Cartilage and Bone

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Cartilage

Supporting tissue

Stronger than other tissues, and more light and flexible than bone

The ECM is produced by the chondroblasts/cytes:

Type 1 & 2 Collagen fibers

Elastin fibers

Proteoglycan & glycosaminoglycans (hyaluronan, chondroitin sulfates & keratan sulfate)

Surrounded by the perichondrium (dense irregular connective tissue) – except in the case of articular cartilage

In adults, cartilage lacks nerves, blood vessels & lymphatic vessels, so cartilage gains its nutrients via diffusion

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Cell types:

Chondroblasts (immature cells):

Location: Derived from mesenchymal cells in the perichondrium

Function: Secrete the ECM of cartilage & become chondrocytes when surrounded by matrix Cell Types 1 2

Chondrocytes: (mature chondroblasts)

Location: In lacunae surrounded by ECM, organised into ‘isogenous groups’

Function: Secrete & maintain the ECM

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Cartilage Growth:

Interstitial Growth:

Where existing chondrocytes divide & make more matrix

Occurs mainly in immature cartilage

Formation of isogenous groups

-If cell division doesn’t keep pace with matrix production → spacing of groups increases. If cell division keeps up with matrix production → distribution of groups more uniform

Chndrocytes devide by mitosis

Daughter cells secrete ECM

Newly divided cells appear as groups of 2-4

Appositional Growth:

Mesenchymal cells in perichondrium differentiate into chondroblasts and eventually chondrocytes

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Perichondrium:

Usually surrounds cartilage except in joints

Dense irregular connective tissue

Consist of chondrogenic cells

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Types of cartilage:

All contain chondrocytes

Differences occur within Extra Cellular Matrix (ECM)

Hyaline cartilage:

Most common

Contains many chondrocytes

ECM contains ~ 40% Type II collagen fibres and 60% glycosaminoglycans

High water contents that makes it good at resisting force

Found in: Nose, Larynx, Tracheal rings, Articular surfaces, Sternal end of ribs

Fibrocartilage:

Least common

Dense type I collagen

Oriented in direction of tensile stress

Found in: Pubic symphasis, intervertebral disc, menisci

Elastic cartilage:

ECM contains some type II collagen, many eleastin fibres

Found in: External ear, external auditory canal, epiglottis

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Cartilage repair:

Avascular

If chondrocytes live, cell matrix will be replaced

Some limited regeneration due to differentiation of cells from perichondrium

Injury to articular cartilage is bad as there is no perichondium present

Usually repair by fibrosis and collagen formation

Calcification may also occur

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Bone:

Bones are organs

Mature bone divided by structural arrangement and work together to form bone:

Compact Bone: Dense and organised into layers, special organisation in long bones to bear the weight

Spongy bone or cancellous bone:

Less organised arrangement

Specialised connective tissue

ECM is mineralised allowing rigidity and strength, compared to cartilage

Function: Support and protect, store calcium to help with homeostasis

2 forms: Woven (immature) less organisation of collagen fibres. Lamellar (Mature) well organised collagen fibres

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Periosteum:

Tough connective tissue layer that surrounds bones

Two layers: Inner osteogenic layer provides cells necessary to form bone, Outer fibrous layer consists of irregularly arranged collagen fibres & blood vessels

Attached to the bone by collagen fibers “Sharpey’s fibers” that penetrate the outer lamellae of compact bone

Woven bone:

Has random organisation of collagen fibers in immature bone

First to be produced and then remodelled

Lamellar bone:

Regular parallel alignment of collagen fibers into sheets (lamellae) and is mechanically strong

Divided into spongy and compact bone

Compact Bone:

Organised fibres

Different layers go in different directions (concentric or parallel in orientation)

Form canals called Volkmann's canal: transverse or horizontal canals. Route by which blood vessels supply osteons.

Organised nto layers/lamellae

In long bone, osteons/haversian systems (cylindrical/concentrical structures) are formed and enable bone to bear weight as well as allow blood vessels and nerves to run through. Composed of concentric lamellae surrounding longitudinally orientated vascular channels. Osteocytes are located within lacunae between each lamella.

Collagen type I fibres layed in precise array

Embedded within a matrix hardened by calcification

Bone cells eventually trapped within interstices of matrix

Fibres and matrix create tensile strength and rigidity

Layers: Outer and inner circumfential plates lamellae, interstitial lamellae, cylindrical lamellae (concentric lamellae)

Spongy Bone (Cancellous):

Spongy and less organised

Empty space is bone marrow

Trabeculae is branching

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Osteoprogenitor cells (stem cells)

Location: Periosteum and endosteum

Function: Mature into osteoblasts

Osteoblasts (bone formng cells)

Location: Beneath periosteum at sites of active growth

Function: Synthesis of the ECM

Osteocytes (mature osteoblast)

Location: Trapped in Lacunae

Function: Maintain ECM

Osteoclasts (bone resorbing cells or "bone eater")

Location: Endosteum

Function: Remodel bones to increase calcium in blood

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ECM:

Osteoid:

Type I collagen

Glycosaminoglycans (GAG's)

Mineralisation

Function: Provides ECM with rigidity and strength

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Ossification or Osteogenesis:

Bone development

divided into 2 processes:

Intramembranous ossification: From mesenchymal stem cells to osteocytes to form bone tissues. Flat bones in facial, clavicles & mandible bones.

Endochondral ossification Start from cartilage which is replaced by bone cells associated with bone growth. In long bones.

Intramembranous Ossification:

Form in areas or mesenchyme

Primary centre of ossification that appears in a "slab" of connective tissue

Mesenchymal cells (osteoprogenitor cells) → osteoblasts → woven bone formation → osteoblasts surrounded by matrix → osteocytes in lacunae → bone mineralisation & remodeled into lamellar bone

Endochondral Ossification:

Cartilage is intermediary stage between mesenchyme and bone

Transformation of the cartilage that gradually enlarges and is replaced by bone

Mesenchymal cells → chondroblasts → cartilage model formation → proliferation & maturation to chondrocytes → form epiphyseal growth plate → at junction with bone, chondrocytes die by apoptosis → osteogenic precursor cells infiltrate (start at the primary ossification center) & produce new bone matrix

Five layers of epiphyseal growth plate:

Resting Zone: Normal chondrocytes in lacuna of cartilage

Proliferative zone: Chondroxytes proliferate and arrange into columns

Maturation zone (hypertrophic zone): Cells enlarge and calcify, chondrocytes finish mineralisation of cartilage, prepare to undergo apotosis

Resorption zone (calcification zone): Insoluble calcium salts become deposited in the cartilage matrix

Ossification zone

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Blood supply:

Bone is living, dynamic tissue with high metabolic demands

Arteries come via periosteum & perforate through foramina

Ramify through haversian & lateral canals

Perforate to marrow cavity: Spongy bone nourished by diffusion. Veins follow return route.