Bone tissue

Cartilage - characteristics

Perichondrium membrane, chondrocytes in lacunae, elastic fibers, matrix

Mesenchymal cells - Function

differentiate into chondroblasts, in bone marrow / mesenchyme

Chondroblasts - function

synthesize ground substance & matrix, in perichondrium

Chondrocytes - function

mature cells, embedded in matrix (self secreted)

Cartilage

semi-rigid dense connective tissue

Appositional

increasing in GIRTH or WIDTH; chondroblasts deposit collagen/matrix on surface of pre-existing cartilage

Interstitial

increasing LENGTH; specific for endochondral bone formation; chondrocytes divide and secrete matrix from within their lacunae

Hyaline Cartilage

– Most abundant cartilage – Provides support through flexibility – Articular cartilages and costal cartilage, larynx, trachea, and nose

Elastic Cartilage

– Able to tolerate repeated bending – Ear and epiglottis

Fibrocartilage

An intermediate between hyaline and elastic cartilage, resists strong tension and compression – Intervertebral discs (knee joint) and pubic symphysis

Types of cartilage, do they repair, can they be damaged

Hyaline - limited repair, easily damaged

Rest - no

damaged cartilage replaced by

fibrocartilage scar tissue

─ cartilage transplantation, no issues of rejection because:

─ antigenic power of cartilage is low ─ immune system cells poorly diffuse cartilage

Arthritis

Degeneration of cartilage in joints (articular cartilage)

Dwarfism

Condition caused by the reduced proliferation of chondrocytes

Herniated disk

Rupture of the disk cartilage ring, which pushes into the spine

Cartilage tumors

Condition where cartilage cells give rise to benign tumors (chondroma), while malignant tumors typically occur in bone rather than cartilage

Scurvy

Lack of vitamin C required to process collagen, resulting in defective cartilage and bone

Cortical Bone (Compact Bone) - Physical description

Dense, compact outer shell that provides strength and protection

Cancellous Bone (Spongy/Trabecular Bone) - Physical description

Porous, lattice-like network of trabeculae; spaces between trabeculae contain bone marrow

Cortical Bone (Compact Bone) - Location

Forms the outer layer of all bones; particularly thick in the shafts (diaphyses) of long bones

Cancellous Bone (Spongy/Trabecular Bone) - Location

Found mainly within vertebrae, flat bones (e.g., pelvis), and at the ends (epiphyses) of long bones

Cortical Bone (Compact Bone) - Skeletal mass percentage

$$\sim 80\%$$

Cancellous Bone (Spongy/Trabecular Bone) - Skeletal mass percentage

$$\sim 20\%$$

Cortical Bone (Compact Bone) - Main function

Provides mechanical strength, protection, and resistance to bending

Cancellous Bone (Spongy/Trabecular Bone) - Main function

Provides strength while reducing bone weight; helps distribute forces and contains bone marrow

Bone matrix - Organic component percentage

$20\%$ (osteoid produced by osteoblasts)

Organic osteoid - Main fiber component

Type I collagen fibers (constitutes $90\%$ of organic osteoid)

Organic osteoid - Non-collagenous components

Glycosaminoglycans and ground substance proteoglycans

Bone matrix - Inorganic component percentage and main minerals

$70\%$ inorganic salts, primarily calcium and phosphate in the form of hydroxyapatite crystals

Bone matrix - Water percentage

$$10\%$$

Osteoprogenitor cells

Bone stem cells that generate osteoblasts and osteocytes

Osteoblasts

Immature, bone-forming cells

Osteocytes

Inactive osteoblasts; most abundant cell type found in bone

Osteoclasts

Phagocytic cells (secrete organic acids and lysosomal proteolytic enzymes) that erode bone; perform bone resorption and remodeling

Formation of bone (4 steps) (see Osteoblasts)

─ 1. osteoblasts synthesise & secrete collagen & organic matrix (osteoid) ─ 2. osteoid then becomes calcified (i.e. calcium deposition) ─ 3. osteoblasts secrete vesicles of alkaline phosphatase (AP) ─ 4. AP causes matrix mineralisation (gives rigidity & strength)

Bone Remodelling

1. Activation

2. Resorption

3. Reversal

4. Formation and mineralisation

5. Quiescence

long

bones like tibia & metacarpals; growth by endochondral ossification

Flat

thin and plate-like; bones of skull & sternum; growth by intramembranous ossification

Woven Bone (Immature)

Bone produced rapidly by osteoblasts with randomly arranged collagen fibers; found in fetal bone development, healing fractures, and Paget's disease

Lamellar Bone (Adult)

Stronger mature bone that replaces woven bone, composed of regular parallel collagen sheets

Intramembranous ossification

direct chondrocytes lay down bone - fractures treated internal fixation with screws/pins

Endochondral ossification

- involves cartilage as a precursor - most common in fracture healing - fractures of long bones treated by plaster of Paris