Full transcript
0:00hi and welcome back to my channel
0:03learning biology with dr vanessa
0:05in today's video we are going to take a
0:08closer look at the skeletal system
0:10looking at different types of bone
0:12formation and healing of fractures
0:15if you haven't already watched my other
0:17video on the introduction to the
0:18skeletal system now is a good time to
0:21stop this video and watch that other
0:23video first so that you have a better
0:25idea of how the skeletal system is laid
0:27out
0:28however if you don't have enough time to
0:30do that i'm going to put a little bit of
0:33background information into this video
0:35so that you're not completely lost if
0:37you're just watching this video as is
0:41in order to understand bone formation we
0:44need to understand the different types
0:45of cells that work in the bone
0:48we have a group of cells that come from
0:50bone cell lineage and then another cell
0:53that comes from white blood cell lineage
0:56in the bone cell lineage we have the
0:58osteogenic cell the osteogenic cell is
1:01basically the stem cell that gives rise
1:03to most other bone cell types
1:06this step this cell is capable of
1:09undergoing mitosis they multiply
1:12continuously making more of themselves
1:15so these stem cells will continue to
1:17multiply making more of themselves
1:21they can then differentiate into
1:24osteoblasts osteoblasts are cells that
1:28form bone or bone-forming cells
1:32these cells are not capable of mitosis
1:35so once a cell has differentiated into
1:38an osteoblast it can no longer undergo
1:41mitosis meaning when those cells die
1:44they're gone and new ones have to come
1:46from these osteogenic cells
1:48now if you looked at the picture of bone
1:50that we talked about in a previous video
1:52or in your class
1:54these osteogenic cells are found in the
1:57industrium as well as in central canals
2:02they then can
2:04turn into osteoblasts and osteoblasts
2:07will be the cells that are going to be
2:09putting down bone okay
2:12they deposit uh down
2:15the bone matrix
2:17and they are going to continue to make
2:20it as they continue to make the bone
2:22matrix
2:24they are going to eventually develop
2:26into osteocytes
2:28now osteocytes are osteoblasts
2:32that have become trapped in the matrix
2:34that they deposited and now they're
2:36going to live in cavities which are
2:38referred to as lacuna
2:41um
2:42these cell these cells cannot move
2:45anymore these osteocytes cannot move
2:47anymore they're trapped within the
2:49matrix but
2:50they live in this little space called a
2:52lacuna and there are little canals that
2:57are formed around them
2:59referred to as canaliculi now the
3:01canaliculi is where their processes
3:03extend out and this also allows them to
3:07talk to other cells while they are there
3:10in this area they're going to contribute
3:12contribute to the homeostatic
3:14maintenance of both bone density as well
3:17as blood concentrations of calcium and
3:20phosphate ions so they are responsible
3:23for
3:24keeping um
3:26the matrix the bone etc happy
3:30and then we have the osteoclasts over
3:33here the osteoclasts are formed from
3:36white blood cells so they don't come
3:39from bone cell lineage they actually
3:40come from white blood cells
3:43and these osteoclasts are bone
3:46dissolving cells they are found on the
3:48surface of bone and they are capable of
3:52what is called resorption where they
3:54release their acidic contents and then
3:56they resorb on the bone so they're going
3:59to break down that extracellular matrix
4:01of bone so we have osteoblasts that are
4:03going to build up the bone and
4:05osteoclasts that are going to break down
4:08bone
4:09let's talk just a little bit more about
4:11these cells and what they specifically
4:13do while looking at these pictures here
4:16these pictures are done with a scanning
4:18electron microscope i really like
4:21pictures done with a scanning electron
4:23microscope because not only do can you
4:26see um really fine details but the
4:30pictures are done in sequence in in a
4:33series
4:34slice after slice after slice after
4:36slice and then they're put together so
4:38that you get this 3d
4:41image
4:42now they're not ever done in color
4:44they're done in black and white and then
4:45an artist can go back in and color the
4:48picture and so you can really see what
4:51the the cell looks like so you can see
4:52the osteoblast in here the osteocyte
4:56right here
4:58deposited in the bone matrix and then
5:01you can see the osteoclast here now this
5:03last picture i believe is um
5:06misnamed because this does not look like
5:09a scanning um electron micrograph as you
5:12see in the other ones
5:14it looks very flat it also is like a
