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- [Voiceover] In this chapter,

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we're looking at the skeletal system.

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So there are three types
of connective tissue

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that make up your skeletal system.

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The most common is gonna be the bones,

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which is made of osseous tissue.

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It's very hard, very solid.

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Now these bones can be held
together with ligaments.

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Ligaments are a second
type of connective tissue.

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And the third is cartilage.

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Cartilage is a lot of times
found between the bones,

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providing a cushioning,

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and helping to protect
the ends of the bones.

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So let's start with the functions
of the bones themselves.

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I'm sure we can all agree
that bones are very hard.

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Well, why are they hard?

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They're hard because of all
the calcium, all the minerals

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that's found within the bone.

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So what are these bones doing?

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Well, one, support.

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Support, as in providing
a framework for your body.

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Support, as in supporting
your various muscles,

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making sure the muscles
have something to attach to.

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Protection.

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Can you think of any organs
that are encased in bone,

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and nicely protected?

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Well, the brain's one.

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The brain's inside the skull,

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nice and protected.

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The spinal cord.

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Again, within bone,

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nice and protected.

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Your heart, lungs are
encased by the rib cage.

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Again, nice and protected.

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So bones can provide a
nice amount of protection

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for various organs.

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Movement.

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Well, the muscles that move your body

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have to have something to pull against.

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If you took all the
bones out of your body,

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what would you be?

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A puddle of blob on the floor.

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Because the muscles and
bones work together.

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Not only for structure,

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to give you your support,

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but they also allow for a lever system

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where the muscles can
pull against the bones,

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creating a movement.

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This one might sound a little odd,

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but blood cell formation.

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Inside quite a few bones in your body,

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the actual inside workings of these bones

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are hollow cavities.

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It's these hollow cavities

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that are going to allow

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the production of blood.

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And then we have our mineral storage.

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Mentioned before that the mineral storage

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is primarily calcium,

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but there are other
minerals stored as well.

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But calcium really is that
nice, hard, solid structure

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that provides the strength
behind your bones.

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But there's also a second
mineral, phosphate.

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Now, phosphate might not
be as prevalent as calcium,

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but it still plays a major roll

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in the functionality of the bones.

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Now, this calcium inside your bones

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is actually located inside
what's called the matrix.

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It's just a grouping of
all these inorganic things.

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You have minerals in there,

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you have some fibers in there,

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but there are no living
materials inside that matrix.

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One of the types is called compact bone.

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Now, compact bone is
primarily what you think of

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when you see a bone.

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It's gonna be solid, it's
gonna be relatively smooth,

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you're not gonna see holes in it.

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It's what covers the
entire outside of the bone,

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and forms the majority
of the shaft of the bone,

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or the middle.

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Spongy bone, well that's
usually inside of another bone.

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Very rarely, you can
actually see the spongy bone

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because the spongy bone is
encased by compact bone.

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Now, I have this term up
there called trabeculae.

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Really, what that is are tiny little lines

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in the spongy bone.

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Those tiny lines are give support

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and give structure to the spongy bone.

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Just think of a sponge,

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like a kitchen sponge or
a sponge out of the ocean.

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Does it look like there's
any real pattern there?

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Probably not.

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The spongy bone also looks very haphazard.

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We're to have a total of
three primary cell types

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in the bone.

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Ostoeblast, oteocytes, and osteoclasts.

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The osteoblast cell

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is going to be a cell
that will start to create

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and make the osseous tissue,

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or make the bone,

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while the osteocytes,

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they get trapped sometimes.

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Once they get trapped in
that solid matrix of calcium,

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they can't get out.

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So they stay there

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and monitor the health of the bone.

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Now this third one might
sound a little bit off,

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because the purpose of an osteoclast

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is to break the bone down,

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remove calcium,

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and try and pull the
calcium into the blood.

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If you're wondering,

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why would you want to break down the bone?

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Well, sometimes your body needs to

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shift around a little bit.

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Sometimes your body needs to remove it

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because you have too much.

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All depends on what signals

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are received in your body.

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The bones themselves,

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we have long bones.

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Think about your upper
arm and your lower arm.

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You have flat bones.

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Things like your skull,

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they're very thin, but long and wide.

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Then you have these irregular bones,

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the ones that don't really match up.

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They have all these
unusual, different shapes.

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Inside your compact bone,

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you have these building blocks,

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these kind of basic structures.

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These are called osteons.

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Now, an osteon, sometimes is referred to

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as the haversian system.

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Osteon's a lot easier and a lot shorter,

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so just stick with that.

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Pretty much what the osteon is

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is a rough, ring shape

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of the cellular arrangement.

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That means that you're
seeing all the cells

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in roughly a ring shape.

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And the very outside,

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periosteum.

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Periosteum is a connective tissue

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meant to cover the outside of the bone,

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helping to protect it

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and keep everything in proper balance.

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So if you look at the
very top of this image,

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you're gonna see a
micrograph, a photograph,

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of an osteon.

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If you notice, it kinda looks
almost like rings there.

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You can see the little
dots inside of the rings

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going around and around
that central canal.

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So that is your compact bone.

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While the spongy bone,

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if you look in the middle
diagram on the bottom,

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you can see the spongy bone

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has all these openings and holes,

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just lines and lines and lines

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all interconnected.

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There are two more types
of connective tissue

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from the skeleton.

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We have our cartilage.

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Main purpose, support.

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Helps to cushion the joints
so they come together.

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And there are three different types,

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called fibrocartilage, hyaline cartilage,

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and elastic cartilage.

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Well, hyaline cartilage
is the most common.

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That's the one you'll usually hear.

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Elastic cartilage,

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well the name kinda gives it away.

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It's going to stretch.

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But what's nice is it'll stretch,

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but then it comes back
to it's original shape.

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Ligaments.

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Ligaments are bone to bone connection.

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You have many, many, many,

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parallel fibers of collagen.

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So you have all these parallel fibers

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giving a ton of strength

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across that ligament.