Full transcript
0:00hi there in this educational video
0:03we're going to talk about how the
0:05majority of your bones form
0:07broadly speaking bone formation is
0:10called osteogenesis
0:11whereby a multi-bony skeleton is formed
0:14during embryonic development
0:17osteogenesis involves ossification
0:20a process of making bone via one of two
0:22processes
0:24first ossification of fibrous membranes
0:28is called intramembranous ossification
0:31and occurs to develop the cranial bones
0:33and the clavicles ossification of
0:37hyaline cartilage
0:39is called endochondral ossification and
0:41occurs to develop
0:43all the other bones of the body now
0:46because
0:46endochondral ossification leads to the
0:48development of most bones of your body
0:51this is what is going to be the focus of
0:53this video
0:56now the importance of using hyaline
0:58cartilage for early skeletal development
1:01is that it is a flexible yet durable
1:03tissue
1:05it allows for rapid mitosis to occur
1:08while being flexible enough for the
1:10development and remodeling of a growing
1:12bone to occur
1:14i want to start by introducing the major
1:17cells or players involved
1:19in endochondral ossification
1:23first let's introduce the origin of most
1:26bone cells
1:27these are called osteogenic cells and
1:30are the progenitors
1:31to osteoblasts you can think of
1:34osteogenic cells as the baby bone cells
1:38second we have osteoblasts
1:41which are actively mitotic just like
1:44closely related
1:45fibroblasts in connective tissue proper
1:48and chondroblasts in cartilage
1:51you can think of osteoblasts as the
1:53growing pre-teenager
1:55bone cells osteoblasts
1:58function in the creation of new bone by
2:01secreting bone matrix
2:03called osteoid and depositing calcium
2:07into this matrix to harden the bone
2:11third we have osteocytes which mature
2:14from osteoblasts
2:15once the bone matrix is fully formed
2:18you can think of osteocytes as the adult
2:21bone
2:22cells osteocytes function in the
2:25monitoring
2:26and maintain maintaining of bone
2:30importantly osteocytes can detect and
2:33respond to
2:33increased mechanical stress and damage
2:36by
2:37communicating with osteoblasts and
2:39osteoclasts
2:42this allows bone to be remodeled as
2:45needed
2:45and also repaired when fractured
2:49fourth and lastly we have osteoclasts
2:53which are derived from the same
2:55hematopoietic stem cells as
2:57macrophages these cells are
3:00giant macrophage-like cells that
3:03function in bone
3:04resorption that is the breakdown of bone
3:08now you may ask why break down bone in
3:11the first place
3:13well breaking down bone can be a
3:16response to low blood calcium levels to
3:18get these levels
3:19back up to a homeostatic level so you
3:22can use calcium
3:24for cardiac function and
3:26neurotransmission
3:27among other important physiological
3:30processes
3:32breaking down bone is also important
3:34when old bone
3:35needs to be remodeled and remade to
3:38ensure
3:38durability and proper functioning
3:42as we go through life
3:45all together these are the four major
3:47cell types involved in endochondral
3:49ossification
3:51making that new bony skeleton out of a
3:54hyaline cartilage skeletal model
3:58let's now turn our attention to the
4:00actual steps
4:01involved in endochondral ossification
4:05the very first step to initiate
4:07endochondral ossification
4:09is the infiltration of small blood
4:11vessels into the perichondrium which is
4:14the outer layer of cartilage
4:16thus delivering the very very first
4:19osteogenic
4:20cells these osteogenic cells
4:24differentiate and develop into
4:26osteoblasts
4:28which will then begin to secrete osteoid
4:31along the outer edges of that
4:33cartilaginous diaphysis
4:37the secreted osteoid ends up encasing
4:40the cartilaginous diaphysis in a bone
4:43collar
4:45the bone collar is an area of
4:47vascularized periosteum
4:49outer layer of bone that is important
4:52for stabilizing
4:53the growing bone shortly after
4:57the bone collar is formed ossification
5:00begins at the primary ossification
5:02center which is located at the center
5:05of the cartilaginous diaphysis
5:09the primary ossification center is so
5:12named
5:12because it is the first area where
5:14extensive
5:15endoconsole ossification and bone
5:18formation
5:19occurs the primary ossification center
5:22is at the center of the cartilaginous
5:24diaphysis and where bone formation
5:26begins
5:28the endochondral ossification that
5:30occurs here