5:17purpley pink so it looks like it's
5:19staying with hematoxylin and eosin
5:22i could be wrong but it does not look
5:24like a scanning electron micrograph
5:27so here we have our osteoblast remember
5:30again that our osteoblasts are our bone
5:33forming cells they actually synthesize
5:36and secrete
5:38collagen fibers as well as other organic
5:41components that are needed to build the
5:44extracellular matrix of osseous tissue
5:47remember osseous is another word for
5:49bone
5:50osteoblasts also initiate calcification
5:54they surround themselves with
5:56extracellular matrix and eventually they
5:59become trapped in their secretions
6:02where they be then become osteocytes a
6:05good thing to understand or note put
6:08into your brain is that when you see
6:10cells that end in blast b-l-a-s-t
6:15that part of their name means that
6:17they're going to secrete some sort of
6:19extracellular matrix okay so osteoblasts
6:23form bone
6:25once they become trapped in that
6:27extracellular matrix they become
6:29osteocytes like i mentioned before you
6:31can see the osteocyte here and it is
6:34surrounded by a hardened calcified
6:38extracellular matrix versus the
6:40osteoblast that is here so the
6:42osteoblasts are either on the very
6:44exterior of the bone or the very
6:47interior of the bone
6:50and they are
6:52forming bone as they get trapped in that
6:54matrix they become an osteocyte so these
6:57are mature bone cells just like
6:59osteoblasts they cannot divide they
7:02cannot undergo mitosis
7:04and they are actually the most numerous
7:07cells that we find in bone tissue they
7:10are responsible for maintaining
7:13um the bone tissue
7:15with its daily metabolism which includes
7:18things like the exchange of nutrients
7:21waste in the blood remember bone is a
7:23living tissue even though it is hardened
7:26there is still blood running through the
7:27bone the cells are receiving nutrients
7:30they're making waste that have to go out
7:33in the blood as well so they're getting
7:35nice fresh arterial blood with oxygen
7:37and nutrients they're putting out their
7:39waste into the venous blood uh to be
7:41taken back to the heart and for those
7:44waste to
7:45be removed from the body okay
7:48so
7:49another type of ending in the cell that
7:52we also see
7:53a lot as we study different types of
7:55cells in different organs is cyte or
7:58site
7:59and that
8:00site when you see that
8:02that is going to mean that that cell is
8:04responsible for maintaining the tissue
8:07okay so we have our blast building up
8:08the tissue we have our site that is
8:11going to be maintaining the tissue and
8:13then finally we have our osteoclasts
8:17now
8:17osteoclasts are actually a really large
8:21cell and as i mentioned before
8:24they are
8:25derived from white blood cells
8:28specifically the white blood cell that
8:30they are derived from are monocytes as
8:33many as about 50 monocytes actually fuse
8:36together in order to form an osteoclast
8:40now if you remember anything about
8:42monocytes you may not have learned about
8:44those yet monocytes are a type of white
8:47blood cell that can eat things or
8:51they phagocytize right so cell eating so
8:54these white blood cells are responsible
8:57for
8:58um eating cellular debris of cells that
9:01have died or um recognizing bacteria
9:05that may come into the body things of
9:07that nature one of their main jobs is to
9:09eat things because of this
9:12monocytes have a lot of lysosomes
9:15within them remember that is a cellular
9:18cellular organelle that has a lot of
9:21digestive enzymes in it so
9:24every cell has
9:25organelles but some cells have more of a
9:28type of organelle than others depending
9:31on their specialty well monocytes have a
9:33lot of lysosomes because they have the
9:37need for a lot of digestive enzymes when
9:39they eat things to break those things
9:41down well osteoclasts are derived from
9:43about 50 or so of these monocytes that
9:46all converge together so osteoclasts
9:49actually really have a lot of lysosomes
9:52within them and so you can see the
9:55osteoclast here and what happens is
9:59where the osteoclast
10:01meets the bone
10:03it has this edge which is referred to as
10:05a ruffled border because it has like a a
10:09folding look where it touches the bone
10:12and from that area is where the
10:14osteoclast is going to release these
10:18enzymes
10:20from the lysosome and these enzymes and
10:23acids when they're released are going to
10:27cause the bone to dissolve
10:30and while the bone is dissolving it's
10:33going to release
10:35uh things like calcium
10:38phosphorus back into the bloodstream
10:40okay so um this can be released
10:44by the osteoclast breaking down the
