1 Bone and Cartilage - Structure and Function Flashcards
Describe Cartilage
Cartilage is specialised connective tissue
- with a support function (often shock absorbers of the body, can be tough or flexible depending on the composition)
What are the cells that makeup Cartilage
Chondrocytes
What is the matrix made of in Cartilage?
- Type II collagen
- Proteoglycans
and others (depending on the type of cartilage)
Describe Cartilage cells
Chondrocytes
- are derived from Embryonic Mesoderm (spindle) - clusters of chondroblasts (rounded), surrounded by a layer of perichondrium (mesenchyme derived fibroblastic cells and collagen)
Growth of cartilage is by:
- interstitial growth (a limited division of chondroblasts in ECM) AND
- Appositional growth (new chondroblasts from perichondrium)
After matrix deposition, cells become less active and become maintaining cells (lacuna is an artefact)
Describe the ECM of cartilage (Fibres and Ground substance)
Made of Type II Collagen and ground substance
- Matrix is 70% water
Has Proteoglycan aggregates
- which contain glycosaminoglycans (GAGs)
Describe the proteoglycans found in the ECM of cartilage
Proteoglycan aggregates
- proteoglycan monomers that are attached to a molecule of Hyaluronin - it is hydrophilic (brings in water)
This provides compressive strength
- as a flexible cushioned surface
Describe the glycosaminoglycans that are found in the proteoglycan in ECM
Proteoglycans contain numerous GAGs
- attached to a core protein (bottle-brush structure - negatively charged chains)
GAG - appear in different forms
- chondroitin-4-sulfate
- chondroitin-6-sulfate
- Keratan sulfate
It is woven with collagen to form an elastic and compressible structure
Describe the different types of Cartilage
It is dependent on the requirements of tissue:
- Hyaline cartilage: Type II collagen only - smooth glistening (glassy) articular surface
- Elastic cartilage: type II collagen + elastin (ear, nose, epiglottis)
- Fibrocartilage: Type I and II collagen (strong, where bones join); e.g. manubrioclavicular joint
Describe the cellular structure of Hyaline cartilage
arrangement of cells, perichondrium, and location
Hyaline
- Chondrocytes are arranged in groups in a matrix
- with Type II collagen
Perichondrium
- Usually present, except at Articular surfaces
Locations
- Articular ends of long bones, ventral rib cartilage, tracheal rings
Describe the cellular structure of Elastic Cartilage
arrangement of cells, perichondrium, and location
Elastic
- chondrocytes compacted in the matrix
- containing Type II collagen and elastic fibres
Perichondrium
- present
Locations
- Pinna of ear, auditory canal, laryngeal cartilages, epiglottis
Describe the cellular structure of Fibrocartilage
arrangement of cells, perichondrium, and location
Fibrocartilage
- chondrocytes arranged in rows in a matrix
- containing Type I collagen bundles in rows
Perichondrium
- absent
Locations
- IV discs, pubic symphysis, joint capsules, ligaments and tendons
Describe hyaline cartilages at joints
- Resists compression due to elasticity and stiffness of proteoglycans
- Tensile strength due to collagen and hydrogel ground substance
- Most are avascular - limits repair and regeneration
Nutrition is by diffusion: limits thickness (it is thin, avascular, so needs nutrients still)
- Articular surfaces of joint has no perichondrium - no source of new chondroblasts (reduces repairability)
- Cartilage atrophy is reversible - but it takes a long time
Describe Bone as a specialised connective tissue
- Structurally strong - mechanical support and proctection
- reservoir for calcium and phosphate in the body
(role in calcium homeostasis) - Supports haematopoiesis - bone marrow
Composed of cells and ECM
- Matrix must be strong enough to support the body, yet light enough to be moved (max strength, low weight)
- The cells produce, mediate, maintain and remodel the matrix
Describe the organisation of bone
- Dense outer shell - compact bone
- Inner spongy/cancellous bone - arranged in interconnecting trabecular with spaces for bone marrow
Max strength, less weight
Describe the organisation and structure of the interconnecting trabecular system of the inner spongy/cancellous bone
Within a single trabecular, there are:
- concentric lamellae
- with osteocytes with lacunae - connected to each other via canaliculi (similar to the tissue arrangement in the osteons of compact bone)
However, unlike other osteons, trabecular do not have central canals or perforating canals containing blood or lymph vessels, and nerves.
Describe how the vessels and nerves travel within the spongy bone
The vessels and nerves of spongy bone travel through the spaces between trabeculae, and do not need separate passageways
- Long vessels - Haversian system
- Branches - Volkmann’s Canal
Describe Trabecular bone
It reduces weight
- provides space for marrow
- struts are arranged to provide maximum resistance to stresses
- Found in e.g. wrists, vertebrae, femoral neck
Describe briefly the effect of osteoporosis on the bone structure
Osteoporosis
- Thinning of both critical and trabecular bone
- but the thin trabeculae are prone to fracture
i.e. FOOSH, hip fracture, dowager hump
Describe the bone matrix
It is made up of 2 parts:
- Organic part (osteoid)
- inorganic part
Together, the tensile and compressive strength of the organic part (osteoid), partnered with toughness of the inorganic part
- the bone has composite properties that make it very tough and incompressible
Describe the Organic part of the bone matrix (osteoid)
The Osteoid is produced by osteoblasts
- collagen type I
- Tensile and compressive strength
- Non collagenous proteins mediate mineral deposition
Without it, the bone is prone to weakness, and turns the bone soft
Describe the inorganic part of the bone matrix
Inorganic part of the bone matrix is made up of: - Calcium phosphate (hydroxyapatite) - It is deposited in the organic matter - 66% of the dry weight of bone - Provides hardness (without it, bone prone to shattering)
Describe the bone cells found in the bone
Bone cells
- derived from mesenchymal stem cells
- they differentiate into osteoprogenitor cells or chondroblasts
- then, osteoprogenitor cells differentiate into osteoblasts
What are osteoblasts?
