hello anatomy colleagues this is Dr. Alsup and in this mini lecture we're going to
talk about the organization of bone tissue and there's really two main types of organization
there's going to be compact bone and there's going to be spongy bone and you'll actually find that
those names are pretty descriptive of what they look like grossly and then we're going to talk
about a few of the specific structures that are going to make up this organization we're not going
to get into the nitty-gritty detail we're going to talk about some of these overarching themes
so again there are going to be two major organizational types of bone tissue named
pretty pretty accurately so you're gonna have a compact bone which is going to appear
very compact pretty dense and then you're going to have spongy bone which is named for its more
sponge-like appearance as you can see here this right here is actually going to be spongy bone
i'll talk about some differing terminologies in a moment and one of those is obviously going to
be trabecular bone as you can see here in this image now if you're looking at the compact bone
you really don't see any holes you don't see any spaces other than obviously this
fairly large marrow or medullary cavity in the middle of the diaphysis but for the most part
you have a very solid portion of bone one other thing that you likely notice as well in terms
of the compact bone is where it is located we're looking at a cross section of a femur femur is the
longest long bone of the body and it is making up the bulk of the diaphysis or the body or the shaft
of a long bone but one other thing that's maybe not quite as easy to see
is that it is lining the ends of the long bone as well so while the majority of the epiphyses
is going to be made up of the spongy type of bone you will always have an outer line or outer
lining of compact bone throughout so even a bone that's made up of almost majority spongy bones
such as what we have with the flat bones of the skull, the vertebrae, the sternum those are mostly
made up of spongy bone but you will always have that outer lining of compact bone which makes
sense because really that compact bone is playing a huge role in terms of protection it's going to
play a role in terms of tensile strength strength and bending associated with it so you will always
have that outer layer of compact bone now if you're looking closer at a spongy bone again these
are going to be located more on the ends or within the ends of the long bones so those epiphyses
so important in terms of formation of joints and then you will have it making up the bulk
of some of the more irregular bones the short bones the flat bones say of the skull those are
going to be made up of majority spongy bone and you can see it's containing these larger spaces
within so you can actually visibly or grossly see the spaces within spongy bone
also this is going to play a role in terms of strength and compression
and those trabeculae or these little beams of bone are really well organized in order
to resist stress from multiple directions which you often have when we're talking about in joints
before we move further i do want to take a terminology break because you can hear
these organizational types referred to in many different ways one of those we just saw in that
previous image the spongy bone is sometimes referred to as trabeculae or trabecular bone
because those little beams of bones that you can visibly see in spongy bone are referred to as
trabeculae in fact trabecular is the term that i used or learned when i was first learning anatomy
so trabecular you may hear for spongy bone cancellous you may also hear cancellous or
cancili in latin means crossbars so really those trabeculae often do look like crossbars
in composition so you may hear spongy, trabecular, cancellous when we're talking about compact bone
sometimes you hear this referred to as cortical when you hear cortical think an
outer layer and that makes sense compact bone is always going to be the outer layer of bone
so that's sometimes you hear referred to as that sometimes you hear it referred to as dense bone
because that's what it looks like it looks quite dense you don't have that those larger spaces
as you do in the spongy bone so i'm kind of buying myself a little leeway here in case
i use some of those terms as i discuss but in terms of any type of assessment i
will use the terms compact or spongy but you may hear these other terms as well
all right so let's take a deeper dive into the microscopic structure of compact bone specifically
and then we'll talk about some of the structures that are shared between the two because recall
if you look with unaided eye at compact bone it just looks like a solid block of bone you don't
see these really complex microscopic canals and spaces that are present even in compact bone and
those are really important in terms of being able to get neurovasculature into this region
now a major difference between compact bone and spongy bone is that compact bone is going to be
organized in what's referred to as an osteon more traditionally you hear it referred to as a
Haversian system and these are just these structural units that are aligned in the same direction along
lines of stress generally these are going to be along the longitudinal axis of a bone
it's really important to note that these lines of stress are dynamic
so these can change throughout life sometimes you have some fairly large changes that occur
say with juveniles that are learning how to walk those lines of stress will change
quite dramatically you can also change them throughout life with differing
differing aspects of walking or not walking weight training can change these lines of stress so
the osteons do change throughout life they do break down throughout life you do develop new
ones throughout life but really compact bone is going to be organized in these
systems and you can see just kind of looking at these it's kind of you can see one that's
kind of pulled up through here but you can see looking at this cross section just the
amount of osteons that you're going to have associated with compact bone
now if you're ever if you've ever seen a microscopic section of compact bone it almost
looks like you're looking at tree rings so you have these tree rings kind of of circling around
a central opening or canal literally referred to as a central canal so you have these tree rings
or maybe even like the layers of an onion that are kind of getting increasing diameter as you
get away from that central canal and these rings or these kind of plates of bone are referred to as
