hello anatomy colleagues this is dr alsup and in this video we will be discussing how bone
formation actually occurs so bone formation is called ossification all right so that is uh the
the correct terminology for bone formation and we will talk about how that term differs from
the term calcification which those two terms are often used interchangeably incorrectly of course
and there are two methods of ossification and we will talk big picture in terms of those processes
and which types of bone will form in these manners so lots of fun stuff to talk about here so let's
get right into it okay all bone formation begins with the formation of an embryonic skeleton
composed of mesenchyme all right so you see this mesenchymal skeleton and remember in our previous
discussions mesenchyme is an embryonic tissue that almost all connective tissues are derived from
and since bone is a type of connective tissue that makes sense
that it's going to form within a mesenchymal skeleton this formation begins around the
sixth week in utero and from there bone formation follows one of two patterns
so these two types of ossification are referred to as intramembranous ossification
as well as endochondral ossification but before we get into the description of these i want to focus
on this question right here because the the terms ossification and calcification are often used
interchangeably which is incorrect these terms are not synonymous ossification which is the focus
of our video here ossification is the process by which bone forms and this can happen in four
specific situations it can happen um with initial formation of bone which is what we're going to
discuss in detail here it also ossification also occurs with growth of bone during the juvenile
period is complete and then there will be adult bone also remodeling of bone and lastly repair
a bone so those are going to be the four specific situations that ossification can occur and as part
of bone formation and ossification osteoblasts remember will lay down that organic material
materials of the extracellular matrix and that will initiate calcification or tissue hardening
okay so i if you think about what is making bone hard it is going to be the calcification process
so calcification is a process within ossification so make sure
to not use these terms interchangeably so calcification happens during ossification
but don't use those terms interchangeably okay returning to the main types of ossification
the first one we are going to discuss is intramembranous ossification which usually
we start with this one because it's the simpler of the two and in this process bone will replace
the mesenchymal skeleton directly whereas in endochondral ossification the mesenchymal skeleton
is is replaced by a hyaline cartilage model which itself will increase in size and then eventually
be replaced by bone so there is an extra step when we're talking about endochondral ossification
so let's explore the big picture ideas regarding the process of intra-membranous ossification and
the first part i really want you to understand is this concept referred to as an ossification
center and ossification centers are areas where cells begin to kind of cluster together
which will eventually become areas that will develop into bone so those cells will
eventually differentiate into osteoblasts that will secrete extracellular matrix
extracellular matrix components of bone so in the case of intra membranous ossification
first those mesenchymal cells will cluster together and then those mesenchymal cells will
differentiate into those osteogenic cells and if you recall the next point in that life cycle
will be changing into the osteoblast so they will kind of collect in this area
after this occurs and osteoblasts are hard at work calcification or hardening of the extracellular
matrix can occur so here right here you have calcification being a process within ossification
now the the bone extracellular matrix will form the trabeculae or those bony
beams that you often see in spongy bone and also during this time red bone marrow will
form around this area and kind of within these spaces
between the trabeculae and lastly the periosteum will form on the outer surface of the bones and
additionally the outer layer of spongy bone will be replaced by compact bone because there always
has to be that outer layer of compact bone but the center of the bone typically stays mostly spongy
so only a minority of bones formed through intramembranous ossification
and these include many of the flat bones of the skull which are kind of that skull cap
region and between the bones of the skull you'll also have font nails which i'm outlining a few
in these images you can have them in different areas of the skull
these font nails are often referred to as soft spots particularly this anterior font now is
the soft spot and these are important in allowing the large skull to exit the relatively small birth
canal so allowing some flexibility there and after birth these areas will undergo
intramembranous ossification and harden and be replaced entirely by bone
other bones that form through intra-membranous ossification include many of the facial bones
as well as the mandible or your lower jaw and the medial part of the clavicle
so when you look at this list particularly these first three intramembranous ossification
is particularly important for the bones of the skull but pretty much all the rest of the bones
of the body of which there are many many more will form through endochondral ossification
now as we mentioned at the beginning endochondral acidification is more complex than intramembranous
and this is all because of the middle step so you have that mesenchymal skeleton
but then you'll have the inclusion of this cartilage model which is not only present it's
not only going to form a model but this model will actually grow before being replaced by
bone and again almost all bones of the body are going to form through this ossification process
so in the region of the body where the bone is going to form chemical messages will cause
those mesenchymal cells to crowd together and it's going to form the general shape of the future bone
so that's that messenger skeleton then those cells would differentiate into chondroblasts
so you see right here in the name you have the blast so you know it is building something
specifically secreting cartilage extracellular matrix matrix components thus it's producing this
cartilage model and any time that you see the chondra within you know we are talking cartilage
all right so that is always a good hint that something cartilage is involved
this cartilage model is going to be composed of hyaline cartilage now the cool thing is
that this cartilage model will actually grow through both interstitial growth or growth from
within which results in the cartilage model growing in length as well as oppositional
growth meaning growth of the outer surface so the cartilage model also will grow in width
eventually there will be the development of the primary ossification center and we talked about
ossification centers with intramembranous ossification and this will also occur with
endochondral ossification but since many of the bones that are being formed here are long bones
which will have a diaphysis and epiphysis there will be multiple ossification centers so the
primary ossification centers are typically the first and they were typically located
in a central loca excuse me a central location associated
with the diaphysis so it's kind of in this middle portion right here and as the primary
ossification center grows towards the end of the bone our good friend the osteoclast will actually
begin breaking down uh some of the middle portions of the bone to start forming the medullary cavity
with time often around the time of birth secondary ossification centers will arise
at the epiphyses once an epiphyseal artery kind of enters into the region and these are going to
be referred to as secondary ossification centers so you can see one here and you can see one here
as i said these are often going to start occurring around the time of birth
and eventually the hyaline cartilage model will be completely replaced with bone
except on the ends where there remains articular cartilage so you still have a remnant
of that cartilage model even into adulthood with that articular cartilage
but before an individual reaches adulthood there is going to be that layer of hyaline cartilage
in the metaphysis remember that epiphyseal plate and so that is going to allow for lengthwise
growth in the diaphysis all right so that was intramembranous and endochondral ossification in
a nutshell there are a lot more details regarding these processes these are actually fairly complex
but i just wanted to focus on the big picture concepts for our purposes
here but before we part let's make sure we are crystal clear on one concept
and see if you can quickly answer this question so the question which type of ossification process
includes the formation of a cartilage model is it a endochondral or b intramembranous
and i hope after all the details we have covered in this video you feel pretty confident that a
endochondral is the correct answer first off it has chondra right within its name and i told you
anytime you see that you just automatically think cartilage is involved so it kind of says it right
here in the name but i also hope you now know that endochondral is the ossification process
that has those additional steps of the formation as well as the growth of the cartilage model
that the intramembranous ossification does not have so that is all i have for this video
in the next video we will delve a bit more into those terms interstitial and appositional growth
as you review please do reach out if you have any questions and have a great rest of your day