Indeterminate Growth And Mammals“. Humans, like most mammals, experience determinate growth, which means that at some point, their growth will be stopped due to bone constraints. The bone constraint is when the bones don’t get any longer because the force to expand the bone volumetrically in any direction is too large for anything reasonable to do.

The reason why indeterminate growth is possible in some creatures is that the tissue that is holding the overall body form in place is cartilage, not bone. Bones are strong, light, and porous but they are too strong as well as being too brittle to be plastically deformed. Even if the necessarily high force was applied in a tensile way, the bones would rather fracture and break in half than to stretch out.

The types of animals which most people know do experience indeterminate growth lare fish and reptiles. The sharks, the largest types of fish in the world actually don’t have bones but cartilage which create the overall body shape. The backbone or vertebrate structure of sharks are cartilage, not bone tissue. This means that with enough nutrients being difussed through the cartilage backbone of a shark, the cartilage can theoretically get larger in size, making each of the cartilage vertebrate bigger, pushing the fish longer and longer. For fish, height for them is length. The bigger the cartilage get, the longer the fish become.

The size of how big fish can get can be huge. A good example is how big some catfish becomes. It may be hard to find catfish these days over 50 lbs and 3 feet long but there were stories of catfish growing to over 12 feet long when the European settlers first came to the Americas. Certain fish can live very long, and they never stop growing. The only way that fish size is usually limited is because the bigger fish are either caught by humans or eaten by something slightly bigger than them. This shows that as long as a fish is not eaten by a bigger animal or caught by humans, certain fish species will grow longer and longer until they become monsters. I am not sure whether some aquatic animals ever grow so big that they have no natural predators and actually die from natural causes like disease and infections. I guess the idea of the squid which grows into the giant squid might answer the issue with an affirmation.

The issue for humans to develop anything like indeterminate growth is to turn their bones into cartilage which can continuously get nutrients through diffusion through cartilage matrix collagen, produce chondrocytes from a mesenchyme location, and never experience growth plates closure.

Of course the likely result is that the cartilage is not strong enough to support the adult weight of the human individual. the cartilage tissue collapses in thickness, and the result is the vascularization of the cartilage area leading to calcification and bone tissue.

The reason why fish can experience indeterminant growth and have cartilage as the main supporting tissue is because they are living in the water, which has a bouyancy force that is pushed upwards. The effect of earth’s gravitational force is actually decrease substantially, if not completely negated. (forgive me if my conceptual physics understanding is completely wrong here)

If we do a fluid statics problem to see how the hydrostatic forces act in water in terms of point to point, we know that from the top of the Earth’s atmosphere to the ground sea level, the average atmospheric pressure is 1 atm or 760 mm Hg (Hg is the shorthand for mercury).

Usually Patm= 1 atm = 101325 Pa

To find the downward pressure at any level of water (or fluid) below the surface of the water, which is assumed at ground level, we use the formula…

Patm + rho*g*h = P

rho is density and the density of water is around 1000 kg/m^3. g is the gravitational constant which is about 9.81 m/s^2. So the pressure exerted on a fish or aquatic creature in the back (assuming that they are swimming in a horizontal direction with a dorsal and ventral side) is determined by the depth at which they live. However, from basic hydrostatic pressure principles, they also have a force that is pushing upwards on their body. This means that instead of in terms of the vertical growth that humans are thinking of, pushing upwards against the gravitational downward force of gravity, aquatic animals focus on growing longer, increasing in the horizontal direction. Since there is no forces that is pushing in the lateral direction, except for the hydrostatic pressures, the fish don’t have a force that is holding them back from growing longer.

They can have cartilage tissue instead of bones because the environment they live in does not have the forces pushing in the direction which would inhibit their growth, which is to grow longer horizontally.

Recently I wrote a post entitled “Growth Plates In Elephants Never Close Or Close Extremely Late Suggesting They Experience Indeterminate Growth (Important)” showing that the largest animals on land, the elephant might actually live to be around 150 years old and some elephant species might have cartilage belts in their legs meaning that they never stop growing throughout life. Even for elephants who are in their 5th to 6th decade, there is still cartilage found in their long bones indicating that they are still growing.

Something that was interesting is that even if the cartilage in the elephant’s legs have ossified, their vertebrate bones still have growth plate cartilage, so they might not be growing vertically longer but they are growing horizontally longer as their vertebrate bones get wider and bigger.

Implications For Height Increase

Humans are a land living, air breathing mammal species. For almost all of us, the cartilage which we were born with which allows us to grow taller eventually disappears. For some creatures, their body is held up in their respective structure by a type of tissue that is not as hard and brittle and strong as the bone.

Sharks and most fish have cartilage and that is okay since they live in the water, which forces them to focus on growing longer. Reptiles like snakes also have vertebrate which are very flexible. Snakes also supposedly never stop growing. However these types of species are all focused on growing longer, not taller.

To grow taller, we have to like the elephant. The elephant supposed have growth plates in their legs for a very long time. They also seem to keep growing horizontally due to unfused vertebrate almost to old age, at least from mammoth and asian elephant studies.

I am not sure which mechanism allows elephants to keep their growth plate for so long, especially since they are so much heavier than humans and there might be more weight or load per square inche of cartilage and chondrocytes than the average human body.

It is suggested that we study the signalling pathway of the growth plate process in elephants and compare them to human vertical growth to see what they are doing right in being able to keep their growth plates around for multiple decades beyond what the human species can do.