Tissues

Question 1

What are the tissue classifications?

What factors are used to put tissues into classifications?

Answer 1

Epithelial - lines surfaces of body cavities/tubes
Muscle - generates motile force through contraction (skeletal, smooth, cardiac)
Nervous - interconnected network allowing communication from CNS to body
Connective - basic supportive tissue for structure, insulation, protection

Put into according to size, shape and function.

Question 2

What is the role of epithelial tissue?

Answer 2

Lines surfaces of body cavities and tubes.

Has a high cellular content (low ECM) so role in protection, transport, diffusion, absorption and secretion.

Question 3

Describe the types of simple epithelial tissue.

Answer 3

Simple Epithelial tissues contain one layer of cells all attached to a basement membrane. There are 3 types:

*Simple Squamous Epithelia:
Squashed shape attached to a basement membrane
Lines heart, blood vessels, lymph vessels, lung alveoli, collecting ducts of the kidney nephron

*Simple Cuboidal Epithelium:
Cube-shaped attached to a basement membrane
Forms kidney tubules and some glands
Play an important role in secretion as well as absorption/excretion

*Simple Columnar Epithelium
Rectangular shaped attached to basement membrane
Found in the stomach, small intestine, trachea, fallopian tubes
Can be ciliated

Pseudostratified Epithelium gives impression of multiple cell layers but each cell is actually attached to basement membrane.
Eg. Pseudostratisfied columnar ciliated epithelium is found within respiratory tract

Question 4

Describe the types of stratified epithelial tissue.

Answer 4

Stratified Epithelial Tissue are several layers of cells that have continual cell division in basal layers, main role is protection from wear and tear.

*Transitional Epithelium are pear shaped and only found in urinary tract.

*Stratified Squamous Epithelium can be keratinised or non-keratinised;

Keratinised Stratified Epithelium are on dry surfaces like skin, hair, nails. Top layer have no nuclei, it contains keratin and creates waterproof barrier.

Non-keratinised Stratified Epithelium is on moist surfaces and protects them from drying out. In mouth, pharynx, oesophagus, vagina…

Question 5

What are tissues composed of?

Answer 5

Cells and an extra-cellular matrix (ECM).

Question 6

What is the Extracellular Matrix function?

Answer 6

It provides the structure for cells to exist in, forms junctions within cells, regulates migration, influences growth and development and can change the morphology/function of the cell. It differs in different cell types.

Question 7

What is connective tissue?

Answer 7

It is the most abundant tissue in body - contains cells within surrounding jelly (ECM) that contains fibres to provide support and structure.

Main functions are binding/structural support, protection, transport and insulation.

Types of connective tissue are loose (areolar) connective tissue, adipose tissue, reticular tissue, dense connective tissue, cartilage, bone and (sort of) blood.

Question 8

What is Loose (Areolar) Tissue?

Answer 8

It is the most abundant connective tissue type, with a semi-solid matrix of collagen and elastin fibres containing fibroblasts, adipocytes, mast cells and macrophages; connects and supports other tissues (under skin, between muscles…)

Question 9

What is Adipose Tissue?

Answer 9

Type of connective tissue that is lipid heavy. The Adipose Tissue matrix contains adipocytes that contains large fat globules and has a high cellular content.

There are 2 types;
White Adipose Tissue - in healthy adults 20-25% is normal; acts as thermal insulator and energy store
Brown Adipose Tissue - found in new-borns; highly vascularised

Question 10

What is Reticular Tissue?

Answer 10

Connective tissue type that is only found in lymph nodes and lymphatic system organs; matrix consists of reticular fibres; cells present are reticular cells and WBCs.

Question 11

What is Dense Connective Tissue?

Answer 11

Dense Connective Tissue has low cell numbers but high fibre content. It can be fibrous or elastic.

Fibrous tissue has collagen fibres in closely packed bundles; fibroblasts sit between these fibres (tendons, ligaments, periosteum…)

Elastic tissue has elastin fibres secreted by fibroblast cells; has a high degree of recoil so in organs where change of shape is common (lungs, blood vesicles…)

Question 12

What is cartilage?

