Full transcript
0:00so now let's take a look into the
0:02histology of osseous tissues like any
0:05other connective tissue bone consists of
0:08cells fibers and ground substance there
0:12are four principal types of cells in
0:15bones
0:15there are osteogenic cells or
0:18osteoprogenitor cells there are
0:23osteoblasts osteocytes and osteoclasts
0:29now these roots are going to come up
0:32over and over
0:33osteo will always refer to bone blasts
0:38will always refer to cells that make
0:42something sites will always be the
0:45mature version of the cell and clasts
0:49will always break stuff down so if we
0:54changed it to mile sites that would mean
0:59mature cells in the muscle tissue but
1:02here we're looking at osteo so
1:04osteogenic or osteoprogenitor cells are
1:07found in the endosteum
1:09and the periosteum so those are the
1:12membranes around the outside of the bone
1:16as well as the membrane that lines the
1:19marrow cavity and it's found in the
1:22central canals of the osteon they arise
1:26from embryonic mesenchymal cells and
1:29they multiply continuously to produce
1:31new osteoblasts so here we have an
1:35osteogenic cell and it will become an
1:38osteoblast will give rise to many many
1:42osteoblasts and the osteoblasts will
1:45eventually become mature osteocytes so
1:52the osteoblasts are the actual bone
1:54forming cells they line up as a single
1:58layer of cells under the endosteum and
2:01periosteum and their non mitotic so they
2:04won't divide again the only source of
2:06osteoblasts is osteogenic cells they'll
2:10synthesize the soft organic matter of
2:13the
2:13matrix which then hardens by mineral
2:16deposition
2:17now stress on the bone such as little
2:21fractures or weight-bearing exercise
2:24will stimulate the osteogenic cells to
2:27multiply more rapidly and increase the
2:30number of osteoblasts to reinforce or
2:33rebuild bone the osteoblasts also
2:36secrete a substance called osteocalcin
2:39this is thought to be one of the
2:40structural proteins of bone it
2:43stimulates insulin secretion from the
2:46pancreas and increases the sensitivity
2:49of adipocytes to insulin and thus it'll
2:54limit the growth of the adipose tissue
2:59now osteocytes used to be osteoblasts
3:05they're now trapped in the bony matrix
3:07that they themselves deposited when they
3:10were osteoblasts the osteocytes live in
3:15little cavities those little cavities
3:18are called lacunae lacunae means little
3:20lakes each of these lacunae are
3:25connected to each other by little canals
3:29or canaliculi that way we can have
3:33constant communication between the
3:35osteocytes you'll notice the osteocytes
3:37have a lot of projections
3:40we'll see where those projections come
3:42into play shortly sometimes we'll see
3:47the osteocytes reabsorbing a little bit
3:50of bone matrix and sometimes we'll see
3:54them depositing it they contribute to
3:57the homeostatic mechanism of bone
3:59density and calcium and phosphate ions
4:03we'll look into this in more detail
4:05later when osteocytes are stressed
4:08they'll produce biochemical signals that
4:11regulate bone remodeling remodeling is
4:15similar to what you would do in a house
4:17taking out old stuff and putting down
4:20new stuff now osteoclasts are bone
4:26dissolving
4:27and they are not in the same lineage as
4:30this osteogenic cell osteoblast osteo
4:34site development the osteoclasts come
4:38from stem cells that are found in the
4:40bone marrow the same ones that give rise
4:43to blood cells these cells are unusually
4:46large and they're formed from the fusion
4:49of several different stem cells so you
4:51can see several different stem cells
4:53coming together here to form this very
4:56large osteoclasts cell they have a
5:01ruffled border on them this ruffled
5:04border is the side that's facing the
5:06bone and they're literally folds in the
5:09plasma membrane that aid to increase the
5:12surface area of the cell and thus the
5:14reabsorption efficiency now these cells
5:18lay on the surface of the bone in an
5:21area called a resorption bay or house
5:25ship lacunae they actually formed pits
5:27on the surface of the bone the
5:29osteoclasts are heavily involved in the
5:32process of bone remodeling they're going
5:35to be doing the dissolving part they
5:38actually secrete chemicals that dissolve
5:40the bone right at this brush border and
5:42they reabsorb the components of the bone
5:45maybe it's because we need to release
5:47some calcium into the blood so they're
5:49breaking down some bone to take calcium
5:52into the cell and then get the calcium
5:54out into circulation so that covered the
5:59cells of osseous tissue now the matrix
6:02of the osseous tissue
6:04it's about one-third organic and
6:07two-thirds inorganic matter by dry
6:10weight the organic component is
