Cartilage and Bone

Cartilage: main cell type

Chondrocytes (sit in lacunae)

Cartilage: % of tissue that is ECM

95% extracellular matrix

Cartilage blood supply / how nutrients arrive

Avascular; nutrients reach chondrocytes by diffusion through the matrix

3 types of cartilage

Hyaline, elastic, fibrocartilage

Hyaline cartilage: locations

Fetal skeleton, articular surfaces (knee), respiratory tract (trachea/bronchi), costal cartilage

Hyaline cartilage: function

Resists compression

Hyaline cartilage: matrix

Collagen II + aggrecan

Hyaline cartilage: perichondrium?

Yes, EXCEPT articular cartilage (no perichondrium)

Elastic cartilage: locations

External ear, larynx/epiglottis

Elastic cartilage: function

Flexible support

Elastic cartilage: matrix

Collagen II + aggrecan + elastic fibers

Elastic cartilage: perichondrium?

Yes

Elastic cartilage: special stain for elastic fibers

Orcein (fibers appear dark brown/branching)

Fibrocartilage: locations

Intervertebral disc (IVD), meniscus, TMJ

Fibrocartilage: function

Resists compression AND shear

Fibrocartilage: matrix

Collagen I + collagen II, versican, aggrecan

Fibrocartilage: perichondrium?

No

Fibrocartilage: histological appearance

Mix of dense CT (collagen I, fibroblast-like rows) + hyaline cartilage (collagen II, chondrocytes)

Which cartilage types lack a perichondrium?

Fibrocartilage and articular hyaline cartilage

Which collagen is common to ALL cartilage types?

Collagen II

Which cartilage type contains collagen I?

Fibrocartilage

Perichondrium: composition / staining

Dense CT rich in collagen I; stains red/pink

Appositional growth (cartilage)

New cartilage added at the surface from perichondrium cells → growth in girth (width)

Interstitial growth (cartilage)

Chondrocytes divide within the matrix → growth in length and girth

Isogenous group

Cluster of chondrocytes that are descendants of a single cell (from interstitial growth)

Territorial matrix

Matrix immediately around an isogenous group; strongly basophilic (rich in proteoglycan)

Interterritorial matrix

Matrix between isogenous groups; less basophilic

Lacuna (cartilage)

Space in the matrix occupied by a chondrocyte

White space around chondrocytes in lacunae

Shrinkage artifact from tissue processing

Chondrocyte organelles (EM)

Rich in Golgi, rough ER, and mitochondria (active matrix synthesis)

Proteoglycan charge & staining

Negatively charged → basophilic (blue/purple)

Collagen charge & staining

Positively charged → eosinophilic (pink)

Aggrecan structure

Core protein with many GAG chains attached ('test-tube brush')

Aggrecan function

GAGs hold water → resists compression

Nucleus pulposus

Jelly-like center of the intervertebral disc

Anulus fibrosus

Ring of fibrocartilage surrounding the nucleus pulposus

Nucleus pulposus vs anulus fibrosus

Nucleus pulposus = jelly center (NOT fibrocartilage); anulus fibrosus = outer ring of fibrocartilage

Disc herniation

Anulus fibrosus ruptures → nucleus pulposus protrudes and presses on a spinal nerve

Articular cartilage: type & key feature

Hyaline cartilage without perichondrium (also no nerves or blood vessels)

Arthritis (in this lecture)

Degradation of articular cartilage

Cartilage vs bone: matrix

Cartilage = unmineralized (collagen II, proteoglycan); Bone = mineralized (collagen I, proteoglycan, hydroxyapatite)

Hydroxyapatite formula

Ca10(PO4)6(OH)2

Cartilage vs bone: function

Cartilage = cushion & support; Bone = support + Ca/P storage

Cartilage vs bone: blood supply

Cartilage = avascular; Bone = vascular

Cartilage vs bone: growth

Cartilage = appositional + interstitial; Bone = appositional ONLY

Main collagen type in bone

Collagen I

Bone as an organ: components

Bone tissue + hemopoietic tissue (marrow) + fat + blood vessels + nerves + articular cartilage

Epiphysis

End of a long bone

Diaphysis

Shaft of a long bone

Metaphysis

Region between epiphysis and diaphysis

Periosteum

Outer CT lining of bone (contains osteoprogenitors)

Endosteum

Inner lining of bone facing the marrow cavity

2 types of bone tissue

Spongy (cancellous/trabecular) and compact (cortical)

Spongy bone: features

Lamellated matrix, NO osteons/Haversian lamellae, nutrients by diffusion from marrow, marrow fat between trabeculae

Does spongy bone have osteons?