Answer 12

Cartilage connective tissue where chondrocytes (cartilage cells) are embedded in a collagen and proteoglycan matrix.

There are three types;
Hyaline - at the end of long bones (articulating cartilage)
Fibrocartilage - in intervertebral discs, menisci
Elastic Fibrocartilage - outer ear

Question 13

What is bone tissue?

Answer 13

Bone is connective tissue that is bone cells (osteocytes) embedded within a mineralised collagen matrix.

Contains 3 main cell types:
Osteoblasts which lay down new bone matrix
Osteoclasts which remove bone matrix
Osteocytes which are mature bone cells that reside within matrix and tell other cells what to do

2 types of bone:
Compact bone which is stronger and heavier
Spongy/Cancellous bone which is softer and lighter

Question 14

What is muscle tissue?

What are the types?

Answer 14

Able to contract and relax, providing movement.

Formed from myoblasts fusing together to form a myocyte (muscle fibre).

Can be skeletal, smooth or cardiac.

Question 15

What is skeletal muscle tissue?

Answer 15

Skeletal Muscle is striated (striped), multinucleated muscle fibres.

It is under voluntary control and moves bones of the skeleton (provide the force of the lever system), only responds to nerve impulses.

Question 16

What is smooth muscle tissue?

Answer 16

It is non-striated, mono-nucleated, responds to nerve impulses, hormones and stimuli, under involuntary control - useful in propelling contents along tubes by peristalsis (rhythmic contractions) so found in gut tube, ureter…

Question 17

What is cardiac muscle tissue?

Answer 17

Cardiac Muscle is striated but under involuntary control, has 1-2 nuclei, responds to nerve impulses, hormones and stimuli, has regular fibres.

It makes walls of heart - intercalated discs between cells help a propagate wave of contraction.

Question 18

What is nervous tissue?

Answer 18

Contains 2 cell types:

Neurons - excitable cells that initiate, receive and transmit information. Cell bodies are within central nervous system and axons are within peripheral nervous system.

Glial Cells - non-excitable cells that support the neurons and are more numerous than neurons (astrocytes, oligodendrocytes, ependymal cells and microglia)

Question 19

What are occluding junctions?

Answer 19

In vertebrates, they are tight junctions. Their main role is to seal cells together to prevent molecules from leaking either to another cell or across a boundary.

Question 20

What are anchoring junctions?

Answer 20

Main role is to mechanically attach cells and their cytoskeleton to adjacent cells or to the matrix.

Within them there are;
Actin Filament Attachment Sites - the cell-cell junctions (adherens junctions) and cell-matrix junctions (focal adhesions)

Intermediate Filament Attachment Sites - the cell-cell junctions (desmosomes) and cell-matrix junctions (hemidesmosomes)

Question 21

What are communicating junctions?

Answer 21

Can be Gap Junctions or Chemical Synapses.

Main roll is to control passage of electrical or chemical signals; most cells linked to their neighbours via gap junctions (like cardiac, smooth muscle cells…)

Question 22

What is histology?

What are common stains?

Answer 22

Histology is when tissues are processed to make them able to study by a microscope. Tissue is sliced into thin sections and stained.

H&E (haematoxylin & Eosin) - most common; stains pink/purple
Masson’s Trichrome - a connective tissue stain; stains green/red/black
Von Kossa - a mineral stain; stains black
Alizarin Red - a mineral stain; stains red
Oil Red O - a fat globule stain; stains red

Question 23

Describe the hierarchical structure of the body.

Answer 23

1. Cells group together to perform specific functions
2. Tissues can be of 4 types, regulate together to perform specific task
3. Organs are different tissue types grouped together
4. Body systems group organs sharing a common function

Question 24

Describe some microscopic techniques.