6:12synthesized by the osteoblasts this is
6:15material like collagen carbohydrate and
6:19protein complexes such as
6:22glycosaminoglycans proteoglycans and
6:25glycoproteins it's also composed of
6:29inorganic matter 85% is hydroxyapatite
6:34which is crystallized calcium and
6:37phosphate salts so hydroxyapatite is a
6:40common
6:41of both calcium and phosphate there's
6:4410% calcium carbonate and then there are
6:48some other minerals such as fluoride
6:50sodium potassium and magnesium so bone
6:54is a composite of two basic structural
6:57materials ceramic and a polymer so it
7:02can finds the optimal mechanical
7:04properties of each component it's sort
7:07of like if we're building a big concrete
7:09building then there's going to be a lot
7:11of rebar the rebar has a little bit of
7:14flexibility and then we pour concrete
7:18into a form around the rebar so that it
7:22makes the rebar not quite as flexible
7:25bones very similar the organic matter
7:29the collagen and these carbohydrate
7:32protein complexes form sort of the rebar
7:36structure it's soft and bendable and
7:39then we fill it in with inorganic matter
7:42like the concrete here in this case
7:46mostly calcium and phosphate with
7:48calcium carbonate in these other
7:50minerals that firms up the structure so
7:54if there's a deficiency in either the
7:57organic part or the inorganic part we're
8:00going to see different effects in the
8:02skeleton rickets is a condition of soft
8:05bones this is when you have enough of
8:08the organic matter the rebar structure
8:11yet not enough of the inorganic cement
8:17type matter and so the bones are soft
8:21and a little bit bendy osteogenesis
8:25imperfecta is the opposite of that we
8:29have not enough of the rebar right the
8:32collagen and the proteins that maintain
8:36some Bend ability to the structure and
8:39plenty of the inorganic matter so the
8:43bones tend to break very easily because
8:47there's not enough structural portion of
8:50the matrix so take a moment here to pull
8:54out
8:54piece of paper and outline all the
8:56different components of bones histology
9:00we'll begin with the different types of
9:03cells of osseous tissue and then what
9:07are the main components of the matrix
9:10remember the matrix is divided into two
9:13portions the organic portion or the
9:16bendy collagenous rebar type portion and
9:20the inorganic concrete or calcium
9:24phosphate portion now we'll move in to
9:29looking at how it all comes together as
9:31you learned in our section on tissues
9:34the main unit of compact bone is the
9:38osteon or the haversian system so here
9:42you can see an osteon the osteon is the
9:47basic structural unit of compact bone
9:50it's formed by a central canal and then
9:54several concentric lamellae the lamellae
9:59are these different layers that we see
10:01here you can see lamellae all around a
10:05central canal they're connected to each
10:09other by canaliculi the canaliculi are
10:13the little stripes here between the
10:16lamellae
10:16within the central canal you'll find
10:19blood vessels and nerves also you'll see
10:22perforating or Volkmann canals they are
10:25transverse or diagonal passages along
10:28the length of the osteon bringing blood
10:31into each of the different
10:33osteons the skeleton receives about half
10:38a liter of blood each minute the blood
10:41vessels will enter into the perforating
10:44canals through the nutrient foramina
10:47they allow them to cross the matrix and
10:50feed the central canals of all the
10:52different osteons in the bone the
10:55osteocytes you can see here live in
10:58these little lacunae as we said so you
11:01can see the outline of the lacunae here
11:04and then a cell the lighter brown
11:06colored structure inside
11:08the little lake and it has lots of
11:11little projections these projections go
11:14out through the canaliculi so that each
11:17cell can communicate with the
11:20neighboring cells you can see although
11:22this is spongy bone you can see the idea
11:25here where you have a little osteo site
11:28inside a lacunae and each of its
11:31projections are reaching out through
11:33these little canaliculi the little
11:35canals in the bone so that this cell
11:37might be able to talk to this cell in
11:43this way the cells can communicate
11:46they're connected to each other by gap
11:48junctions and they'll not only receive
11:51nutrients from their neighboring cells
11:53but pass waste back to them on those
11:56osteocytes that are closest to the blood
11:59vessels will be the ones that are
12:00receiving the nutrients first this way
12:03each of the cells has a two-way flow of
12:06nutrients of waste into and out of the
12:08cells so it goes from the blood vessel
12:11to the osteocytes closest to the central
12:14canal and then it gets passed literally
