hello anatomy colleagues this is dr alsup and in this mini lecture we will be discussing
how bone tissue is actually organized forming two major organizational types
and these two major types are referred to as compact and spongy bone and while they are
made up of the same general components they are organized differently and have some very
characteristic differences that can be that can be seen both microscopically as well as
grossly or with the unaided eye we will also describe the basic composition of a long bone
specifically the compact portion of the lung bone in other words what is
the microscopic organization that helps form the robust hard bone that we normally think of
so again the two major organizations of bone tissue form compact and spongy bone and you can
see in this image here that the compact bone which i am outlining kind of all around this region here
is um is very dense looking you see no holes or spaces other than this very large space in the
middle of the the long bone which is actually the medullary cavity we'll talk about that
in a bit more detail in another video this is where the yellow bone marrow sits during life
so that's going to be the compact bone you can see it here and here
whereas with the spongy bone you can see these little beams of bone kind of
all over the place all this area right in here is going to be made up of spongy bone
and there's considerably more space between these little beams of bone
one other thing you can notice is that the compact bone is most present in the middle
portion of the long bone or the shaft or the body whereas the spongy bone is much more prevalent on
the ends of long bones so we're looking at the the proximal portion of the femur here so that's
going to be the end of the femur but you can also have spongy bone in many of the short or flat or
irregular types of bone and higher frequencies but it's important to note and you can't really see
it very well here but it's important to note that there's always a layer of compact bone
you can see a little bit of it here there's always a layer of compact bone around the edges
even around the spongy bone so spongy bone is always protected by a covering of compact bone
bone as you may have guessed is important functionally in support and protection
but it also really is important in terms of resisting stresses produced by movement and
weight so really important in terms of that strength and bending spongy bone is located
as we mentioned most frequently at the ends of long bones which are very vital
in joint formation so it really makes sense that trabeculae while appearing not to be as organized
are actually very well organized to help reduce stress and over compression of the bone
before we move into breaking down these types of bone bones into their microscopic parts i do
want to take a quick terminology break and let you know that you may hear these types of bones being
referred to by different names maybe even by me in this very same video now you may hear compact bone
referred to as cortical which means referring to an outer layer of tissue or dense bone both
pretty descriptive names whereas spongy bone is sometimes referred to as trabecular in fact i
often use that term that's what i learned when i was in school trabecular it's called
trabecular because those beams of bone that we saw visible to the unaided eye are called trabeculae
you may also hear the term cancellous bone cancelae in latin means crossbar so those
trabeculae look like crossbars okay so i've bought myself some cover here in case i use these terms
and you may hear it if you're in another course or in some other textbook so i did want to present
these but for your purposes on any assessment i will use the terms compact and spongy bone
all right so let's talk about the microscopic organization of compact bone
that makes it look like what we saw in the last image so that's looking like there's not a
tremendous amount of space so at that gross level compact bone looks dense completely solid but when
viewed under a microscope you actually begin to see this beautiful and richly complex organization
of both bony tissue but also importantly there's a lot of spaces and canals
a major difference between compact and spongy bone is that only compact bone is organized into
these repeating structural units called osteons or more traditionally referred to as aversion systems
osteons are aligned in similar directions and are parallel to the diaphyseal length which we like
to refer to as lines of stress or stress lines which is important in resisting bending forces
these lines of stress are dynamic and can and will change throughout life sometimes considerably
as in when we're talking about juveniles learning how to walk so that can tremendously
change the li the lines of stress or even to a lesser extent would say with targeted
weight training you can have changes so it's really key to understand that
how these osteons are organized are going to be dynamic through life
each individual osteon consists of concentric lamellae and i am underlining concentric here i'm
going to even make it a nice box because i want you to understand we're talking about concentric
here because there are different type of lamellae so you have concentric lamellae surrounding
and organized around a central canal so here is a central canal here in the middle and then the
concentric lamellae will be organized around the central canal all right so you can see that here
as well so here in the middle is the central canal and then concentric lamellae surrounding it
the central canal is the space so that's key the canal is a space that the neurovasculature
that serves the osteon run and returning to the term lamellae
lamellae means plate like okay so that's what this term is we'll see it in another
in a moment as well so what you have are these concentric or circular plate like components
of mineralized or calcified bony extracellular matrix and these will increase in diameter the
further you get away from the central canal and it's really these concentric lamellae that give
bony tissue that very distinct ring-like appearance you see micro when you see these
microscopic sections of a bone that's what we're looking at here is a microscopic section of the
bone you can see its central canal say here and then the concentric lamellae are just
following around getting progressively larger the further you get away from the central canal
but what are the rest of these little dots throughout what are those
well these are mostly spaces referred to as lacunae all right so lacunae or lacuna if you're
talking in the singular and these are going to be located between the concentric lamellae so that is
what you're seeing interspersed throughout these rings here so all these little dots right here