Answer 24

• Light microscopy (LM) - stain like H&E stain which stains cell contents pink but nucleus purple
• Fluorescent microscopy (FM) - stain like DAPI fluorescent stain makes nucleus blue and rounded an cytoplasm green
• Scanning electron microscopy (SEM) - can only see surfaces but more detailed
• Transition electron microscopy (TEM) - no dye so can’t tell organelles apart but can see specific structures in great detail

Question 25

What are the three types of connective tissue?

Answer 25

Connective tissue proper (loose/dense), fluid connective tissue (blood/lymph), supporting connective tissue (cartilage/bone).

Question 26

Describe connective tissue proper.

Answer 26

Two types:
Loose - fibres create loose, open framework (areolar, adipose, reticular tissues)

Dense - fibres densely packed (dense regular (tendons, ligaments), dense irregular (capsules), elastic)

Question 27

Describe fluid connective tissue.

Answer 27

Two types:
Blood - contained in C.V. system

Lymph - contained in lymphatic system

Question 28

Describe supporting connective tissue.

Answer 28

Two types:
Cartilage - load bearing resilient ‘rubbery’ matrix (hyaline (end of bones), fibrocartilage, elastic)

Bone - solid crystalline matrix (cancellous/compact)

Question 29

What are the components of connective tissue?

Answer 29

Cells - resident cells include chondrocytes (in cartilage), fibroblasts (in most CTs), tenocytes (like fibroblasts in tendons), osteoblasts (build new bone), osteoclasts (remove bone), osteocytes (maintains bone matrix/assists bone repair); immigrant cells include macrophages, lymphocytes, neutrophils (defence), mast cells.

Extracellular matrix (ECM) - collagens and proteoglycans.

Interstitial fluid

Question 30

What are integrins?

Answer 30

Transmembrane receptors that facilitate cell-cell and cell-extracellular matrix (ECM) adhesion and physical signalling (mechanotransduction).

Question 31

What are chondrocytes?

Answer 31

Cells in cartilage exclusively responsible for synthesis / breakdown of ECM components.

Can synthesise the full range of ECM proteins (collagens/proteoglycans, degradative enzymes and inhibitors of enzymes etc.) - highly specialised cells.

Normally synthesise cartilage-specific ECM components (collagen type II - basket-weave network; aggrecan)

Cell metabolism highly sensitive to physico-chemical environment.

Has specialised matrix surrounding cells called lacuna/chondron.

Chondrocyte shape distinctive (rounded/elliptical) but phenotypically unstable - can de-differentiate to fibroblasts.

No cell division in healthy tissue - years/decade life span, but during osteoarthritis cells start to divide causing issues

Question 32

What are fibroblasts?

Answer 32

Activated connective tissue cells - abundant rough endoplasmic reticulum and synthesis of fibrous matrix proteins, particularly collagen (type I - forms rope-like network).

Fibroblasts metabolically active state, fibrocyte relatively inactive.

Cell metabolism highly sensitive to physico-chemical environment.

Tissue damage stimulates production of fibroblasts - critical in wound healing.

Live for months.

“-blast” means donate stem cell/cell in activated state of metabolism

Question 33

What are differences in chondrocytes and fibroblasts cytoskeleton and morphology?

Answer 33

Chondrocyte - cytoskeletal proteins (actin, vimentin) localized to cell periphery; control morphology.

Fibroblast - extensive actin stress-fibre network throughout cell;linked to integrins on substrate surface.

Question 34

What are components of the ECM in supportive connecting tissue?

Answer 34

Collagens - fibrillar proteins resist tensile stresses like ‘rope’

Proteoglycans - composed of negatively-charged glycosaminoglycans (GAGs) which attract cations and SWELL like partially inflated ‘balloons’ and resist compressive forces.

ECM turnover (synthesis/degradation) by cells throughout life.

Question 35

What is connective tissue interstitial fluid influenced by?

Answer 35

Complex composition – influenced by the negatively charged GAGs of proteoglycans in ECM.

Question 36

What are the functions of skeletal muscle tissue?

Answer 36

Generates movement, maintains posture, provides protection, heat production, promotes blood flow.

Question 37

What are the 5 key properties of skeletal muscle?