10:46bones so the osteoclasts break down that
10:48bone by the releasing of those enzymes
10:51acidicness
10:52and this is termed resorption so you
10:55heard me mention that before and
10:58resorption
10:59is um
11:01when that matrix is being broken down by
11:03the osteoclast now this is a very normal
11:06process
11:07and throughout our whole lives there is
11:09going to be some sort of building up
11:12bone with the osteoblasts
11:14and breaking down bone with the
11:16osteoclasts so that's going to be a
11:19lifelong in this video we're going to
11:21talk about that
11:23in relation to bone formation
11:26bone remodeling bone healing um in
11:29another video that i'm going to make
11:31we'll talk more about the physiology of
11:33why this would happen in maintenance of
11:36um
11:37minerals that are in the in the blood
11:40one other thing to note is that when we
11:42see cells with the ending clast
11:45c-l-a-s-t
11:47that means that this type of cell is
11:49going to break down extracellular matrix
11:54so one more time for recap osteoblasts
11:57build bone osteocytes are mature
12:01osteoblasts that are now trapped in the
12:04extracellular matrix and osteoclasts
12:08break down bone
12:11bone development so bone development as
12:14i had mentioned in my other video is
12:17the development of bone just like it
12:19sounds
12:20um but it is termed ossification so
12:23ossification
12:24is bone tissue formation
12:26and it occurs in four different
12:28situations one
12:30in the formation of bone that happens in
12:32an embryo
12:33two the growth of bones until adulthood
12:37three the remodeling of bone and four
12:39the repair of fractures
12:41when we think about bone and bone growth
12:45and um bone as a living tissue we
12:48probably really only think about those
12:50first two we don't really think about
12:52the remodeling of bone or the repair
12:54fractures but ossification or bone
12:57tissue formation is going to occur in
13:00some
13:01form or fashion
13:02all throughout your life obviously
13:06in the embryo in
13:09childhood etc it's going to be much more
13:11dramatic
13:13but it is a process that's going to
13:14occur so our osteoblasts and our
13:16osteoclasts are always going to be
13:19working
13:21in the bone
13:22if we talk about ossification or bone
13:25tissue formation there are actually two
13:28types of ossification
13:30the first type is referred to as
13:31intramembranous ossification and this is
13:34the ossification that produces the flat
13:37bones of the skull
13:38and most of the clavicle or collar bone
13:42the other type of ossification is
13:44referred to as endochondral ossification
13:48this is
13:50the process by which most of the bones
13:52of the body develop the vertebrae ribs
13:55scapula pelvis bones of limbs etc
13:59we are going to take a closer look at
14:01how both of these types of ossifications
14:04happen
14:05and
14:07what's the difference between them
14:08because obviously they are very
14:11different from one another
14:13first let's take a look at
14:14intramembranous ossification
14:17this is the bone formation of the flat
14:19bones of the skull and the clavicle if
14:22you think about these types of bones
14:24they are not very thick
14:26this type of bone formation occurs when
14:29there is a condensation of mesenchyme
14:32into a soft sheet
14:34here i have a picture of what mesenchyme
14:37looks like
14:38what is mesenchyme mesenchyme is a
14:41connective tissue it is very loosely
14:44organized as we can see here the cells
14:46are very spread apart the cells in
14:50mesenchyme tissue are referred to as
14:52mesenchymal cells
14:54and it is very again it is very loosely
14:56organized and we find this tissue type
15:00in embryos it is actually an embryonic
15:03tissue type
15:04undifferentiated
15:06from which connective and skeletal
15:08tissues
15:09can be made so in order for inter
15:12membranous ossification to occur we need
15:15a sheet of this mesenchyme tissue that
15:18has condensed
15:20in the area so again
15:22these bones are not very thick they're
15:24pretty thin bones so think about that
15:27compared to other bones we'll talk about
15:29with a diff different type of bone
15:31formation
15:33so in intra membranous ossification
15:35we're going to have that condensation of
15:38the mesenchyme into a soft sheet
15:41eventually there is going to be this
15:43formation of an ossification center and
15:46what we'll also see during this
15:48development is that um
15:50blood capillaries are going to start to
15:53permeate into the area in order to
15:55deliver nutrients um to those cells
16:00the next thing that's going to occur in
16:02intramembrane is ossification
16:05is that
16:07there is going to be calcification
