Based on anatomy and physiology

1. PART I
The Skeleton
human mouse
206 bones > 200 bones
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2. Bones can differ
dramatically in their
size and shape
Bones of the inner ear
Human femur
What is bone?
• Specialized form of connective tissue:
mineralized collagen matrix, therefore very
rigid and strong while still retaining some
degree of flexibility
• Other types of connective tissue:
- Cartilage: semi-rigid form, glycoprotein rich
- Ligaments: flexible bands, rich in collagen fibers,
contribute to stability of the joint
- Tendons: strong flexible bands,
rich in collagen fibers, connect muscles
with bone
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3. Diverse functions of bone
• Support
• Protection (Skull)
• Mineral storage (e.g. calcium homeostasis)
• Hematopoiesis (bone marrow - postnatal)
• Locomotion - muscular-skeletal system
• Hearing
Mechanisms of bone formation
• Membranous ossification
intramembranous bones: flat bones of the skull,
clavicle, periosteum
how: direct differentiation of cells within mesenchymal
condensations into bone forming cells (osteoblasts)
• Endochondral ossification
endochondral bones:axial and appendicular skeleton,
some bones in the skull
how: replacement of a cartilagenous template by bone
3

4. Membranous bone formation
osteoid
compact bone
Hartmann, TCB, 2006
Endochondral Ossification
Hartmann, TCB, 2006
4

5. Different types of bone cells
involved in bone homeostasis
• osteoblasts
bone forming cells
• osteocytes
terminal differentiated
osteoblast
• osteoclasts
bone resorbing cells
How do these cells look?
Osteoblast
mononucleated
positive for AP
(AP= alkaline phosphatase)
Osteocyte
mononucleated
trapped within lacunae
serve as mechanosensors
Osteoclast
multinucleated
positive for TRAP
(TRAP =
tatrate-resistent alkaline phosphatase)
5

6. Origin of bone cells
• osteoblasts: mesenchymal
e.g. the first ob differentiate
within the periosteum and
form the bone collar
postnatal: bone marrow
• osteocytes: mesenchymal,
terminal differentiated
osteoblasts
• osteoclasts: hematopoietic lineage;
bone marrow
Postnatal bone cell differentiation
Hartmann, 2006
Encyclopedic Ref.
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7. Bone architecture changes with age
Vertebral body of Vertebral body of
a young person an ederly person
Outside factors affecting bone mass
• Exercise: muscle contractions stimulate osteoblast
function - increased production of bone
peak bone mass reached at the age of 30
• Body weight: obesity can protect from osteoporotic
bone loss
• Diet affects bone: minerals and vitamins
• Menopause in women: decrease in hormone level
can lead to osteoporosis (treatment: HRT)
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8. Terminology of changes in bone
• osteopenia: decreased calcification or density of bone
• osteoporosis: progressive reduction in quantity of bone
• osteopetrosis: excessive formation of dense trabecular bone
• osteosclerosis: abnormal hardening or eburnation of bone
• osteohypertrophy: overgrowth of bone
• osteosarcoma: tumor of the bone
• osteochondrodysplasia: extreme bending of long bones
• osteochondroma (exostosis): benign cartilaginous
neoplasma
• osteoblastoma: benign tumor of osteoblasts
Changes in bones
(associated with bone cells)
• osteopenia: osteoporosis: osteopetrosis:
(mild) (severe)
wt wt
wt
Cause: osteoblasts are Cause: osteoblasts are not Cause: osteoclast deficiency
not active enough active enough or no active OC
and/or osteoclasts
are too active
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9. Congenital bone disorders
• Achondroplasia • Osteogenesis • Marfan syndrome • Osteochondro-
(Fgfr3 act. mutations) Imperfecta (skeletal overgrowth, matosis
Dwarfism (brittle bones; lengthening of long bones
prenatal form of osteoporosis) mutations in Fibrillin gene) (multiple exostoses)
Skeleton formation begins during
embryogenesis
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10. Cartilage
Types of cartilage (avascular tissue):
• hyaline cartilage: • fibro-cartilage: • elastic cartilage:
(e.g. trachea, nose, (e.g. intervertebral disc) (e.g. ext. ear, epiglottis)
articular ends of bones,
embryonic skeleton)
The skeleton is derived from three
different compartments
Craniofacial skeleton
Appendicular skeleton
Axial skeleton
Neural crest
Somite
Neural tube Lateral plate mesoderm
Intermediate
mesoderm
Notochord
Dorsal
Medial Lateral
Ventral
10

