hello anatomy colleagues this is dr alsup and in this video we will be discussing the histology
of bone tissue or in other words the structure of bone at the microscopic level now i know the
majority focus of this course is gross anatomy or the anatomical structures that you can see with
the naked eye but i feel that having knowledge of the microscopic building blocks can really help in
the understanding of these gross structures and really additionally help with the understanding
of the functions which is kind of the main point anyway so we need to review these things we need
to get these concepts before really getting into this separate bone so let's get started
as you can see here we have quite a bit to discuss here including cells the extracellular matrix
which is very important in terms of all connective tissues but certainly in terms of osseous tissue
and we'll even touch on a few pathological areas in the case of rickets and osteomalacia and how
that is going to be affected by changes or issues with the extracellular matrix so let's get going
there are going to be four types of cells present in osseous tissue and we are going
to review each one starting with the osteogenic cells and sometimes you hear this referred to as
the osteoprogenitor stem cells because they they are that that's exactly what they are
they're these unspecialized bone stem cells osteogenic cells are derived from mesenchyme
which is the embryonic tissue from which almost all connective tissues are formed and bone is a
connective tissue so it makes sense that it will be derived from the mesenchyme
now this is an important point so let's put a little star next to it
these are the only bone cells so these osteogenic cells are the only bone cells to undergo
cell division so none of the other three that we discuss will be able to do that
and as these cells divide they will eventually develop or convert into osteoblasts
which are very important in terms of bone tissue deposition osteogenic sounds can be found
throughout life so that's an important uh idea as well you do have osteogenic cells throughout life
and these will be found within the inner portions of the outer periosteum you'll
also found that find them around the endosteum or the inner portions of the bone or surrounding the
inner portions of the bone and you'll also have them in many of the neurovascular canals of bone
okay so now we know that the osteogenic cells will develop into osteoblasts let's talk about what an
osteoblast is so this is the second cell that we are going to talk about simply put osteoblasts
are bone building cells all right so osteob blasts build alright so we'll talk about
how this is going to be almost an opposite function of the osteoclasts bone building cells
are going to produce and secrete collagen fibers and other organic components so this is important
these are going to be the organic components of the extracellular matrix
and it is these organic components are needed to initiate calcification or tissue hardening
so making that tissue hard and in this case we're talking bone tissue hardening which we know
is going to be very important in terms of the structure of bone now osteoblasts will
continually secrete and surround itself with these components of the extracellular matrix
eventually becoming essentially trapped in their own secretions and at the point that these cells
are completely surrounded and trapped in extracellular matrix there will be the
stopping of secretion of the organic materials and the osteoblasts will transition into an osteocyte
but before we move on to osteocytes i want to take a little break
and really explore the extracellular matrix of bone making sure we are comfortable with
its basic components extracellular matrix is the material located between widely spaced cells and
it will differ considerably between different types of tissues and it's
really the structure of the extracellular matrix that determines much of the tissues qualities
so for example in cartilage the extracellular matrix is firm but more pliable or flexible
while the extracellular matrix of bone will be much harder and less flexible and it's all due
to the components within so the major components of bone extracellular matrix will be water it will
be organic materials mainly collagen fibers and importantly you will also have inorganic materials
that are going to be these crystallized mineral salts so let's talk a little bit
about this inorganic material the most abundant mineral salt is going to be calcium phosphate
which will combine with calcium hydroxide and this will form crystals of hydroxyapatite
right here now i don't want you to worry about this chemical formula here i just put it here
because it looks so cool and if you're into that kind of thing i wanted you to be able to see
really of what it's composed of but i do want you to understand that this inorganic material
is mostly going to be hydroxyapatite which will combine with other mineral salts and ions and this
will be deposited amongst the organic framework formed by the collagen fibers that were secreted
by those osteoblasts now as more inorganic material is deposited it will crystallize
and the bone tissue will harden in other words the bone tissue will calcify and it's important
to note you need both organic and inorganic materials for the process of calcification
to occur now the bones hardness is dependent on those crystallized inorganic mineral salts
just as you would probably think whereas a bone's flexibility is going to depend on that organic
the organic material or the collagen fibers
now you may be thinking to yourself flexibility in a bone sounds like a bad thing i don't want
that but you do need some flexibility in bone to allow for tensile strength which is going to to be
that resistance to being stretched or torn apart and it also helps to combat a bone brittleness
which you can see kind of over here in a in a cartoonish image here and so this bone
brittleness can really lead to fragile bones which may fracture or shatter more easily
now if you soak a bone in acidic solution something as simple as vinegar this will dissolve
the mineral salts or the inorganic material and leave only the collagen fibers making the bone
extremely flexible or almost rubbery so having that correct balance of organic and inorganic
materials in the bony extracellular matrix is vital for proper bone health and function but what
happens if you don't have these proper balances or say the calcification process is insufficient
you can have various pathological issues and two of these are going to be rickets and osteomalacia
and these are going to these two disorders are going to be two forms of the same disorder
these are going to result from inadequate calcification or hardening of that
extracellular matrix and it's usually caused by an insufficient amount of vitamin d rickets
is the name for this order for this disorder excuse me in juveniles all right so in children