16:10so those osteogenic cells which were the
16:13stem cells are going to differentiate
16:16into osteoblasts
16:18osteoblasts are going to start laying
16:21down the matrix of bone and as they do
16:25so they're going to become trapped
16:27within the matrix and turn into
16:29osteocytes so we'll start to see some
16:32osteocytes in lacuna
16:34and then developing that canaliculi
16:37those little canals
16:38to be able to talk to one another so
16:41that matrix is going to calcify in the
16:43middle as bone forms
16:48and then we have the formation of the
16:49trabeculae which is that um
16:53spongy bone that airy middle portion of
16:57the bone to make bone a little bit
16:59lighter so we see that honeycomb
17:02formation that is going to occur
17:05the blood vessels are going to
17:07intertwine more in there
17:09and they are going to uh continue to
17:12form as the
17:14mineral
17:15of bone is going to be deposited in that
17:18extracellular matrix and again that is
17:21going to create that spongy bone within
17:24the middle portion of the bone so you
17:26can see their spongy bone and then there
17:29is um compact bone as well
17:34finally the surface of bone is going to
17:37be filled in
17:39with
17:40the deposits of the extracellular matrix
17:42by those osteoblasts forming a nice
17:46layer
17:47of um
17:48compact bone and then inside we can see
17:51that spongy bone again remember these
17:54bones are not very thick you can see
17:56they're very um
17:58thin compared to the other bones we're
18:00going to talk about
18:02and so this bone again was formed on a
18:04mesenchyme tissue
18:07and now we have we're at the point where
18:09we now have compact bone that has been
18:11formed and the spongy bone
18:14tissue that we see in the middle at this
18:16point the periosteum has also been
18:18formed on the outer layer that
18:20connective tissue that's going to be
18:23where the
18:25osteoblasts are going to
18:27be
18:28forming bone because they're going to be
18:29derived from those osteogenic cells so
18:31the osteogenic cells are going to hang
18:33out in this area giving rise to more
18:36osteoblasts so that they can continue to
18:38deposit
18:39more bone
18:40within there in this area here if we
18:43were to zoom in this is where you would
18:45see the osteocytes that are now stuck
18:48within the compact bone there would also
18:50be osteocytes within the spongy bone as
18:53well
18:55endochondral ossification is the other
18:58type of bone formation
19:00this is the ossification by which most
19:03of the bones of the body develop
19:06it begins about the sixth week of fetal
19:08development and this process continues
19:11on into a person's twenties
19:14we just finished talking about
19:16intramembranous ossification which
19:18occurred directly on a sheet of
19:20mesenchyme tissue
19:22and a ossification however
19:25begins on a pre-existing model
19:29that is composed of hyaline cartilage
19:32let's talk about how this process begins
19:35and then how this process can actually
19:37continue on into a person's 20s
19:42in endochondral ossification we start
19:45with the development of the cartilage
19:46model now this hyaline cartilage
19:49develops on
19:51so mesenchyme tissue is actually the
19:53precursor to this but instead of bone
19:56being developed straight from that
19:57mesenchyme tissue that mesenchyne tissue
20:00is going to differentiate into hyaline
20:02cartilage forming this
20:05model of bone and then from this
20:08cartilage model the bone will be
20:11formed now you can notice
20:13this um
20:15the hyalum cartilage actually takes the
20:17formation of a bone where there's going
20:19to be the proximal epiphysis the distal
20:22epiphysis and then the diaphysis that is
20:26in the middle
20:27just like we saw in um
20:30intermembranous ossification we also
20:33have the development of the ossification
20:35center right in the middle of the bone
20:39as this initial cartilage model
20:41continues to grow
20:43the cells in the mid region are going to
20:46increase in size or undergo hypertrophy
20:51this is going to stimulate the
20:52surrounding extracellular matrix to
20:55start to calcify
20:57so you can see here there's going to be
20:59some calcification that is going to
21:02occur around these cells that have
21:04gotten bigger
21:06as development continues there's going
21:08to be a nutrient artery that is going to
21:11penetrate through the periosteum that
21:13outer layer
21:14into
21:16the middle portion of the bone now as
21:19this happens we initially had the cells
21:21in the periosteum that were
21:23differentiating into chondroblasts
21:27chondroblasts
21:29lay down
21:30cartilage
21:31and when this nutriar nutrient artery