11. Different origins of membranous
bone in the mouse skull
pariental
frontal
nasal IP
Cranial neural crest
mesoderm
According to: Jiang et al. 2002
Axial skeleton
Origin: somites
young somite intermediate somite differentiated somite
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12. Limb skeleton
Origin: lateral plate Condensing cartilage
mesoderm anlagen in the limb
Sclerotome
Dermomyotome
Starting to be distinguishable around
day 4 in chick and day 11 in mouse
Formation of the cartilagenous
template in the limb
naive signals initiating condensation & lineage cartilage template
mesenchyme condensation lineage specification commitment &
differentiation
growth
pre-chondroblast chondroblast chondrocyte
perichodrial
cell
12

13. Growth of the cartilage template
Two mechanisms:
• Appositional growth: new cartilage is added on the
surface by recruiting chondroblasts from the inner
layer of the perichondrium
• Interstitial growth: new cartilage is formed within the
cartilagenous template by chondrocytes dividing and
producing additional matrix
After initiation and formation
of the cartilage template
chondrocytes within the
template undergo a controlled
maturation program
13

14. Mitotically active postmitotic
flattened Pre-
proliferating hypertrophic hypertrophic
Round proliferating chondrocytes chondrocytes chondrocytes chondrocytes
PC: proliferative chondrocytes
PHC: pre-hypertrophic chondrocytes
HTC: hypertrophic chondrocytes
MHTC: mature hypertrophic
Bone collar chondrocytes
periosteum
transforming region
PC
PHC
HTC MHTC
Cilia are found on chondrocytes (uni-ciliated)
Possible functions:
•“Anker” for oriented secretion of extracellular matrix
• Mechanosensor
• Signaling center, as some receptors have been shown to
localize to the cilium (for example smoothened the receptor
required for transmitting the hedgehog signal)
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15. Continuous growth of skeletal element
is ensured by the growth plate
Blood
vessel
invasion
The Growth Plate
round,
proliferating
chondrocytes
flattened, stacked
proliferating
chondrocytes
prehypertrophic
hypertrophic β1 integrin -/-
Wt
transformation zone
- defective cytokinesis
osteogenic front - decreased proliferation
- decreased cell adhesion
- increased apoptosis
newly formed
bone
15

16. Integrin-(outside-in) signaling
Replacement of cartilage by bone
Blood
vessel
invasion
16

17. Histology of a juvenile long bone
articular cartilage
secondary
ossification
epiphysis center
growth plate
epiphysis
primary
spongiosa
trabecular bone
endosteum
secondary
spongiosa
periosteum
cortical bone
bone marrow
periosteum
endosteum
cortical bone
Replacement of HTC by Bone
Changes happening in the transformation zone:
• Mineralization of cartilage matrix
• Apoptosis of hypertropic chondrocytes
• Phagocytosis of old cartilage matrix by
osteo/chrondroclasts
• Metalloproteases
• Invasion of vascular system
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18. Segmentation of the skeleton
Skeletal elements are separated from each other by so-called joints
Different types of joints (arthrosis):
• synovial joints (diarthrosis): has a joint cavity that is enclosed by a
fibrous capsule, which is lined by the synovial membrane.
e.g. joints separating skeletal elements in limbs - free movement!
• Non-synovial joints (synarthroses):
• fibrous joints: skeletal elements are directly linked by fibrous tissue
e.g. sutures between the skull bone - don’t allow movement!
• cartilaginous joints: two skeletal elements are linked by cartilage
e.g. joints between vertebral bodies - limited movement!
Patterning of appendicular skeleton
during embryonic development
Alcian blue stained chicken hindlimbs
Sequential process, proceeds from proximal to distal
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19. The skeletal elements of the limb form by a
process of branching and segmentation
prechondrogenic
(Sox9+, Col2a1+)
chondrogenic
(Sox9++, Col2a1++)
joint
(Sox9- , Col2a1- )
Development of the synovial joint
by
apoptosis
Modified after P. Francis-West
19

20. Adult mouse knee joint
Secondary ossification center meniscus
Joint
cavity
bursa
Articular
cartilage
Ligament
cruciatum
synovium
subchondral
bone
Growth plate Ligaments
of tibia of joint
capsule
Pathological changes of the joints
• arthrosis: general term for degenerative affection of a joint
• rheumatoid arthritis: systemic disease affecting connective
tissue of joint, accompanied by inflammation and erosion
of cartilage and bone due to synovial overgrowth
• osteoarthritis: destruction of joints due to erosions of articular
cartilage, accompanied by inflammation, eburnation of
subchondral bone
• gout: inflammation of the joint
• synovitis: inflammation of synovial membrane
• bursitis: inflammation of bursa (german: Schleimbeutel)
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21. Histological changes in RA and OA
Rheumatoid arthritis (RA) Osteoarthritis (OA)
wt
Part II
1. Steps involved in the initiation of
cartilage formation
2. Signals regulating maturation of chondrocytes
within a cartilage template
3. Factors controlling osteobastogenesis
4. Factors involved in osteoclastogenesis
5. Bone homeostasis
6. Bone repair - fractures
7. Joint formation
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