No

Osteon (Haversian system)

Structural unit of compact bone: concentric lamellae around a Haversian canal

Haversian canal

Central canal of an osteon; runs longitudinally (vertical); contains blood vessels/nerves

Volkmann's canal

Transverse (perforating) canal; connects Haversian canals of osteons, and the inside and outside of the bone

Where are transverse (Volkmann's) canals found?

Compact bone

Concentric lamellae

Rings of matrix within an osteon around the Haversian canal

Interstitial lamellae

Remnants of old osteons between intact osteons

Outer circumferential lamellae

Lamellae just under the periosteum, encircling the whole bone

Inner circumferential lamellae

Lamellae lining the marrow cavity, just under the endosteum

Lacuna (bone)

Space in the matrix occupied by an osteocyte

Canaliculi

Tiny channels between lacunae containing osteocyte processes

How do osteocytes communicate?

Through cytoplasmic processes in canaliculi, connected by gap junctions

Osteoid

Unmineralized bone matrix laid down first by osteoblasts, then mineralized

Ground section of bone: dark spots with radiating lines

Lacunae (spaces osteocytes occupied) with canaliculi

4 bone cell types

Osteoprogenitor, osteoblast, osteocyte, osteoclast

Osteoprogenitor cells: location

Periosteum and endosteum

Osteoblast: function

Secretes osteoid: collagen I, osteocalcin, osteopontin, alkaline phosphatase (ALP), bone sialoprotein (BSP)

Osteoblast: relation to matrix

Lines the bone surface; NOT surrounded by matrix

Osteocyte

Mature bone cell surrounded by matrix in a lacuna; star-shaped (processes); maintains matrix

Osteoclast: appearance

Large, multinucleated cell on the bone surface

Osteoclast: origin

Monocyte lineage

Howship's lacuna

Resorption pit on the bone surface created by an osteoclast

Osteoclast resorption: 3 steps

1) Acidify: pump out H+ to dissolve mineral

2) Lysosomal enzymes degrade organic matrix

3) Clean up by endocytosis

What does an osteoclast pump out to dissolve bone mineral?

H+ (protons) — acidification

Why do osteoclasts have many mitochondria?

Energy (ATP) for proton pumping/resorption

Bone remodeling

Ongoing resorption (osteoclasts) + formation (osteoblasts); the two cells communicate

Effect of mechanical load on remodeling

Faster remodeling / more bone formation

Osteoporosis

Low bone mass → fragile bone; caused e.g. by high-dose corticosteroids; partly genetic; helped by weight-bearing exercise

Osteopetrosis

Increased bone mass due to defective osteoclasts → still fragile; unerupted teeth

Dental sign of osteopetrosis

Unerupted teeth

Paget disease

Overactive osteoclasts → increased remodeling → softer, weaker bone

Which disease: defective osteoclasts?

Osteopetrosis

Which disease: overactive osteoclasts, increased remodeling?

Paget disease

Identify the cartilage

Hyaline cartilage — not bone, no obvious fibers, not elastic

Which statements about cartilage are INCORRECT?

A, D, F

Which pairings are INCORRECT?

C, D, E

Which of the following incorrectly describe this tissue?

B, E

• elastic cartilage

Inappropriately matched pair?

B

Antibodies find aggrecan, collagen II, hyaluronan. What is the sample?

C

Transverse canals are found at A, B, or C?

B

What are the arrows pointing at (ground section)?

A

• not live osteocytes (dead in ground section)

Which INCORRECTLY describes this tissue?

c) Contains osteons

b) Osteocyte

e) Communicates with osteoblasts to maintain bone homeostasis

What is known about osteoporosis?

B, C

The 3 types of cartilage with locations?

Hyaline: fetal, articular, respiratory, costal.

Elastic: ear, larynx (epiglottis).

Fibrocartilage: IVD, meniscus, TMJ