One of the distinguishing features of vertebrates is the presence of bones. We learned in an earlier post that not all vertebrates have bones, but if you do find an animal with bones, you can be confident you’re dealing with a vertebrate.
Bones are a combination of small crystals of the minerals in the apatite group supported in a matrix of collagen. We refer to the mineral collectively as bioapatite, as it can be any of several different minerals in the apatite group. A bone has a blood supply, some nerves, and living cells that can make the bone grow and others that can break down the bone. This is important because it makes it possible for bones to heal if they’ve been broken.
Two other tissues exist in vertebrates that are made of bioapatite, dentine and enamel. These are found in teeth and in the scales of fishes. Both dentine and enamel are made of bioapatite, but lack the blood vessels and other cells so they can’t heal if they get broken. That’s why you get a crown or a root canal if you break a tooth.
Bones and the use of bioapatite for a skeleton, are expensive propositions for animals. Phosphate minerals, like bioapatite, take a lot of energy to make, and the materials, particularly the phosphorus, are less common than carbonate that typically comprises animal skeletons.
So then, why bone? What’s so great about it?
For one, it is stronger and more stable than other materials. The phosphate is less likely to break down if say, for example, the ocean becomes more acidic.
It also has the ability to heal and grow, even if slowly. Other skeletal materials either have to be molted at regular intervals (like insects), or they can only grow by adding onto the edges (like clams).
Bioapatite can also store important ions, like calcium and phosphorus, that the animal can use for other purposes. When the body runs short of these important elements, part of the bone can break down to release the ions for use.
Perhaps the most interesting benefit for bone is the fact that apatite is a decent electrical insulator. By growing apatite crystals around nerve endings in the skin of an animal, it allows the nerves to detect other electrical impulses in the animal’s environment. These external electrical signals may come from other animals in the vicinity, permitting the animal to detect prey, or escape from predators. Such a system is employed today in modern fishes in what is called the lateral line system. This is what makes it possible for schools of fish to move in apparent synchrony. Each fish can sense its neighbor through the lateral line system and can react when the other fish in the school move.
The first bones were bones on the outside of the bodies of fish, like scales. These later developed to huge armor plates. These early fishes did not have an internal skeleton made of bone. Bone was only found on the outside. Such an external skeleton is called an exoskeleton. We as humans have an internal skeleton called an endoskeleton.
Early fish only had an endoskeleton. The bones of the endoskeleton formed initially around nerve endings in the skin, apparently to assist in sensory functions, but later developing into substantial armor. Referring back to yesterday’s post on embryology, these so-called dermal bones are ectodermal in origin.
The term for bones that grow in a particular area (say for example insulating a nerve ending in the skin), and frequently forming plate like structures is perichondral.
The development of the more familiar vertebrate endoskeleton came later. First with supporting structures made of cartilage. These cartilage structures later changed to bone. Sharks still have a cartilage skeleton. Other vertebrates with a bony endoskeleton start life with cartilage templates for all the bones that later grow to become completely bone.
The term for bones that start from a cartilage precursor is endochondral.
Most of the endoskeleton of vertebrates is derived from mesoderm in the embryo. Part of the braincase, the jaws, and gill (branchial) arches come from endoderm in the embryo.
It turns out that teeth and fish scales are constructed in very similar ways. On the outside is enamel (or enameloid). Under this is dentine, and then there’s often a pulp cavity. The similarity in structure has lead to the understanding that teeth have the same origin as scales, and over evolutionary time moved from the skin into the mouth.
The original bones in vertebrates were scales and external armor. The internal skeletone came later and are derived from different parts of the embryo. Over geologic time, the external scales were modified to become the teeth that we’re familiar with today.