21:34comes in through the periosteum is going
21:36to stimulate those cells to now form
21:40osteoblasts
21:41rather than chondroblasts so now we're
21:44going to have osteoblasts that are going
21:46to start to lay down the extracellular
21:49matrix of bone
21:51there is also going to be the primary
21:53ossification center formation where bone
21:56is also going to be laid down in that
21:59middle area taking over those
22:02areas that cartilage had been initially
22:06as the primary ossification center
22:09continues to develop
22:11osteoclasts will work in there too and
22:14break down some of the bone
22:15and as we can see here this is going to
22:17give rise to this middle area where red
22:20bone marrow will be found
22:23where blood formation and blood cell
22:25formation can take place within the
22:27middle of those long bones eventually
22:29that red bone marrow will turn into
22:32mostly yellow bone marrow as you age
22:37eventually the epiphyseal arteries will
22:39enter into the epiphyseal portion of the
22:42bone this then forms the secondary
22:44ossification center
22:47this is going to be very similar to what
22:49we saw in the primary ossification
22:51center where
22:53bone formation is going to continue by
22:56osteoblasts laying down the
22:57extracellular matrix
23:00in this case though instead of hollowing
23:02out the bone we'll see spongy bone that
23:05develops in the middle while compact
23:07bone will stay on the outside
23:11and then finally we'll see the formation
23:12of the articular cartilage this will
23:15cover both
23:17ends of the bone
23:19and this is going to give the bone some
23:22protection when um bone uh touches
23:25against bone so that there is some
23:27protection of the bone that they're not
23:28rubbing each other directly that they're
23:30rubbing the articular cartilage instead
23:33and we also see the formation of the of
23:36the epiphyseal plate now the epithelium
23:38epithelial plate will be the only area
23:42where cartilage still remains in the
23:43bone as you can see here we have spongy
23:45bone we have compact bone but then we
23:48still have the epiphyseal plate where
23:49there is still hyaline cartilage the
23:51this is going to be where the bone can
23:53grow in length and we'll see this on
23:56both ends of the epiphysis even though
23:58only one um end is shown here the other
24:01end will look exactly the same where it
24:03has this epiphyseal plate
24:05this is where bone is going to be able
24:08to
24:09grow in length now
24:13in an adult okay once you've reached
24:16about your 20s or so this epithelial
24:18plate
24:20gets all covered with bones so that
24:22there's no more cartilage here anymore
24:24and growth in length can no longer occur
24:27in the bone and instead of being
24:29referred to as an epiphyseal plate it is
24:31then referred to as an epiphyseal line
24:36if we zoom in and take a closer look at
24:39the epiphyseal plate
24:41okay
24:42this is going to they're actually
24:43showing it to you upside down so here's
24:45the epiphyseal side here's the
24:48diaphyseal side okay so the epithelial
24:50side and the diaphyseal side
24:53so
24:54this is showing you what the epiphyseal
24:56plate looks like
24:58blown up in proportion
25:00and again this is only going to be seen
25:03in children that are growing this is
25:05what allows bone to grow in length
25:10and so there's different um zones here
25:12the zone of resting cartilage
25:15these cells are not responsible for the
25:18function in bone growth that's why it is
25:21called resting cartilage
25:23they are going to basically anchor this
25:26epiphyseal plate
25:28to the epiphysis of the bone so they're
25:30just kind of holding things in place
25:33then you see the zone of proliferating
25:35cartilage that proliferating term should
25:38give that away that these cells are
25:41going to be actively working so the
25:44chondrocytes in this area chondrocytes
25:47are cartilage cells
25:49they are going to be secreting
25:51extracellular matrix they're going to be
25:54um
25:55dividing to replace cells that may have
25:57died and so this area is going to be
26:00able to be growing the cartilage is
26:03actually growing in this area
26:05as we move down we get into the zone of
26:08hypertrophic cartilage and this is going
26:11to be where the chondrocytes are
26:14maturing so you can see how they're
26:15smaller in the zone of proliferating but
26:18then as we get into the zone of
26:20hypertrophic cartilage hypertrophic they
26:23are getting bigger
26:25finally we have the zone of calcified
26:27cartilage and this is going to be
26:30um
26:32where the chondrocytes are mostly dying
26:35in this area
26:36and then the osteoclasts are going to
26:39help dissolve