Answer 37

Excitability – responds to nerve stimulus

Conductivity – stimulus spreads throughout whole muscle

Contractability – muscle cells can forcefully shorten

Elasticity – ability to return to original length (recoil)

Extensibility – capable of stretching

Question 38

Describe the hierarchical arrangement of muscles.

Answer 38

Regular arrangement of tissue in ‘packets’ with each layer surrounded by connective tissues.

1. Muscle - the whole tissue; surrounded by epimysium.
2. Fascicle - small ‘packet’ of muscle cells; surrounded by perimysium.
3. Muscle Fibre - muscle cells: myocytes (multinucleated); surrounded by endomysium.
4. Myofibrils - cylinder contractile unit; each myocyte contains many.
5. Sacromeres - pattern of repeating proteins within myofibril; smallest funcional unit

Question 39

What is the difference between tendons and ligaments?

Answer 39

Structurally similar but:

Tendons usually connect muscle to bone, are tough and inelastic.

Ligaments usually connect bone to bone, are flexible and highly elastic.

Question 40

What is hyaline cartilage?

Answer 40

Articular cartilage on surfaces of moveable joints – glassy/shiny appearance, provides low friction surface in joints, alongside synovial fluid.

Withstands compressive and tensile forces – load-bearing but not elastic.

Pliable - spreads load over ends of bones, protecting them.

Made of mainly basketweave collagen (type II held together with type IX).

Question 41

What is the main cell of hyaline cartilage?

What do they do?

Answer 41

Chondrocytes.

Exist in a lacuna (chondron) bubble within the ECM. Their orientation changes as you get deeper. Job is to make and break down matrices in response to changes, highly metabolically active.

Make type 2 collagen mainly but also type 9 and others - producing a basket-weave matrix.

Also produce large proteoglycan (PG) (Aggrecan), matrix-degrading enzymes and TIMPS (tissue inhibitors of metalloproteinases - cathepsins, metalloproteinases, gelatinases, collagenases).

Question 42

What is fibrocartilage function and structure?

Answer 42

Supports and prevents bone-bone contact, spreads load, limits movement. Can withstand tensile and compressive forces.

Found in intervertebral disc, meniscus…

Collagen fibres are thick & normally have clear parallel orientation and structure.

Cells often in rows, mainly fibroblasts but some chondrocyte.

Question 43

What are the main cells in fibrocartilage?

What do they do?

Answer 43

Fibroblasts - flat irregular spindle-shaped processes.

They mainly make type 1 collagen but also many other types. Collagen is aligned with direction of force in tissue - rope-like.

Also produce small proteoglycans (PGs) (Decorin, Biglycan), elastin, fibrillin, matrix-degrading enzymes and TIMPS (tissue inhibitors of metalloproteinases - cathepsins, metalloproteinases, gelatinases, collagenases).

Question 44

What is elastic cartilage?

Answer 44

Histologically very similar to hyaline but contains elastin - highly & reversibly deformable. Ideal for a flexible skeleton; in auricle of ear and epiglottis.

Fibroblasts synthesise elastin (which makes it elastic), collagens, small proteoglycans.

Question 45

What is synovial fluid?

Answer 45

Ultrafiltrate of plasma with hyaluronic acid – lubricant and primary source of nutrition & removal of waste for cartilage cells.

Produced by synoviocytes of synovial membrane.

Viscous when joint is immobile. ‘Warming-up’ exercises increase production/secretion, reducing viscosity. Lower viscosity - more lubrication less friction.

Sometimes phagocytes in synovial fluid for protection.
Rheumatoid arthritis - autoimmune disease where immune system attacks synovium.

Question 46

What is the relationship between load and biological properties of connective tissues?

Answer 46

Appear simple and homogeneous - but complex and heterogeneous, not inert but living and responsive to mechanical environment.

Load-bearing cartilage thicker & stronger than non-load-bearing. In immobilised joints cartilage thins and is lost - usually reversible.

Passive cycling does not maintain cartilage health. Long-term static load does not maintain cartilage health. Need clear physical stimulus - the right signals.