5:31is sort of a dark process for the cells
5:34involved
5:35because essentially what happens is the
5:38killing
5:38and replacement of chondrocytes
5:40cartilage cells
5:42with calcified osteoid in osteocytes
5:45bone cells as osteoblasts
5:49secrete osteoid it wraps around the
5:53chondrocytes and becomes calcified
5:56the calcification cuts the chondrocytes
5:59off
5:59from nutrients and oxygen therefore
6:03they die and this will leave open spaces
6:06in the bone called lacunae at this point
6:10this is compact bone
6:14the lacunae become occupied by the
6:17osteocytes
6:19the quote-unquote mature osteoblast
6:22which will now monitor and maintain the
6:24newly formed compact bone
6:28the process of wrapping chondrocytes
6:31with osteoid
6:33calcifying it and making new compact
6:36bone
6:36occurs continuously therefore
6:40as time continues more and more compact
6:43bone is made
6:45throughout this diaphysis
6:48importantly as new compact bone is made
6:52new cartilage grows at the proximal and
6:55distal ends of the diaphysis
6:57thereby elongating the entire structure
7:02throughout this entire process the bone
7:05collar is there
7:06to stabilize the structure to prevent
7:08collapse
7:10eventually the next major event occurs
7:13which is the invasion of the periosteal
7:16bud
7:17the periosteal bud contains a large
7:20artery
7:21a large vein nerve fibers
7:25red bone marrow elements and even more
7:27osteogenic cells
7:29which will develop into even more
7:31osteoblasts
7:32meaning more bone formation
7:36what are also now arriving are
7:39osteoclasts
7:40the bone resorbers which are responsible
7:43for the breakdown
7:44of bone osteoclasts are important here
7:48because they will erode the recently
7:51made compact boneless diaphysis
7:53thus forming numerous small holes that
7:56ultimately
7:57fuse together to begin forming the
8:00medullary cavity
8:02the medullary cavity will be lined by
8:05spongy bone
8:07essentially partially eroded compact
8:09bone
8:11you can think of this process in regards
8:13to a slice of delicious
8:15cheese after some erosion in the center
8:18of the cheese slice
8:21nibbling little bites it will begin to
8:23develop
8:24holes now it's swiss cheese this is like
8:27spongy bone
8:29if enough erosion occurs like taking
8:32multiple little bites
8:35in the center of the cheese slice
8:36eventually a
8:38complete hole will develop this is just
8:41like
8:42the medullary cavity so altogether
8:46the entire process just described
8:49of osteoblasts secreting osteoid around
8:51chondrocytes and making new bone
8:54as osteoclasts resorb the newly formed
8:57bone
8:58continues throughout the diaphysis
9:02this continuation allows for the
9:04continued development of the bony
9:06diaphysis
9:07and the continued elongation and opening
9:10up
9:10of the medullary cavity which will
9:13ultimately run the length of the entire
9:15diaphysis
9:18as more and more bone is created in the
9:21diaphysis
9:22and the medullary cavity continues to
9:24elongate
9:26eventually secondary ossification
9:28centers appear
9:30at the epiphyses which are at each end
9:33of the growing bone
9:36these secondary ossification centers
9:38appear just
9:39following birth at this point
9:42the epiphyses begin the processes of
9:45endochondral ossification
9:47thus beginning to gain bony tissue
9:52ossification of the epiphyses occurs in
9:55much the same way that it does in the
9:57diaphysis
9:59osteoblasts secrete osteoid wrapping
10:02around the chondrocytes which then
10:04becomes calcified
10:06this initially results in a development
10:08of new
10:09compact bone however
10:13as this new bone is made osteoclass
10:16partially erode or resorb
10:19the bone away this process continues
10:23in each epiphysis ultimately leading to
10:26the development of spongy bone
10:27throughout
10:28each epiphysis eventually
10:32you end up with two epiphyses composed
10:34of spongy bone
10:35each having a single epiphyseal plate
10:41it is here at the epiphyseal plates
10:44where the remaining hyaline cartilage
10:46rapidly divides making more and more
10:49layers of cartilage
10:51that can then be ossified into bone
10:55as all these layers of cartilage are
10:58ossified
10:59more and more bone is made and the
11:01entire bone
11:02elongates in mammals
11:06such as humans this process called
11:09epiphyseal bone growth
11:10continues up to puberty at which point
11:14maxwell bone length has been achieved
11:16and
11:17no further bone growth or ossification
11:20will occur