This lecture explains the general concepts of connective tissue, cartilage and bone, along with their composition and types.
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Images of Compact bone
Images of Spongy bone
Outline of Lecture
CONNECTIVE TISSUE CARTILAGE AND BONE
CARTILAGE
Is a specialised type of connective tissue.
Consists of cells and extracellular components.
Does not contain vessels or nerves.
Is surrounded by a layer of dense connective tissue, the perichondrium
Cartilage
Embryo
More prevalent than in adult
Skeleton initially mostly cartilage
Bone replaces cartilage in fetal and childhood periods
Location of cartilage in adults
External ear
Nose
“Articular” – covering the ends of most bones and movable joints
“Costal” – connecting ribs to sternum
Larynx – voice box
Epiglottis – flap keeping food out of lungs
Cartilaginous rings holding open the air tubes of the respiratory system (trachea and bronchi)
Intervertebral discs
Pubic symphysis
Articular discs such as meniscus in knee joint
Types of cartilage
Hyaline cartilage
Elastic cartilage
Fibrous cartilage
Functions of Cartilage Tissue
Firm consistency of the extracellular matrix allows the tissue to bear mechanical stresses without permanent distortion
Supports soft tissues.
Shock-absorbing because it is resilient.
Smooth surface allows sliding .
Essential for growth, development of bone.
Characteristics
Chondrocytes
Located in lacunae
Extensive extra-cellular matrix
Fibers, Collagen, &elastic
Ground substance
Fibers bind together and give firm, flexible properties to tissue
Chondrocytes are mature cells found in cartilage.
These cells make up the cellular matrix of cartilage, performing a number of functions within the cartilage, including facilitating the exchange of fluids through the gelatinous layers which make up cartilage.
The progenitors of chondrocytes arise in the bone marrow, in a form of stem cell..
When they differentiate into cartilage cells, they start out as chondroblasts, actively producing secretions of chondrin, the primary substance in cartilage, to build and repair cartilage.
Once a chondroblast becomes totally surrounded, it is a mature chondrocyte.
Chondrocytes can be found in small gaps within the cartilage known as lacunae. Chondrocytes are not capable of cell division.
Chondrocytes
Produce and maintain extra cellular matrix.
Either single or in isogenous groups .
Fat droplets, glycogen granules are found in cytoplasm.
Active ones are more basophilic.
Cartilage matrix
Contain high concentration of bound sulfate So stains with basic dyes.
Rich in water(70-80%)
Three classes of molecules are present; Collagen, Proteoglycans and Multi adhesive glycoproteins.
Aggrecans
Large proteoglycan molecule
Protein core Glycosaminoglycans (chondroitin4-sulfate and chondroitin 6-sulfate) attach to protein cores
100 to 200 aggrecan molecules linked non-covalently to hyaluronic acid
Perichondrium
Dense CT that covers cartilage
Contains blood, nerve supply, lymphatics.
Source of new cartilage cells
Divided into two layers
Inner cellular
Outer fibrous
Hyaline Cartilage
Extra cellular matrix
The matrix near the isogenous groups of chondrocytes is termed territorial matrix or capsule. In H&E stained sections the territorial matrix is more basophilic, i.e. it stains darker.
The remainder of the matrix is called the interterritorial matrix.
Articular Cartilage
Hyaline cartilage of articular surfaces do not posses a perichondrium
ELASTIC CARTILAGE
Similar to hyaline cartilage but has elastic fibers running in all directions in addition to collagen.(resorcin stain)
Found in auricle of ear, walls of external auditory canals, eustachian tubes, epiglottis, larynx
Maintains shape, deforms but returns to shape; flexibility of organ; strengths and supports structures.
Fibrocartilage
Chondrocytes may lie singly or in pairs, but most often they form short rows between dense bundles of collagen fibres. In contrast to other cartilage types
It is difficult to define the perichondrium because of the fibrous appearance of the cartilage and the gradual transition to surrounding tissue types.
Types of cartilage: 3
Hyaline cartilage: flexible and resilient
Chondrocytes appear spherical
Lacuna – cavity in matrix holding chondrocyte
Collagen the only fiber
Elastic cartilage: highly bendable
Matrix with elastic as well as collagen fibers
Epiglottis, larynx and outer ear
Fibrocartilage: resists compression and tension
Rows of thick collagen fibers alternating with rows of chondrocytes (in matrix)
Knee menisci and annunulus fibrosis of intervertebral discs
Chondrogenesis
Growth of Cartilage
Growth is attributable to two processes:
Interstitial growth
Mitotic division of preexisting chrondrocytes
Synthesis of matrix
Expands cartilage matrix from within
Occurs in epiphyseal plates, articular cartilage
Appositional growth
Differentiation of perichondrial cells ® chondroblasts
Synthesis of matrix
Increase in girth
Epiphyseal Plate (Interstitial Growth)
Appositional Growth
Repair of hyaline cartilage
Can tolerate considerable amount of stress
Limited ability to repair Because of
1. Immobility of chodrocytes
2. Less ability of chodrocytes to proliferate
3. Avascularity
4. When hyaline cartilage calcifies it is replaced by bone
Anatomy of a Long Bone
DEFINITION
Bone is a specialized connective tissue characterized by mineralized extracellular matrix.
