At 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 minerals. But one thing 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. This system includes bones and also
connective tissue like: (1) ligaments - generally connecting bone to bone, (2) tendons – which
generally connect muscle to bone – and also (3) cartilage which we’ll get to later.
I also emphasize “human” to point out: there are 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 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 (those are called ossicles),
bone in your throat (called the hyoid), vertebral column, and the ribcage. 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. And part of the hands, you have metacarpals and
phalanges - and 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 wrists and tarsals of the ankles 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 (aka cranial bones) and shoulder blade bones (aka 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 in the inside of 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 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 cell 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 parts 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 a 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 the body may require. A major mineral
bones 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 signals 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 also is a template for bone placement. In fetal development,
it is 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:
it involves learning about different zones, including the growth plate zone. 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 osteoblasts – 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? Osteoclasts, right? And they do. And
osteoblasts will get to work on the remodeling – which means – they ultimately will make new
bone there. It takes time for this 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 kinds of fractures. If a
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? This 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 and more
conditions that can affect the skeletal system.
Well, that’s it for the Amoeba Sisters, and we remind you to stay curious.