Question 47

How do tendons/ligaments deal with mechanical forces?

Answer 47

Tendons transmit load from muscle to bone.
Ligaments transmit load/give stability from bone to bone (hold skeleton together).

Cells adapt to prevailing mechanical forces by modifying ECM synthesis so there’s more like cartilage where mechanical load is beared.

Extracellular matrix of tendon passing over bone is more cartilaginous, with more PGs, and less collagen - cells are ‘chondrocytic’.
Compared to tensile region where ECM is fibrocartilagenous, with less PGs and more collagen - cells are ‘fibroblastic’.

Question 48

What is Wolff’s Law?

Answer 48

‘Form follows function.’

Physiological loads are essential for health of musculoskeletal tissues (bone, cartilage, tendons…)

Cartilage thickness proportional to prevailing load –thicker where load is greater, thinner where load is less.

Question 49

What can excessive load/impact cause in connective tissues?

Answer 49

Matrix damage and chondrocyte death.

Wave of chondrocyte death away from lesion after chondrocyte death & cartilage damage from inappropriate mechanical stress.

Subjecting cartilage to un-physiological stress causes permanent damage to chondrocytes/matrix.

Question 50

What does load do to chondrocytes?

Answer 50

Static load depresses synthesis, dynamic load stimulates synthesis.

Fluid flow / streaming potentials / ionic composition is static load.

High hydrostatic pressure (200x on standing) is dynamic load.

Load squeezes fluid out of tissue.

Question 51

How do chondrocytes sense mechanical load?

Answer 51

Integrins and stretch-sensitive ion channels.

Mechanotransduction - the link between changes to the mechanical environment of cells and the cell’s response.

Question 52

How is cartilage adapted to withstand load?

Answer 52

Articular cartilage has no vulnerable structures; it’s avascular, aneural, alymphatic, no epithelium at cartilage surface, low cell density (1-10%), complex ECM (fibre-reinforced gel) is highly resilient – adapted to compressive and tensile forces.

Question 53

What are proteoglycans?

Answer 53

Key component of articular cartilage.

Includes aggrecan - monomers of GAG, highly sulphated and acidic with fixed negative charges.
They attract cations and water, repel anions and swell (but only swollen to 20% ish of total).

Fives negative charges attract cations (mainly Na) - these are hydrated causing the swelling.

Also small proteoglycans (decorin, fibromodulin…) but these tend not to be retained effectively by the hyaline cartilage matrix.

Question 54

Describe the interstitial fluid of articular cartilage.

Answer 54

Complex, challenging and changing environment for chondrocytes - controlled by PGs.

Positive ions make it acidic and very high pressure and less oxygen pressure so poor environmental condition for chondrocytes but can still make ECM. Includes Na+, K+, Ca2+, Cl-…

It is hypoxic, hyper-osmotic, acidic, higher cation concentrations, lower anions.

Changes during static load as fluid extruded.

Question 55

When do chondrocytes divide?

Answer 55

There is no chondrocyte division in skeletally mature healthy cartilage.

Chondrocyte proliferation only occurs in late stage OA. Cartilage produced is very feeble and weak and eventually is worn away. This is because the molecules are smaller and less spherical so more permeable.

Question 56

What type of joint is more permeable?

What makes this possible?

Answer 56

Mobile joints more permeable than immobile. This means nutrients (like glucose) diffuse more in mobile joints.

Closer parallel weave of surface collagen fibrils gives smoother tensile-resisting surface and filters out large molecules, prevents ingress of inflammatory mediators from synovial fluid and loss of aggrecan.

Question 57

What is scurvy?

Answer 57

Vitamin C (ascorbic acid) deficiency.

Causes poor wound healing, hair and tooth loss,capillary weakness, stunted growth. May be fatal because of increased risk of infection/bleeding.

Vitamin C in diet is essential as humans(and some animals) do not produce it and it is a key co-factor for the hydroxylation of collagens.

Reduced hydroxylation of prolyl amino acids leads to reduced collagen cross-linking and tissue weakness.