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By the end of this section, you will be able to:
  • Explain the function of cartilage
  • List the steps of intramembranous ossification
  • List the steps of endochondral ossification
  • Explain the growth activity at the epiphyseal plate
  • Compare and contrast the processes of modeling and remodeling

In the early stages of embryonic development, the embryo’s skeleton consists of membranes hyaline cartilage. By the sixth or seventh week of embryonic life, the actual process of bone development, ossification    , begins. There are two ways that bones undergo ossification— intramembranous ossification    and endochondral ossification    —but bone is the same regardless of the pathway that produces it.

Cartilage templates

Throughout fetal development and into childhood growth and development, bone forms on the cartilaginous matrix. By the time a fetus is born, most of the cartilage has been replaced with bone. Some additional cartilage will be replaced throughout childhood, and some cartilage remains in the adult skeleton.

Intramembranous ossification

During intramembranous ossification    , compact and spongy bone develops directly from sheets of connective tissue. The flat bones of the face, most of the cranial bones, and the clavicles (collarbones) are formed via intramembranous ossification.

The process begins when cells in the embryonic skeleton gather together and begin to differentiate into specialized cells ( [link] a ). Some of these cells will differentiate into capillaries, while others will become osteogenic cells and then osteoblasts. Although they will ultimately be spread out by the formation of bone tissue, early osteoblasts appear in a cluster called an ossification center    .

The osteoblasts secrete osteoid    , uncalcified matrix, which calcifies (hardens) within a few days as mineral salts are deposited on it, thereby entrapping the osteoblasts within. Once entrapped, the osteoblasts become osteocytes ( [link] b ). As osteoblasts transform into osteocytes, osteogenic cells in the surrounding connective tissue differentiate into new osteoblasts.

Intramembranous ossification

Image A shows seven osteoblasts, cells with small, finger like projections. They are surrounded by granules of osteoid. Both the cells and the osteoid are contained within a blue, circular, ossification center that is surrounded by a “socket” of dark, string-like collagen fibers and gray mesenchymal cells. The cells are generally amorphous, similar in appearance to an amoeba. In image B, the ossification center is no longer surrounded by a ring of osteoblasts. The osteoblasts have secreted bone into the ossification center, creating a new bone matrix. There are also five osteocytes embedded in the new bone matrix. The osteocytes are thin, oval-shaped cells with many fingerlike projections. Osteoid particles are still embedded in the bony matrix in image B.  In image C, the ring of osteoblasts surrounding the ossification center has separated, forming an upper and lower layer of osteoblasts sandwiched between the two layers of mesenchyme cells. A label indicates that the mesenchyme cells and the surrounding collagen fibers form the periosteum. The osteoblasts secrete spongy bone into the space between the two osteoblast rows. Therefore, the accumulating spongy bone pushes the upper and lower rows of osteoblasts away from each other. In this image, most of the spongy bone has been secreted by the osteoblasts, as the trabeculae are visible. In addition, an artery has already broken through the periosteum and invaded the spongy bone. Image D looks similar to image C, except that the rows of osteoblasts are now secreting layers of compact bone between the spongy bone and the periosteum. The artery has now branched and spread throughout the spongy bone. A label indicates that the cavities between the trabeculae now contain red bone marrow.
Intramembranous ossification follows four steps. (a) Mesenchymal cells group into clusters, and ossification centers form. (b) Secreted osteoid traps osteoblasts, which then become osteocytes. (c) Trabecular matrix and periosteum form. (d) Compact bone develops superficial to the trabecular bone, and crowded blood vessels condense into red marrow.

Intramembranous ossification begins during fetal development and continues on into adolescence. At birth, the skull and clavicles are not fully ossified nor are the sutures of the skull closed. This allows the skull and shoulders to deform during passage through the birth canal. The last bones to ossify via intramembranous ossification are the flat bones of the face, which reach their adult size at the end of the adolescent growth spurt.

Endochondral ossification

In endochondral ossification    , bone develops by replacing hyaline cartilage. Cartilage does not become bone. Instead, cartilage serves as a template or blueprint to be completely replaced by new bone. Endochondral ossification takes much longer than intramembranous ossification. Bones at the base of the skull and long bones form via endochondral ossification.

Questions & Answers

where we get a research paper on Nano chemistry....?
Maira Reply
nanopartical of organic/inorganic / physical chemistry , pdf / thesis / review
what are the products of Nano chemistry?
Maira Reply
There are lots of products of nano chemistry... Like nano coatings.....carbon fiber.. And lots of others..
Even nanotechnology is pretty much all about chemistry... Its the chemistry on quantum or atomic level
no nanotechnology is also a part of physics and maths it requires angle formulas and some pressure regarding concepts
Preparation and Applications of Nanomaterial for Drug Delivery
Hafiz Reply
Application of nanotechnology in medicine
what is variations in raman spectra for nanomaterials
Jyoti Reply
ya I also want to know the raman spectra
I only see partial conversation and what's the question here!
Crow Reply
what about nanotechnology for water purification
RAW Reply
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
yes that's correct
I think
Nasa has use it in the 60's, copper as water purification in the moon travel.
nanocopper obvius
what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
How we are making nano material?
what is a peer
What is meant by 'nano scale'?
What is STMs full form?
scanning tunneling microscope
how nano science is used for hydrophobicity
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
what is differents between GO and RGO?
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
analytical skills graphene is prepared to kill any type viruses .
Any one who tell me about Preparation and application of Nanomaterial for drug Delivery
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
The nanotechnology is as new science, to scale nanometric
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
what school?
biomolecules are e building blocks of every organics and inorganic materials.
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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Source:  OpenStax, Skeletal system. OpenStax CNX. Apr 17, 2015 Download for free at https://legacy.cnx.org/content/col11779/1.1
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