Bone development and growth Flashcards
What are the tissues and cell types in the development of the mammalian skeleton?
Two tissues = cartilage + bone
Three cell types = chondrocytes, osteoblasts and osteoclasts
What is the osteochondral progenitor?
A common mesenchymal progenitor cell
What is the embryonic origin of bone?
Mesenchymal cells in different parts of the embryo come from different cell lineages
1 = neural crest (neuroectoderm) - craniofacial
2 = paraxial mesoderm (somites) - axial skeleton
3 = lateral plate mesoderm - limb skeleton
What is intramembranous ossification?
Flat bone formation - mesenchymal condensation - differentiate into osteoblasts
What is endochondral ossification?
Long bone formation - mesenchymal condensation - differentiate into chondrocytes producing cartilage. Bone is formed on the cartilage template
Why is angiogenesis important?
Brings the vascular endothelial growth factor where it is needed for vascularisation
What is mesenchymal condensation?
Aggregation of mesenchymal cells at future skeletal locations
- positional identity = locations of intital skeletal formation determine which of the three mesenchymal cell lineages contribute to the future skeleton
What is axial patterning and what controls it?
Somites (segmented mesenchymal structures located in either side of the neural tube) bud off from the anterior tip of the presomitic mesoderm
- Notch, Wnt, FGF pathways control patterning of the axial skeleton
What is a limb bud and how does it form?
Small buds of mesodermal cells covered by an ectodermal cap
- requires three signalling molecules and three signalling centres that interact with each other
What is proximal - distal patterning (P-D)?
Shoulder → digit tip
3 limb segments
- stylopod, zeugopod, autopod (proximal - distal)
What happens if you remove the AER early?
Only the stylopod region forms
What happens if the AER is removed late?
The autopod region is not formed but the stylopod and zeugopod regions are formed
What is the AER?
Apical Ectodermal Ridge
- thickened epithelium at the most distal end of the limb bud
- signalling centre that direct P-D patterning
- FGF-4, FGF-8, maintain FGF10, important for the proliferation of the mesoderm
What is the zone of polarising activity (A/P patterning)?
Thumb to little finger
- area of mesenchyme that contains signals which instruct the developing limb bud to form along the A-P axis
- Shh expressed in ZPA and mediates ZPA activity
- other signals include: bHLH transcription factors (twist 1)
What does a mutation in the twist 1 gene cause?
Premature fusion of the caluarial bones and limb abnormalities
What is dorsal/ventral patterning?
Back of the hand - palm
- non AER limb ectoderm that covers the limb bud - this tissue controls D-V polarity of the ectoderm and underlying mesoderm
- Signals such as Wnt and BMP activate LIM homeobox transcription factors that determine dorsal identity
How is embryonic cartilage and bone formed?
Endochondral progenitor in the mesenchyme condensation expresses both Runx2 and Sox9
Mesenchyme forms either cartilage (endochondral, controlled by Sox9 with low levels of 𝛃-catenin) or bone (membranous, controlled by Runx2 with high levels of 𝛃-catenin)
- once cartilage is formed Ihh signalling mediated by Runx2 forms bone
Why is embryonic bone and cartilage formed dependant on canonical Wnt signalling?
- induces expression of genes required for osteoblastic differentiation
- inhibits the expression of genes required for chondrocyte differentiation
How does intramembranous ossification occur?
Occurs within mesenchymal condensations that are rich in blood vessels
- flat bones
- Wnt signalling in condensations is high - promoting osteoblastic differentiation rather than chondrocyte differentiation
- Mesenchymal cells = osteogenic cells = osteoblasts
How does bone within a limb form?
Endochondral ossification
What is endochondral ossification?
- Mesenchymal cells proliferate, differentiate in prechondrocytes → chondrocytes forming a cartilage matrix
- This causes the cells at the centre to become hypertrophic (stop proliferating)
- Perichondrial cells at the periphery differentiate into osteoblasts forming a bone collar
- Hydrotrophic chondrocytes at the centre direct formation of bone by attracting blood vessels bringing in osteoblast precursors and then the hydrotrophic chondrocytes will die by apoptosis
- Chondrocytes still proliferate and lengthen the bone
- Once chondrocytes become hypertrophic and attract blood vessels bringing more osteoblasts to form bone creating a secondary ossification centre
- In the middle, bone replaces the cartilage matrix that disintergrates because chondrocytes die by apoptosis
- New bone and woven bone will be replaced by lamellar bone
What is secondary ossification in the epiphysis?
- Occurs at time of birth in humans
- Epiphysis initially lacks perichondrium (no osteoblasts)
- Blood vessels invade the centre of the epiphysis
- Transport osteoclasts and osteoblasts
- Osteoblasts initiate bone growth and formation of secondary ossification centres
Which regions do not have their cartilage replaced by bone?
- End of epiphysis - joint cartilage
2. Region between diaphysis and epiphysis - epiphyseal growth plate (EGP)
What is the importance of VEGF in endochodral ossification?
Immunohistochemical analysis of mammal embryo for presence of VEGFR-2
- resting and proliferating chondrocytes have strong anti-angiogenic effects - not labelled with the antibody against VEGF receptors with hypertrophic chondrocytes which strongly express VEGF receptors and elicit revascularisation
What are the important signalling pathways in chondrocyte differentiation in developing cartilage?
BMP - (Bone Morphogen Protein)
Ihh - (Indian hedgehog - synthesised by chondrocytes leaving the proliferative pool)
PTHrp - (PTH - related peptide - keeps proliferative chondrocytes in proliferative pool and delays production of Ihh, no PTHrp chondrocytes become hypertrophic)
Wnt
How does bone grow in diameter?
Periosteal apposition exceeds endocortical reabsorption (thicker cortex)
- bone diameter increases during growth
What are the requirements for reabsorption during bone growth?
Cortex in diaphysis must be reabsorbed inside and reformed outside
- increase space for bone marrow whilst limiting weight of bone
- in response to muscle activity and mechanical loading
What are factors affecting bone growth?
- Vitamin C: collagen synthesis
- Vitamin D: absorption of Ca2+ in the intestine
- Hormones: (thyroid, estrogen)
- Growth hormone: secreted by pituitary - stimulates division of cartilage cells
- Insufficient GH = dwarfism
- Excessive GH = gigantism or acromegaly in adults
- IGF-1: expansion of proliferating chondrocytes
What effect do thyroid hormones have on bone growth (length)?
Growth arrest in hyperthyroidism (replacement of cartilage by bone)
What effects does oestrogen have on bone growth?
Expression is induced at the end of puberty which causes epiphyseal growth plate cells to hypertrophy and die
- growth plate closure and a lack of oestrogen function
What effect do thyroid hormones have on bone growth (width)?
+ =
- mechanical forces
- Androgen
- PTH
- = estrogens
