Even though humans look different from other animals like fish, birds, or snakes, we all have a skeleton that gives our bodies shape, protects our internal organs from harm, and helps us move. Animals with a skeleton and a backbone are called vertebrates. Because all skeletons are built from bones, scientists can learn a lot about human bones by studying them in other vertebrates, including fish. Zebrafish are one vertebrate used by scientists to study how bones are made, age, and are repaired by the body. One cool fact about zebrafish is that they can regrow their tails, including the bones, if they get bitten off by another fish. Scientists can also use zebrafish to learn how new medicines can maintain bone health, or how to fix bones after an accident or disease. In this article, we will discuss how scientists can work with zebrafish to learn about bone biology and health.
Skeletons Are Made of Bones
The skeleton is the structure that gives shape to your body and allows you to stand and move (Figure 1A). The adult skeleton is made of about 206 interconnected bones, which come in various shapes and sizes (You can read more it in this Young Minds article). For example, some bones are long, like the humerus in your upper arm and the femur in your upper leg, while others are short, like the carpals of the wrists and the tarsals of the ankles. Despite differences in shape, size, and function, bones must be strong and healthy to work properly. A healthy lifestyle that includes nutritious food and regular exercise is the best way to keep your skeleton in good shape.
How Bones Are Made
Through a process called ossification, bones start to form before birth and continue to develop throughout childhood, teenage years, and early adult life. Before birth, bones begin forming in one of two ways (Figure 1B). One way is for cells called osteoblasts to invade a pre-made cartilage mold of the bone and deposit proteins and minerals (specifically calcium) in the jello-like scaffold. Cartilage is a tissue that can give shape to body parts such as your nose or outer ear, but it does not provide a lot of strength. By depositing additional proteins and calcium, osteoblasts transform the soft cartilage into strong bone [1, 2]. Scientists call this type of bone formation endochondral ossification. A second way to make bones that does not require a cartilage mold is called intramembranous ossification. In intramembranous ossification, the osteoblasts invade a tissue and start making bone by depositing proteins and calcium directly around them. Regardless of how they are made, bones must be further shaped and sculpted (remodeled) to perform their functions in the body [1, 2].
The job of remodeling the bones is the responsibility of cells called osteoclasts. When necessary, osteoclasts can remove excess proteins and calcium from bones, for example when bones get damaged [1, 2]. Afterwards, osteoblasts can return, fill in the gaps, and reshape or strengthen the bone [1, 2]. This teamwork between osteoblasts and osteoclasts continues throughout a person’s life. During the teenage years, osteoblasts work faster than osteoclasts to increase the length of bones. This is how you and your classmates get taller as you grow up! Later, in adult life, osteoblasts and osteoclasts work equally hard to repair and restore areas of bones that have been weakened by everyday wear and tear (Figure 1C). The cooperation of osteoblasts and osteoclasts keeps a person’s bones strong and healthy through their lifetime [1, 2].
Bone Biology: From Fish to Humans
All animals that have a backbone, made up of small vertebrae bones stacked on top of each other, are called vertebrates. Vertebrates come in all shapes and sizes—from elephants and whales to mice and fish. Their internal skeletons give vertebrates their unique shapes. Even though vertebrates may look very different from each other on the outside, their bones serve similar functions, such as keeping fragile organs, like the brain, spinal cord, and heart, safe. Zebrafish, a small fish native to India that can now be found in pet stores all over the world, have 74 bones in their skulls. That is many more than the 22 bones in the human skull (Figure 2) . Despite this difference in number, skull bones in both zebrafish and humans fit together to make a case that holds and protects the brain. The skull bones of both zebrafish and humans are built without a cartilage mold, through intramembranous ossification [1–3].
Interestingly, both zebrafish and humans have a similar number of vertebrae in their spinal columns (about 33), which help maintain body posture and protect the spinal cord (Figure 2) . Despite the similar number, the way that these bones are made in zebrafish and humans is very different: while human vertebrae form from a cartilage mold, zebrafish vertebrae do not [1–3]. Regardless of their form, number, or how they were made, ossified bones are strong to protect internal organs and to give animals their shapes.
Bones in Sickness and in Health
Accidents, disease, and age can damage bones. Most of the time, if bones break during an accident, doctors can repair the damage. They do this by putting the bones back in place with the help of an X-ray machine, then stabilizing the area with a cast for 4–8 weeks. Once the bones are set back into place, osteoblasts can come in and repair the damaged area, by depositing proteins and calcium to fuse the broken pieces back together. This results in a healthy, strong bone, with the same shape and function as before the accident.
Sometimes, however, doctors have a difficult time repairing bones because the bones are sick. Sick bones are fragile because they have lost proteins and calcium that normally make them strong. This can happen when osteoclasts remove proteins and calcium faster than osteoblasts can replace them. Doctors do not fully understand why this happens, but they know that this imbalance tends to be associated with age. For example, people over 50 years old are more likely to have weak bones. In fact, two out of 10 adults are diagnosed with weakened bones . While there are some medications that can stop bones from getting weaker, doctors currently have very few medications that help rebuild bones and restore their function. Doctors and scientists are working together to develop new medicines that can be used after accidents or disease, to encourage osteoblasts to restore bones to their original state.