Bones are the organs of skeletal system; bone tissue is the structural component of bones.
CHARACTERISTICS
Bone Matrix
35% collagen – organic
Some chondroitin sulfate, hyaluronic acid and polysaccharides
65% calcium phosphate minerals – hydroxyapetite -inorganic
Combination provides strength and rigidity
Laid down by osteoblasts
Osteon
Long cylinder parallel to long axis of diaphysis
Consists of:
Haversian canal with nerves, blood vessels; lamellae with osteocytes
Haversian canals communicate with marrow cavity, periosteum, other canals through Volkmann’s canals
BONE MATRIX
90% Collagen fibers
Type 1, Type 3, Type 5, Type11, Type13
10% non collagenous protein
1. Proteoglycans
2. Multiadhesive glycopoteins
3. Bone specific vitamin k dependent proteins
4. Growth factors and cytokines
BONE CELLS
1. Osteocytes
2.Osteoblasts
3. Bone-lining cells
4 Osteoclasts
Osteoprogenitor cells
Located in the periosteum and endosteum. They are very difficult to distinguish from the surrounding connective tissue cells.
Osteoblasts
Osteoblasts may form a low columnar “epitheloid layer” at sites of bone deposition. They contain plenty of rough endoplasmatic reticulum (collagen synthesis) and a large Golgi apparatus
Osteocytes
Osteocytes. Mature bone cells.
Stellate.
Surrounded by matrix,
can make small amounts of matrix to maintain it.
Lacunae: spaces occupied by osteocyte cell body
Canaliculi: canals occupied by osteocyte cell processes
Nutrients diffuse through tiny amount of liquid surrounding cell and filling lacunae and canaliculi. Then can transfer nutrients from one cell to the next through gap junctions.
Canaliculi between Osteocytes
States of osteocytes
Quiescent osteocytes
Paucity of rER & decrease golgi apparatus
Formative osteocytes
More abundant rER& golgi apparatus
Resorptive osteocytes
Abundant Rer, golgi apparatus and lysosomes
Bone lining cells
On external surface are called periosteal cells
Oninternal surface are called endosteal cells
Mantainance and nutritional support of osteocytes
Osteoclasts
Large multi-nucleated bone-resorbing cells.. Osteoclasts attach themselves to the bone matrix (howships lacunae) and form a tight seal at the rim of the attachment site.
Osteoclasts are stimulated by parathyroid hormone and inhibited by calcitonin
The cell membrane opposite the matrix has deep invaginations forming a ruffled border. Osteoclasts empty the contents of lysosomes into the extracellular space between the ruffled border and the bone matrix. The released enzymes break down the collagen fibres of the matrix.
Classification of bones
Compact Bone
Cancellous (Spongy) Bone
Periosteum
Bone is, again like cartilage, surrounded by a layer of dense connective tissue, the periosteum. A thin layer of cell-rich connective tissue, the endosteum, lines the surface of the bone facing the marrow cavity. Both the periosteum and the endosteum possess osteogenic potency.
Compact Bone
Central or Haversian canals: parallel to long axis
Lamellae: concentric, circumferential, interstitial
Osteon or Haversian system: central canal, contents, associated concentric lamellae and osteocytes
Osteons (Haversian systems)
Blood vessel-filled central canal (Haversian canal)
Concentric lamellae of bone surround central canal
Lacunae and canaliculi contain osteocytes and fluid
Perforating or Volkmann’s canal
perpendicular to long axis. Both perforating and central canals contain blood vessels. Direct flow of nutrients from vessels through cell processes of osteoblasts and from one cell to the next.
Circumferential lamellae on the periphery of a bone
Interstitial lamellae between osteons
Circulation in Bone
Blood vessels from periosteum penetrate bone
Vessels of the central canal
Nutrients and wastes travel to and from osteocytes via
Interstitial fluid of lacunae and canaliculi
From osteocyte to osteocyte by gap junctions
Cancellous (Spongy) Bone
Trabeculae: interconnecting rods or plates of bone. Like scaffolding.
Spaces filled with marrow.
Covered with endosteum.
Oriented along stress lines