12:17from cell to cell through gap junctions
12:19that connect the cells and the cells can
12:23connect to each other by sending out
12:25projections through the little
12:26canaliculi the cells are arranged in
12:29layers called lamellae and the
12:32canaliculi span from one lamellae to the
12:35next lamellae not all of the matrix
12:39however you'll notice is arranged in two
12:41osteons though the osteon is the basic
12:44unit there's some space here that
12:46doesn't have the same circular
12:48arrangement these are called
12:52interstitial lamellae in this area
12:54they're the remains of old osteons that
12:57have been broken down as the bone grows
12:59and remodels itself will also see around
13:03the very edge of the bone
13:06circumferential lamellae they go around
13:09the whole circumference of the diaphysis
13:11of the bone and run parallel to the
13:15bones surface
13:20again you can see a little bit more
13:22detail on the blood vessels in the bone
13:25you can see that they have perforating
13:27canals make their way to the central
13:30canals the nutrient foramina are the
13:32holes in the outside that the
13:34perforating vessels will go into you can
13:39also get a good vision here of the
13:42circumferential lamellae here's an
13:44osteon each of these little dots is an
13:48osteocyte residing in a lacunae
13:52connecting to other osteocytes through
13:56their canaliculi and we'll also see the
14:01interstitial lamellae between each of
14:05these circular osteons spongy bone is a
14:10little bit different in that it has the
14:12appearance of sponginess it's not dense
14:15down here in the bottom portion of the
14:17picture this is the spongy bone the
14:20spongy bone consists of trabeculae these
14:25projections and these trabeculae have a
14:28structure very similar to the osteon but
14:30they lack a central canal the spaces
14:34between the trabeculae are filled with
14:37red bone marrow so most of the blood
14:39cells are produced in here they do have
14:42the same arrangement of osteocytes in
14:45lacunae forming a lamellar layer and
14:48each of these lamellar layers talk to
14:51each other through canaliculi now in
14:54spongy bone trabeculae will develop
14:57along the bones lines of stress so if we
14:59look at this picture you can see the
15:01main lines of stress in the bone and
15:03you'll notice that the trabeculae are
15:05aligned along those lines of stress this
15:09makes the bone be much more strong along
15:12the lines that receive the most stress
15:16okay it's time to diagram again this
15:20time we're highlighting an osteon as a
15:22component of a piece of compact bone the
15:26things that you'll need to include are
15:29the osteo site the lamellae
15:33the canaliculi the central canal some
15:40circumferential lamellae as well as some
15:42interstitial lamellae perhaps you might
15:49identify perforating canals also and
15:52then you'll want to diagram some spongy
15:56bone
15:56show me the trabeculae of spongy bone
15:59and there
16:01lacunae and canaliculi and lamellae and
16:06osteocytes on these diagrams you may
16:10also add in where you most likely see
16:13osteoclasts and where you see
16:16osteoblasts at least think about it so
16:19that you can put it all in perspective
16:23now we'll move on and talk about bone
16:26marrow bone marrow is a general term for
16:28this soft tissue that occupies the
16:30marrow cavity of a long bone and the
16:33small spaces between the trabeculae of
16:36spongy bone there's two types of marrow
16:39there's red marrow and yellow marrow red
16:44marrow or myeloid tissue is in nearly
16:47every bone of a child this is the
16:49hemopoietic tissue that means it
16:52produces blood cells and it's composed
16:54of multiple tissues that are in a
16:57delicate but intricate arrangement this
17:00is an organ in itself in adults we
17:04primarily find this red marrow in the
17:06skull in the vertebrae in the ribs in
17:09the sternum and in parts of the pelvic
17:13girdle and the proximal heads of the
17:16humerus and femur all these areas are
17:19shown in red yellow marrow is found only
17:23in adults in adults most of the red
17:26marrow in the long bones is going to
17:29turn into a fatty yellow marrow this
17:32fatty yellow marrow no longer produces
17:35blood it's the marrow you're probably
17:36familiar with seeing in a long bone that
17:39you might give to a dog often it's the
17:42femur cut into sections and you can see
17:44that fatty yellowish material in the
17:47center
17:47they loved so much so that brings us to
17:50the end of this section of lecture we'll
17:53move on next to bone development but be
17:56sure to spend some time with these
17:57diagrams here while this materials fresh
18:00in your brain because this is the part
18:02of the diagramming assignment for this
18:04chapters material