V.C - J - Factors in Initiation of Ossification (intramembranous & endochondral); Role of Mechanobiology in Development Flashcards
What are the two different ways which bone is created during foetal development?
Intramembranous ossification and Endochrondral ossification - this is where bone originates from cartilage
What is the definition of intramembranous ossification?
This is the immediate conversion of mesencyhmal cells to bone
Where does intramembranous ossification primarily occur? There is an exception where intramembranous ossification also occurs, try and name this site?
Intramembranous ossifciation occurs primarily in the skull, including cranial suture lines The exception includes the periosteum surrounding the outer surface of long bones: its growth is also considered as intramembranous
Intramembranous ossification occurs in 4 stages Describe stage 1?

Stage 1: Ossification centre formation The mesenchyme condense to form a membranous sheet permeated with blood capillarie The osteoprogenitor cells found here differentiate into osteoblasts and collagen fibres surround the cells

What forms the osteoprogenitor stem cells found in the ossification centre of the connective tissue?
The osteoprogenitor cells arise from the mesenchumal cells differentiating

Describe stage 2 of intramembranous ossification
Stage 2: Calcification The osteoblasts secrete osteoid and some are enveloped in intracellular matrix forming lacunae and converted into osteocytes The osteoid calcifies to form spicules of bone

Describe stage 3 of intramembranous ossification
Stage 3: Trabecular and Periosteum formation The remainder of cells synthesise and secrete osteoid, then calcify to become woven bone trabeculae Cancellous bone forms from interconnecting network of trabecualae, including spaces for separating - form red bone marrow Vascularised membranous sheet condenses to become perisoteum

Describe stage 4 of intramembranous ossification?
Stage 4: Periosteum Development The periosteum, which contains osteoblasts, constructs a woven bone layer just deep to its surface which later becomes lamellar bone forming the outer compact layer of bone (the cortical bone)

Describe the stages of intramembranous ossification
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We have now discussed intramembranous ossification (Ossficiation centre formation, calcification, trabecular and periosteum formation, periosteum development) We will now discuss endochondral ossification Define both intramembranous and endochondral ossifcation?
Intramembranous ossifcation - the immediate conversion of mesenchymal cells to osteoblasts - occurs primarily in the skull, including cranial suture lines Endochondral ossification - the process involves cartilage - the conversion of mesenchymal cells to chondroblasts producing the cartilaginous template and then the mineralised mode is replaced by bone
What is the main mode of ossification within the centra of the vertebrae?
Endochndral ossification is the main mode of ossification within the centra of the vertebrae
Which type of cartilage is most likely to ossify to become bone? Hyaline Fibrous Elastic
Hyaline cartilage is the most likely to ossify to become bone
Before step 1 of endochondral ossification begins where the hyaline cartilage model is formed, describe how the foetal hyaline cartilage model is formed? (from mesenchymal stem cells to hypertrophic chondrocytes)
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Hypertrophy and division of chondrocytes occurs, hyaline model begins to calcify. As calcification progresses, diffusion of nutrients through calcified matrix dramatically reduces What happens in step 2 of endochondral ossification? (how does the periosteum form)
The chondrocytes apoptose and the cartilage calcifies now serving as a framework for the deposition of bone Due to the osteogenic potential of the perichondral cells, a thin layer of bone surrounds the diaphysis of the bone - known as the periosteum
What is the periostuem bone collar continuous above and below at stage 2 of endochondral ossification?
Above and below the periosteal bone collar, it is continuous with the perichondrium
There are a few steps occurring before stage 3 of endochondral ossification What is stage 3 of endochondral ossification?
This is where the primary ossification centre forms in the centre of the diaphysis with blood vessels and capillaries supplying the diaphysis and epiphysis
What are the few steps happening between the cartilage calcifying, periosteal bone collar forming around the midpoint of the diaphysis and the primary ossification centre forming in the diaphysis?
Mesenchyme near the periosteum will differentiate into osteoprogenitor cells and blood vessels will invade the hyaline cartilage model Osteoprogenitor cells will differentiate into osteoblasts which will secrete osteoid that is quickly mineralised to bone. The oseoblasts surrounded in lacunae are now known as osteocytes
What is the lacunae? What conects adjacent lacunae?
Lacunae is a small gap containing an osteocyte in bone or chondrocyte in cartilage Lacunae are connected by canaliculi

Discuss stages 4 and 5 of endochondral ossification? Which ossification centres are important for longitudinal and diametric expansion until bone growth stops?

Secondary ossification centres form in the epiphyses - these are important for longitudinal and diametric expansion until growth stops Bone replaces cartilage, except at the epiphyseal growth plates and articular surfaces
What is the final step of endochondral ossification?
The epiphyseal growth plate ossifies leaving epiphyseal lines and the cartilage at the articular surfaces remains

We are now going to look at factors initiating ossification Primarily focused on is endochondral ossification as this is the type that takes place during vertebral column development What two transcription factors are vital in the ossification process?
Sox9 - SRY-box containing gene 9 and Runx2 - run-related transcription factor 2

What is Sox9 essential in? What can happen if it is damaged or absent?
Sox9 (SRY-box containing gene 9) was found to be essential in the proliferation, differentiation and maturation of mesenchymal cells to chondrocytes Absence or damage of Sox9 can cause achondroplasia - a type of dwarfism characterized by short proximal limbs
What cells express Runx2 in the chodnrogenesis/ossificaiton process? What can happen if it is absent?
Runx2 is expressed by prehypertrophic and hypertrophic chondrocytes and osteoblasts Consequences of Runx2 removal are fatal as they are needed to regulate development of a mineralized skeletion
What is Sox9 essential in? What can happen if it is damaged or absent? What cells express Runx2 in the chodnrogenesis/ossificaiton process? What can happen if it is absent?
Sox9 is essential in proliferation, differentiation and maturation of mesechymal cells into chondrocytes Damage or absent Sox9 can cause achondroplasia Runx2 is expressed by prehypertrophic and hypertrophic chondrocytes as well as osteoblasts. Absence is fatal as you will not be able to mmineralize bone