concentric lamellae so let's turn let's discuss the term lamellae before we move on because
it's going to come up again as well lamellae means plate-like and what you have are these
concentric or circular plate-like components of mineralized or calcified extracellular
matrix and this is going to be dispersed and dispersed throughout you will have osteocytes
kind of within so if you're looking at this microscopic section here all these dots throughout
here are going to be osteocytes that are sitting within these little pits or these little
divots within referred to as lacunae so you'll have that mineralized or calcified extracellular
matrix that's going to form these ever increasing in diameter concentric lamellae around
what's referred to as a central canal and a central canal is the means by which
neurovasculature so nerves arteries veins as well as lymphatics are going to get
to these deeper portions of the bone and supply or innervate these individual osteons
now along the length of a central canal so remember that central canal is right here in
the middle of an osteon along its length you're going to have these perforating
canals which are getting the the branches from outside of the bone into these deeper portions
and these are referred to as perforating sometimes you hear them referred to as Volkmann's canals
and so how this happens is on for each bone at least you'll have at least what's one nutrient
foramen and this is a hole by which branches from outside of the bone actually enter
into the bone and the nutrient foramen will open into these perforating or Volkmann's canals
these perforating or Volkmann's canals will get that neurovasculature to that central canal in
order to supply those osteons so one thing i do want to please please please don't forget
how important neurovasculature is to a bone's function it's remember bone is dynamic it's living
it needs that arterial supply it needs venous drainage to get rid of waste it needs innervation
in order for it to function properly so these perforating canals and these central canals
are central to the to the function and the survival of bone
now there's another type of lamellae that i want to mention before we move on
that's associated with compact bone and this is referred to as circumferential
lamellae so not concentric lamellae circumferential going around the entire
circumference outer circumference as well as the inner circumference of of a bone
so you'll have again you'll have these external or outer so the surrounding the outside of the bone
as well as an internal or inner so kind of surrounding the medullary cavity region as well
and really these are the both of these types are important but this external or
outer portion is actually going to have an attachment as well to the periosteum
via what's referred to as perforating or Sharpey's fibers and this connection is important because
these fibers are not only going to interweave between the periosteum and the bone but it will
interweave the fibers associated with ligaments and tendons so maybe making this musculoskeletal
functional unit that's important in terms of movement as well as in terms of growth
this circumferential lamellae is going to also play a role in terms of what's referred to as
appositional growth or growth of the outer surface and this is really important in terms of bone
width growth so if you're going you'll obviously have bone length growth that we'll talk about
in more detail in a moment but you also can have bone that's going to grow and increase the width
of bone and so these circumferential lamellae play an important role here so if you're looking here this
is indicating the periosteum and then you will have these long lamellae surrounding the entirety
of the bone so the circumferential lamellae is not organized in these osteon units it's going to
surround the outside of the bone as well as the inside or around that medullary cavity as well
now spongy bone has many of the same strong structures that we just described associated with
compact bone so there will be concentric lamellae it's going to be organized in these trabecular
trabeculae you'll obviously have osteocytes you have to have osteocytes in terms of the
communication between the two but spongy bone is not organized in osteons okay so you don't
have that specific functional unit you don't have central canals if you think about it
any osteocyte in the spongy bone is not very far away from any of these spaces in between which is
filled with marrow marrow has a very rich vascular supply so that's how it's getting its nourishment
and drainage in that region now it does if you're looking at a spongy portion of bone it does look
like these trabeculae are kind of just willy-nilly kind of all over the place but they are very
functionally organized along lines of stress and it's going to be in areas where you have stress
kind of coming from multiple directions often and so it while it does look like it's unorganized
it's actually quite organized in terms of how it is it's going to present and again these larger
spaces that are visible to the naked eye are going to be filled with bone marrow it could be red bone
marrow depending on what age that individual is or the type of bone or it could be yellow bone marrow
all right excellent so we've covered the basics of compact and spongy bone now let's review a concept
through a question all right so the diaphysis of a long bone is formed predominantly by which
type of bone is it compact or is it spongy to pause the video if needed and when you are ready
i hope you have landed on compact and in order to answer that you have to first of all understand
where the diaphysis is so recall you have to go back to your knowledge from a previous video
that diaphysis sometimes it's referred to as the body of a long bone or the shaft of the long bone
basically it's the long portion of the long bone in the middle you have that medullary
cavity that's going to be hollow other than during life it's going to be filled with marrow
but the actual bony portion of it is is going to be exclusively composed of compact bone
if you get more into the marrow cavity you may have some spongy bone there but the majority is
going to be made up of this compact bone you'll also have compact bone recall along the the ends
of the epiphyses so the majority of an epiphysis or the ends of the bone will be spongy bone but
you'll always have that protective layer of compact bone on the outside all right excellent
thank you so much for your time and attention here and i hope you have an excellent rest of your day