Osteoblasts lay down the organic bone matrix
- and they mediate the mineralisation of the osteoid
Osteoblasts become osteocytes when surround by mineralised bone
What are osteocytes?
Osteocytes maintain the matrix
- they are formed from osteoclasts, when surrounded by mineralised bone
Describe the mineralisation of the osteoid
Osteoblasts secrete collagen and matrix vesicles
- Matrix vesicles contain enzymes and proteins to control the availability of calcium and phosphate, so that the mineral is precipitated
Describe the Immature osteoid (the formation and structure)
Woven bone
- haphazard
- fibre arrangement - mechanically weak - foetal development/fracture repair (rapid osteoid formation)
Describe Mature osteoid (the formation and structure)
Lamellar bone
- remodelled woven bone - regular parallel collagen
- strong, all adult bone
- arranged as osteons (aligned with the direction of force)
Describe Mature lamellar bone
Concentric circles can be seen
- ground bone - between 2 plates
- Here, in the middle of the concentric circles, there is a Haversian canal, with osteocytes surrounding them
- There are also the canaliculi (connect different cells; allow nutrients to flow
Describe the role of Osteocytes
They are mature osteoblasts - and surrounded by mineralised matrix
- Long cytoplasmic processes connecting to each other and osteoblasts (via gap junctions)
- They maintain matrix - no osteocytes; matrix is reabsorbed
In lacunar surrounded by extracellular bone fluid, that allowed nutrient diffusion through the bony channels (canaliculi)
- stress information: responsive to tiny currents generated when bone is deformed
- connected to both osteoblasts and osteoclasts
What are osteoclasts?
They exist to destroy bone
- It is a bone absorbing cells
- A phagocytic cell from monocyte macrophage cell line
- Multinucleate mobile cell, which attaches to the bone surface and resorbs bone leaving a pit behind (Howships lacuna)
They work with osteoblasts to regulate to:
- REGULATE BONE TURNOVER AND REMODELLING
Describe how osteoclasts destroy bone
Large multinucleate cells
- Actin clear zone zone and interns adheres it to the bone surface
- Ruffled border increases surface area of the cells
- Mineral is dissolved by acids; outside of cells is low pH
- Lysosomal enzymes resorb organic matrix
Describe what could affect the number of osteoclasts/function
Number and function is affected by PTH (parathyroid hormone)
- inhibition of osteoblasts leads to
- increase in the function of osteoclasts - regulate calcium level and calcitonin (does opposite)
Oestrogen also reduces activity (in menopause)
- keeps calcium in the bone
(protects this - lack of steroid hormone may lead to osteoporosis)
Describe bone remodelling
Bone is constantly remodelled through the coordinated actions of osteoblasts, osteocytes, and osteoclasts, to adjust the stresses and strains
- it affects density, orientation, and responds to micro-fractures and wear and tear
- AND keeps bone healthy
The aim is to place bone struts at the location and direction of maximum stress,
- while keeping the structure open to minimise weight
Describe why and how bone remodelling can increase
In adults. bone turnover is slower than in children, but can increase due to:
- change in function (onset of wailing)
- New demands (running, tennis, jumping)
- Repair of fractures
- Disease (e.g. Paget’s disease)
Describe Osteoporosis
A rare group of inherited conditions
- Reduced osteoclastic activity (defective bone remodelling)
Osteoclasts cannot excrete H+ ions to dissolve bone mineral (H+ needed for the acidic environment)
- Dense ‘stone bone’ but brittle and easily fractured
Clinical effects:
- Fractures, spinal nerve compression (excess bone) and recurrent infection (reduced bone marrow capacity), Hepatosplenomegaly due to haematopoiesis outside of the bone
Potential treatments:
- bone marrow transplant needed to provide healthy osteoclast precursors - can be effective
Describe the relationship between osteoclasts and osteoblasts
PTH (parathyroid hormone) stimulates bone resorption by osteoclasts activation
- the receptors for PTH are located on osteoblasts
- Osteoclast precursors have RANK (receptor activator of Nuclear Factor Kappa B) receptor on cell membranes
- Osteoblasts have the ligand for this receptor on their cell membranes (RANKL)
PTH up-regulates RANKL which binds to RANK and stimulates the differentiation of osteoclasts
- Osteoblasts also produce Osteoprotegrin, which prevent resorption by binding to RANKL (covering receptors; preventing binding to RANK)
The ratio of RANKL:osteoprotegrin determines bone resorption (which sit on the surface of osteoblasts, which then are presented to osteoclast precursor)
