If first thought of the skeletal system, you might think of a lot of bones, with a lot of names to memorize. That's fair. You might know bones consist of a protein known as collagen, as well as many minerals.
But one thing that many don't realize is that bones are a living tissue, and there are so many things going on in bones. No bones about it, this video is going to be interesting. Now our focus is going to be on the human skeletal system.
The system includes bones and also connective tissue, like ligaments, generally connecting bone to bone, tendons, which generally connect muscle to bone, and also cartilage, which we'll get to later. I also emphasized human to point out there are many different kinds of skeletal systems in different organisms. For example, in our animal video, we mention earthworms. Earthworms have a hydrostatic skeleton, a skeleton that is filled with liquid and kept under pressure. We also mention insects in our animal video.
Insects have an exoskeleton, a hard outer skeleton made of chitin. In order to grow, most insects have to molt, which involves shedding their old exoskeleton. And then many other animals have a hard skeleton on the inside, called an endoskeleton. Humans, for example, have an endoskeleton.
The skeletal system has an important job of supporting your body and protecting the organs inside. But your skeletal system also provides storage of important minerals, involves the making of red and white blood cells, and, along with the help of muscles, of course, movement. A reminder that body systems do not have to be don't work in isolation. In fact, the term musculoskeletal system used in a lot of textbooks combines the skeletal system with the muscular system, highlighting their work together.
An adult human generally has 206 bones. A few more when they were a baby, but some of the bones fuse together by adulthood. The human skeleton can be divided into the axial skeleton and the appendicular skeleton. The axial skeleton, think central axis of a human, includes the bones in the skull, in your ears, bone in your throat, called the hyoid, vertebral column, and the rib cage.
As you can imagine, muscles attach to bones in the axial skeleton to let you move parts of your body like your head and neck as well as breathe. It also offers a foundation to the other division, the appendicular skeleton. All the bones of the arms and the shoulder girdle where they attach are included, essential for the functioning of arms and hands, and all the bones of the legs and the pelvic girdle where they attach, essential for movement.
Let's talk about bone shape because That can be used to classify bones. Long bones. You know, it's not that they have to be long, per se. It's more about their cylinder shape and the fact that they're longer than they are wide. These bones tend to be moved in muscle contraction.
The femur in the leg is a great example of a long bone. The tibia and fibula in the legs are also long bones. In the arms, you can have the humerus, ulna, and radius.
In part of the hands, you have metacarpals and phalanges. In part of the feet, you have metatarsals and phalanges. Short bones.
Unlike long bones, these are more like a cube as their length and width are close to equal. These bones are also moved using muscle contraction, and these bones can offer stability. Carpals in the wrist and tarsals of the ankle are examples.
Sesamoid bones. Like a sesame seed, they are roundish. They're great about handling pressure. The patella is an example. Flat bones.
So unfortunately, you don't want to just go on the name because they aren't necessarily flat. They're often curved and thin. Skull bones, otherwise known as cranial bones, and shoulder bones, otherwise known as scapulae, are examples.
And irregular bones, a shape that is… irregular. Basically, there isn't a specific shape description you can use to define them. Irregular bones can be protective from an assortment of forces. Vertebrae are a great example.
Let's talk a bit about what's going on inside bones. There are two general types of bone tissue. Compact bone tissue, an outer layer that is hard, and then spongy bone tissue, a layer where you will find… bone marrow.
Some of it is yellow marrow, and it stores fat, which is important for long-term energy storage. And then red marrow, that's where blood cells can be made. Red blood cells, which transport gases, and white blood cells, which are important for defending the body.
Red marrow also involves platelet production. Platelets aren't really cells, but cellular fragments that are critical in allowing blood to clot. So overall, it makes sense that we say bones are vascular because they have a rich blood supply.
Interesting fact, in emergencies, there is even a way to administer fluids and medications into the bone in a similar way that they can be delivered by an IV. Let's talk about cells involved with bone or cartilage. First, osteoblasts.
These are cells that make bone. They can mature into another type of cell called osteocytes. These cells maintain bone structure and make up the majority of bone cells present.
Osteoclasts. These cells actually break down bone structure and the way they do it is cool. Remember the organelles called lysosomes?
Well, osteoclasts have a lot of lysosomes. In lysosomes, there are enzymes there, and together with acids, they can help dissolve bone. You might wonder, why would you want to normally break down bone? Well, the body is always responding to stresses.
And some part of old bone is removed, and then some parts are rebuilt in what is called bone remodeling. So osteoclasts are needed for this normal process. It's estimated that every year, approximately 5-10% of the skeleton is remodeled.
So remodeling in the skeletal system means bone that has been damaged or is just old is reabsorbed and replaced. If you get injured or if you do a lot of exercise, this can also impact remodeling. In addition, the process of breaking down bone also releases minerals that the body may require. A major mineral bone store is calcium.
Specific hormones from the endocrine system actually can signal whether calcium in the bones may need to be released. Specific hormones also signal when calcium is deposited. Calcium has a lot of functions for body processes.
For example, in our muscle contraction video, we show how important calcium is in order for muscle contraction to occur. Bone cells work alongside other cells. For example, chondroblasts.
Chondroblasts make the connective tissue cartilage. Chondroblasts can mature into chondrocytes, which maintain this cartilage. They make up the cartilage.
Cartilage supports bone. It's important in joints where bones are connected. It's also a template for bone placement. In fetal development, it is the cartilage, made by chondroblasts, that is meant to serve as the matrix for the bones. Eventually, bone will replace most.
but not all of the cartilage matrix. With the help of osteoblasts, bones grow in length, usually until early adulthood. Bones can also grow in diameter even after they stop growing in length.
The process for bone growth is complex and involves learning about different zones. We have a suggested read in our video description to learn more. What happens when a bone breaks?
So typically after a break, there is a fracture hematoma, which is a collection of blood that forms at the site of the break because again, bones are very vascular. And because blood is blocked from getting to the bone cells that exist near the fracture, those bone cells generally die. But thanks to the help of chondrocytes and the osteoblast, an internal callus and external callus is made there to keep it all together.
The internal callus is mainly cartilage, and the outer callus is a mix of cartilage and bone. So what's going to remove the damaged part of the bone? Osteoclast, right? And they do.
And an osteoblast will get there working on the remodeling. which means they ultimately will make new bone there. It takes time for that to occur though.
Many fractures heal in 6-8 weeks, but it can be longer depending on an assortment of variables and the type of bone that was broken. It can also matter on the fracture. There are a lot of different types of fractures. If the fracture is from a severe trauma, let's say to a long bone like the femur, not only can this cause excessive internal bleeding, but remember how we mentioned that bone has yellow marrow which stores fat?
the fat can also be released into the blood. And while usually the body can clear this, it is possible, though rare, to develop what is called fat embolism syndrome, which needs emergency care. While many often are familiar with fractures, there are also conditions that can affect bone development and growth.
One example is osteogenesis imperfecta. There are multiple forms of this genetic disorder, but generally collagen production is impacted. If you recall from the very beginning of our video, collagen is the major protein in bone.
With this condition, bones tend to be brittle and easily fracture. While there is not a cure yet, treatment plans aim to help with mobility and also to prevent fractures. Orthopedic research continues to expand to address this in more conditions that can affect the skeletal system.
Well that's it for the amoeba sisters and we remind you to stay curious.