Using Animals to Study Bone Biology
Before new medicines are given to people, scientists must test them to make sure they are safe. For example, doctors and scientists must know how much of a medication to give to a patient, how long a patient should take a medication for, and whether it should be given as a pill or as a shot. Scientists can answer all these questions by testing new medicines in animals. Non-human animals that can be used to test medicines because they mimic key aspects of human biological processes and diseases are called animal models. Before starting an experiment, scientists carefully select the best animal model for the question they are asking.
Over the last 20 years, Zebrafish have become a popular animal model to test medications and to learn about bone biology and disease (Figure 3A). Zebrafish are useful for studying bones because the way their bones are made is very similar to the way human bones are made [1–3]. For example, both humans and zebrafish have bones that are built by osteoblasts and osteoclasts using endochondral and intramembranous ossification methods (Figure 3B). The proteins and minerals that osteoblasts use to build and that osteoclasts remove to dismantle bones are also similar. Interestingly, as zebrafish age, their bones can become brittle just like human bones. Humans can live over 100 years, but zebrafish only live about 3 years, making it much easier to study age-related bone breakdown. One additional fun fact that makes zebrafish a useful animal model is that, unlike humans, they can regrow bones. If a zebrafish gets into a fight and one fish bites the tail off the other, the injured fish can regrow its tail fin, including all its bones (Figure 3C) [1–3]. One zebrafish success story involves the discovery of a new class of drugs to treat a human disorder in which muscle is slowly turned into bone. This class of drugs is now being tested in humans . For all these reasons, zebrafish have become a popular animal system to study the effect of medicines on bone formation, healing, and repair.
The skeletons of all vertebrates are made of bones. Bones are strong, they protect the animal’s internal organs, and they give the animal its unique body shape. But even the strongest bones can break or become sick. Doctors and scientists are looking for ways to cure people with sick bones and to prevent people from getting sick bones in the first place. Because all animal skeletons are built using the same methods and materials and by the same cells, scientists can learn a lot about how bones are made, how bones get sick, and what medications might cure bone diseases from studying animals, including zebrafish.
Ossification: ↑ The natural process of hardening a tissue into bone.
Osteoblasts: ↑ Cells that deposit proteins and minerals to build bones.
Cartilage: ↑ A firm but flexible tissue that connects parts of the body together, builds structures like the outer ear, and provides a mold to build bones.
Endochondral Ossification: ↑ Type of bone building method in which a jello-like cartilage mold is transformed into strong bone.
Intramembranous Ossification: ↑ Type of bone building methods in which cells start depositing proteins and calcium around them to build up bone.
Osteoclasts: ↑ Cells that remove proteins and minerals to break down bones.
Vertebrates: ↑ Animals that have a backbone including mammals, birds, reptiles, amphibians, and fish.
Animal Models: ↑ Living, non-human animals that allow scientists to do experiments to improve human health without putting people in danger.
Conflict of Interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
We want to thank past and current members of the lab: Esmail Miyanji, Zach Perkins, and Quan Chau. Special thanks to Phinn Hilliker for reviewing early versions of the manuscript. We also want to thank the organizations that supported our research: the National Science Foundation (DMR 1809419), the National Institute of Health (NIAMS R21AR072226), and the University of Richmond School of Arts and Sciences.
 ↑ Tonelli, F., Bek, J. W., Besio, R., De Clercq, A., Leoni, L., Salmon, P., et al. 2020. Zebrafish: a resourceful vertebrate model to investigate skeletal disorders. Front. Endocrinol. 11:489. doi: 10.3389/fendo.2020.00489
 ↑ Busse, B., Galloway, J. L., Gray, R. S., Harris, M. P., and Kwon, R. Y. 2019. Zebrafish: an emerging model for orthopedic research. J. Orthophaedic Res. 38:925–36. doi: 10.1002/jor.24539
 ↑ Dietrich, K., Fiedler, I. A. K., Kurzyukova, A., Lopez-Delgado, A. C., McGowan, L. M., Geutzen, K., et al. 2021. Skeletal biology and disease modeling in zebrafish. J. Bone Miner. Res. 36:436–58. doi: 10.1002/jbmr.4256
 ↑ Wright, N. C., Looker, A. C., Saag, K. G., Curtis, J. R., Delzell, E. S., Randall, S., et al. 2014. The recent prevalence of osteoporosis and low bone mass in the United States based on bone mineral density at the femoral neck or lumbar spine. J. Bone Miner. Res. 29:2520–6. doi: 10.1002/jbmr.2269
 ↑ Cully, M. 2019. Zebrafish earn their drug discovery stripes. Nat. Rev. Drug Discov. 18:811–3. doi: 10.1038/d41573-019-00165-x