and what you will observe is often a bowing
of the legs due to the bones being more flexible or too flexible due to insufficient calcification
osteomalacio which sometimes you hear referred to as adult rickets is going to be the same disorder
but as you can see here in adults so making sure an individual has sufficient vitamin d supplies
which can be helped with increased exposure to moderate amounts of sunlight can really
help in terms of lessening the effects of the this disorder or preventing
okay that was a fun clinical vignette but now let's return to the cells of bone tissue
recall that we know that we have the cell dividing
osteogenic cells that will develop into osteoblasts those osteoblasts
will secrete collagen fibers into the extracellular matrix eventually surrounding
and trapping itself within the matrix and at that point when it is trapped in the matrix and it is
stopped secreting organic materials that osteoblasts will become an osteocyte and these um
you can just think of as your mature bone cells kind of the main cells in the the bony tissue
now it's important to note that these cells these osteocytes are not
dead they are very much functioning in monitoring and maintaining the bones metabolism
through exchanging nutrients and getting rid of waste and you can see in this illustration
how these cells have these fairly long extensions or processes that are um that are important in
terms of being able to better communicate with other osteocytes that are widely separated
so be able to send information um and certain components to osteocytes that are fairly far away
all right so the fourth bone cell is a big one and this is the osteoclast
clast means break or breakdown and this is exactly what an osteoclast
does you can see here kind of this mnemonic osteoclast with the c consume or break down bone
and it will break down the extracellular matrix which at first blush i know actually sounds kind
of pathological but this is very much a natural and necessary process in maintaining a healthy
skeleton as long as these processes are in line with the osteoblasts or bone building cells
so you want them kind of working in unison you don't want one working much faster than the other
whereas the other three bone cells are part of a life cycle the osteoclasts are going to have
a different origin developing from the fusion of many monocytes which are a type of white
blood cells and this is actually going to be a really large or comparatively large cell
osteoblasts can be found in numerous areas of the bone but they are most concentrated
in the endosteum or that thin membrane lining the internal portion of the bone and the cell
has this very distinctive ruffled border which i'm underlining right here and this ruffled
border will be facing the actual bone surface and when needed it will release lysosomal enzymes and
acids that will be the the component that will actually break down and digest the extracellular
matrix components and so this breakdown is what is referred to as bone resorption
so as i mentioned previously this resorption is a part of typical bone development
it will be important in terms of bone growth bone maintenance and also repair
and it will also play a role in increasing blood calcium levels when necessary
all right so as a quick review you can see these osteogenic cells are
kind of the first component of the life cycle these will develop into osteoblasts which play
an important role in terms of formation of the bone matrix once these osteoblasts are trapped
in extracellular matrix and after organic material secretion ceases an osteoblast
will convert to an osteocyte which is important in terms of maintaining the bone metabolism
so you can see that these three cells are part of one life cycle whereas the osteoclast has a
different origin but is also important in the bone tissue specifically in terms of bone resorption
okay let's work through two questions as there were a lot of concepts covered here
and i suggest you stop and try to answer this question on your own
either writing it down or saying the answer out loud so pause after i read here
and to kind of kind of think this through so the question is you notice in a patient that
there is a pathological amount of flexibility or softness in some of their bones which components
of the extracellular matrix may be affected here all right so pause and when you are ready
you can unpause the video and we will discuss this kind of together so flexibility of bone
a flexibility of bone i'm going to underline that here is a function
of what is it a function of the organic or the inorganic components of the extracellular matrix
hopefully you are feeling good that this is part of the organic portions or components
of the extracellular matrix so if a bone is too flexible it is typically not the organic material
that is insufficient this would there could be issues with the calcification or hardening process
of the inorganic material or that hydroxyapatite so really what we were going for here is that
there is some issue with either the inorganic material being calcified or hardened within
the extracellular matrix all right and we saw that this can often occur with rickets and osteomalacia
due to most commonly vitamin d deficiency all right so hopefully this is this is something
you're feeling good with if not go back and review that portion or you can always reach out to me but
i do want one more question usually i only have one but i do have two in this particular session
all right so question is which cell associated with osseous tissue
plays the largest role in bone resorption is it osteoblasts osteoclasts osteocytes
or d osteogenic cells pause the video if you need a moment to think but once you are ready
let's discuss how b or osteoclasts is the correct answer it is these large osteoclasts
that will break down bony extracellular matrix and this is referred to as bony resorption
osteoblasts are this is not correct because these are going to be the porsche these cells
are the ones that are going to lay down the organic material of the extracellular matrix
so these will be bone building kind of the opposite right of the osteoclast so these two
kind of work in unison to do opposite functions osteocytes these are not correct either
because these are those mature bone cells that play a role in bone maintenance not breakdown
and lastly osteogenic cells are not correct these are the beginning of this life cycle
here and important in terms of cell division okay so this is likely one of my longest mini lectures
but i felt it was really important to keep these structures and concepts together
please make sure you have these concepts down before moving on to other areas in this session as
the concepts here really help make sense of some of the ones we will discuss in subsequent videos
please always feel free to reach out to me with any questions and have a great day