Bone Biology: From a to Zebrafish

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.

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 di erences 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. Building bones from a cartilage mold is called endochondral ossification. Building bones without a cartilage mold is called intramembranous ossification. During ossification, cells called osteoblasts (blue) deposit minerals and proteins, and cells called osteoclasts (magenta) remodel the bone to shape it. (C) Osteoblasts and osteoclasts cooperate to maintain bone health. Age or disease can cause osteoclasts to remove bone tissue faster than osteoblast replace it, causing bones to become brittle and sick (Created with BioRender and Adobe Illustrator).

HOW BONES ARE MADE
Through a process called ossification, bones start to form before OSSIFICATION The natural process of hardening a tissue into bone. birth and continue to develop throughout childhood, teenage years, and early adult life. Before birth, bones begin forming in one of two ways ( Figure B). One way is for cells called osteoblasts to invade OSTEOBLASTS Cells that deposit proteins and minerals to build bones. a pre-made cartilage mold of the bone and deposit proteins and 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. minerals (specifically calcium) in the jello-like sca old. 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 [ , ]. 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 [ , ].
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 [ , ]. Afterwards, osteoblasts can return, fill in the gaps, and reshape or strengthen the bone [ , ]. 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 C)

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. 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 di erent 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 bones in their skulls. That is many more than the bones in the human skull ( Figure ) [ ]. Despite this di erence 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 [ -].

Figure
Humans and zebrafish look very di erent, but their skeletons share many structural and functional similarities. One example is the skull, which protects the brain. The skull is built from a jigsaw puzzle of bones shaped using the same ossification method in both zebrafish and humans, yet the skulls look distinct because the shape and number of pieces di er. Another example is the spinal column, which is made of vertebrae stacked on top of each other. Humans and zebrafish have the same number of vertebrae, but they use di erent ossification methods to build them (Created with BioRender and Adobe Illustrator).

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

ANIMAL MODELS
Living, non-human animals that allow scientists to do experiments to improve human health without putting people in danger. experiment, scientists carefully select the best animal model for the question they are asking.

kids.frontiersin.org
February | Volume | Article | Over the last years, Zebrafish have become a popular animal model to test medications and to learn about bone biology and disease ( Figure A). Zebrafish are useful for studying bones because the way their bones are made is very similar to the way human bones are made [ -]. For example, both humans and zebrafish have bones that are built by osteoblasts and osteoclasts using endochondral and intramembranous ossification methods ( Figure B). 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 years, but zebrafish only live about 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 o the other, the injured fish can regrow its tail fin, including all its bones ( Figure C) [ -]. 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 e ect of medicines on bone formation, healing, and repair. Scientists can use zebrafish to study general bone biology because the way humans and zebrafish build, grow, and repair their bones is similar. (C) Zebrafish have the remarkable ability to regenerate bones after injury, particularly in their tails. If scientists learn how zebrafish regenerate their bones, they could use this information to help treat people with bone diseases (Created with BioRender and Adobe Illustrator).

CONCLUSION
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.

ACKNOWLEDGMENTS
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 ), the National Institute of Health (NIAMS R AR ), and the University of Richmond School of Arts and Sciences. . doi: . /frym. .

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.
COPYRIGHT © DuMez and Skromne. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original kids.frontiersin.org February | Volume | Article | author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

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