At first thought of the skeletal system, you  might think of a lot of bones with a lot of names  
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  
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  
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. 
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  
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  
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.
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  
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  
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  
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  
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  
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  
exoskeleton. And then many other animals  have a hard skeleton on the inside called  

an endoskeleton: humans, for  example, have an endoskeleton.
an endoskeleton: humans, for  example, have an endoskeleton.

The skeletal system has an important job  of supporting your body and protecting  
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,  
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  
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,  
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  
the term “musculoskeletal system” used in  a lot of textbooks combines the skeletal  

system with the muscular system –  highlighting their work together.
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  
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  
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  
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),  
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,  
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  
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:  
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  
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  
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.
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.
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  
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  
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.  
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,  
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  
ulna, and radius. And part of the  hands, you have metacarpals and  

phalanges - and part of the feet,  you have metatarsals and phalanges.
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  
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  
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.
offer stability. Carpals in the wrists  and tarsals of the ankles are examples.

Sesamoid bones: like a sesame seed, they are  
Sesamoid bones: like a sesame seed, they are  

roundish. They’re great about handling  pressure. The patella is an example.
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  
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  
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.
bones (aka cranial bones) and shoulder  blade bones (aka scapulae) are examples.

And irregular bones: a shape that  is…irregular. Basically there isn’t  
And irregular bones: a shape that  is…irregular. Basically there isn’t  

a specific shape description you can  use to define them. Irregular bones  
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.
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.
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  
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  
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 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  
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  
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  
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  
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  
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.
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  
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;  
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.  
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:  
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,  
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:  
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  
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  
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,  
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  
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  
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,  
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  
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  
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  
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  
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.
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  
Bone cells work alongside other cells. For  example, chondroblasts. Chondroblasts make  

the connective tissue cartilage. Chondroblasts  can mature into chondrocytes which maintain this  
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  
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,  
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  
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.
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  
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:  
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  
it involves learning about different zones,  including the growth plate zone. We have a  

suggested read in our video  description to learn more.
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  
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-  
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,  
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  
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 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.  
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  
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  
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  
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 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  
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  
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  
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  
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.
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  
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  
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  
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,  
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  
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.
yet - treatment plans aim to help with  mobility and also to prevent fractures.

Orthopedic research continues to  expand to address this and more  
Orthopedic research continues to  expand to address this and more  

conditions that can affect the skeletal system.
conditions that can affect the skeletal system.

Well, that’s it for the Amoeba Sisters,  and we remind you to stay curious.
Well, that’s it for the Amoeba Sisters,  and we remind you to stay curious.