and sitting within excuse me these lacunae are our old friends those mature bone cells the osteocytes
which i hope you are recall you recall are vital to bone maintenance and metabolism
so the the osteocyte cell body is sitting within the lacunae
and their cellular extensions or processes sit in extensions of the lacunae so they sit in
extensions of those spaces and these are referred to as cannuliculine so here is
the cell body sitting in the lacunae and then the processes are sitting in the cannuliculi
cannuliculi of other lacunae will meet with other cannuliculi and
spread out actually quite prolifically to allow communication and exchange between osteocytes
throughout and this system provides many avenues for nutrients and oxygen to reach the osteocytes
as well as a means of waste removal so when we're talking about the osteocyte being a living cell
and playing a role in terms of maintenance this is how that communication is allowed because these
processes can sit within those cannulically so just to review both lacunae and their extensions
cannuliculi are spaces all right i have that underline but that is a concept that i have found
most students have some difficulty with so these are spaces where the osteocytes and their cellular
processes sit between concentric lamellae and the those concentric lamellae remember
surround the central canal so these structures and spaces are what make up the osteon
now in areas between neighboring osteons there are often fragments of
older osteons that are partially destroyed either due to bone growth or bone remodeling
and these are called interstitial lamellae and you can see them just kind of in between the osteons
these will have structure these may still have some structures of an osteon so there
there may be lacunae as you can kind of see in these regions here you don't have the organization
of an osteon but you can still see components of broken down osteons there and that makes sense
if we return back to this it makes sense that you're going to have some broken down osteons
because remember bone is dynamic it's changing throughout life so you may have the breakdown of
certain osteons and the building of new osteons throughout life not just when bone length ends
now neurovasculature needs to make its way from the outside of the bone which you can see in
this image here it so it needs to make its way from outside to the inner portions to eventually
run through central canals as you can see here of osteons or even to the larger medullary cavity
as you can see here and it does the does so through transverse or perforating canals
often revert referred to as volkman's canals
lastly let's discuss one more type of lamellae so i told you that term would come up again so
we're not talking the concentric lamellae this time we're talking circumferential lamellae all
right so circumferential lamellae surround the entire outer all right which you can see really
nicely right here oops yeah it's going all the way around the entirety of the bone and then you'll
also have inner concentric lamellae that are going to kind of surround that industrial region
the outer circumferential lamellae is just deep to the periosteum or that connective tissue
lining around the bone and actually connected to the periosteum by perforating excuse me my voice
is trying to give out on me it's uh surrounded by those perforating or sh or not surrounded
the perforating or sharpey's fibers are going to kind of be interspersed throughout there
and these sharpey's fibers are very important as well because muscle tendons will often intertwine
in certain areas of bone and periosteum to form that extremely close connection between these
structures which is you certainly need for muscle contractions to really lead to bone movement
it is these circumferential lamella that play a huge role in acquisitional growth and we'll
have a whole video talking about interstitial and appositional growth appositional growth is growth
of the outer surface so particularly important in increases in bone width
so that was compact bone in a nutshell and as a reminder spongy bone has the same structures that
we just described but the key here is that they are not arranged in osteons all right they are
not arranged into those structural units of an osteon they still have the same components just
not organized in that way and they will have these very distinct trabeculae which means little beams
little beams of bone or columns of bone that are separated by much larger spaces and these
macroscopic spaces are filled with red bone marrow and some bones and yellow bone marrow and others
now i know it looks uh like the bones are kind of formed in a willy-nilly all over the place way
but they are exactly organized along lines of stress so um they're often in places where stress
comes from multiple directions and that's why those trabeculae look like they're irregular or
disorganized but they're very much formed wherever their stress coming most typically with that bone
also these spaces help to reduce the weight of the bone so that helps in terms of
just the weight of the skeletal system all right so those are the basics of compact and spongy bone
let's take a minute to review a concept all right so question osteocyte processes sit
within which area of an osteon is it a canuliculi b central canal c interstitial lamellae d lacunae
or e perforating or volkman's canals all right so take a minute and pause it
and when you're ready to go i want you to note that the key to these questions
are osteocyte and processes sit so they're sitting within
and while osteocytes do sit in the cunei so those osteocyte cell bodies sit in the lacunae
this is not correct because of this term here the processes those processes sit in the spaces called
cannuliculi so a cannuliculi is the correct answer recall recall cannulicular extensions
of the lacunae that allow for the osteocyte processes to sit and communicate with other
osteocyte processes the central canal is not correct this is where neurovasculature resides
within the central portion of an osteon interstitial lamellae is also incorrect these are
remnants of older osteons located between those functioning osteons and oops sorry and perforating
volkman canals are not correct either those are those transverse tunnels by which neurovasculature
enters the deeper portions of bone all right so if you miss this that's no big deal
you just now know that this is an area you need to review again and just to let you know this
particular video and the concepts within are a little conceptually difficult so you may
need to kind of sit down and write down your own definitions for each one of these terms
so take it as a learning opportunity thank you for kind of working through this material with me and
you can always please feel free to reach out with any questions and have a great rest of your day