26:40um that calcified cartilage and the
26:42osteoblasts are going to start laying
26:45down the bone to eventually we're
26:47getting into the area
26:49where the bone is forming and so this is
26:53going to happen throughout life
26:54basically where more cartilage is being
26:57made to lengthen this area
26:59eventually developing into bone more
27:01cartilage is being laid eventually
27:04developing into more bone
27:06until there comes a point where this
27:09whole area gets calcified over in
27:12adulthood and then this no longer occurs
27:15so bone will no longer grow in length
27:18at that point
27:20let's talk about the healing of
27:22fractures because in the healing of
27:23fractures we're going to have the work
27:25of both the both the osteoblasts and the
27:28osteoclasts
27:30in here as well as the healing
27:32goes on so what happens you break a bone
27:35okay remember bone is a living tissue so
27:37if you break a bone that bone is going
27:39to bleed because you're going to break
27:41through
27:42blood vessels that are feeding into that
27:44bone tissue and so when you break a bone
27:47the bone is going to bleed and so one of
27:49the first things that is going to happen
27:50is that you're going to have a blood
27:51clot or hematoma formation
27:54and this is going to clot in that area
27:57obviously
27:59if the
28:00fracture is very great
28:03you are going to have to get a cast and
28:05so the healing of the fracture would
28:06then occur within the cast itself if it
28:09is very very great then there may be
28:12surgery and pins etc involved but these
28:15are the steps that the bone is going to
28:17go through as it heals so we get that
28:18blood clot formation and that's going to
28:21kind of hold all those areas of tissue
28:23together
28:24then you have what is called a soft
28:26callus formation where collagen is going
28:28to be deposited
28:30and fibrocartilage is going to um
28:34help form this soft callus so it's
28:36basically holding everything together
28:38with a very soft tissue callus in the
28:41area that originally had the blood clot
28:45formation
28:46and then there's a hard catalyst
28:47formation and this is where that soft
28:50callus had initially been made again
28:52over the initial blood clot and this is
28:55where is going to be where the
28:56osteoblasts are going to start to lay
28:59down
29:00um the extracellular matrix of bone
29:04and then it's going to get hard
29:06okay
29:07at this point when they take an x-ray of
29:09the bone they're going to see that hard
29:12callus formation and that the bone is
29:14essentially healed to the point where it
29:16can start to be used
29:18so this is where the cast would come off
29:20at this point
29:22now
29:23what's going to happen is you're going
29:25to have these protrusions of the bone
29:26around that area that healed and so now
29:29the bone is going to have to start to be
29:30used because it hasn't been able to be
29:32used while it was in the cast then it
29:35comes off and what's going to happen is
29:37as the bone gets used and as we put
29:40different stresses on that bone bone
29:43remodeling is going to occur now bone
29:45remodeling occurs because of the
29:47stresses that are put on the bone and it
29:49involves both osteoblasts and
29:52osteoclasts so osteoclasts are going to
29:55help start to break down the bone and
29:57osteoblasts are going to build up the
29:59bone depending on where the stresses are
30:03happening on that bone so eventually
30:06this um hard callus will go away
30:10as bone remodeling occurs and then the
30:13bone will go back to normal as it is
30:16used but in order for bone remodeling to
30:18occur
30:20the bone has to be used because bone
30:22remodeling
30:24happens again based on the stresses that
30:27are put on bone so if the bone doesn't
30:29get moved then the bone remodeling isn't
30:32going to happen so this happens
30:35bone remodeling will happen then as the
30:37bone is used and as we're putting
30:39different stresses and osteoclasts help
30:41to break down that bone and osteoblasts
30:44build up that bone so that there's um
30:48it starts to remodel to the way it's
30:49being used and there's not so much
30:51stress on that bone
30:54thank you so much for watching my video
30:56i really hope that this video helped you
30:58to understand the difference between
31:00intramembranous and endochondral
31:02ossification as well as the healing of
31:05fractures my main goal in bringing you
31:07these videos is to take really difficult
31:09topics and make them easy to understand
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31:30please make sure that you put them down
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31:34thanks for watching