Connective and skeletal tissues

Question 1

Describe the nature and properties of collagen

Answer 1

One tropocollagen molecule is formed of a triple helix of two alpha 1 collagen helices and one alpha 2 collagen helix-the ends of tropocollagen are tightly wound as its precursor, procollagen (which has loose ends made of propeptides), has it ends cleaved by peptidases.

The tropocollagen molecules self assemble end-to-end longitudinally, and in a staggered parallel arrangement horizontally. These fibrils self assemble to form banded fibres, banded due to the gaps formed in their staggered arrangement. This fibre arrangement gives it tensile and structural strength.

Sequence is a repeat gly-X-Y, with X usually being proline and Y usually being hydroxyproline

Question 2

Describe the nature and properties of elastic fibres

Answer 2

Elastin core, fibrillin surface

Elastin:

• random coiled tropoelastin chains
• covalent cross links to form a meshwork
• coiled chains can stretch and recoil reversibly, giving the fibres elastic properties

Fibrillin:

• glycoprotein that forms microfibrils
• acts as a scaffold onto which elastin is deposited

Question 3

Describe the nature and properties of reticular fibres

Answer 3

Thin fibres of type 3 collagen which are covalently cross linked to form a highly ordered meshwork called reticulin. This acts as a supporting structure for internal organs

Question 4

Where is type 1 collagen distributed, where is type 2 collagen distributed, and where is type 3 collagen distributed?

Answer 4

Type 1 = bone, tendons, ligaments, cornea (90% of body collagen)
Type 2 = cartilage, intervertebral disc, notochord
Type 3 = skin, blood vessels, internal organs such as spleen and lymph node

Question 5

Which molecules make up the ground substance in the ECM (just give names not descriptions)?

Answer 5

Glycosaminoglycans
Proteoglycans
Glycoproteins such as fibronectin and laminin

Question 6

Describe the nature and properties of GAGs.

Give two examples

Answer 6

• Long, unbranched polysaccharides
• Form highly hydrated gels due to being very anionic, thus hydrophilic, attracting sodium ions and water making them suitable to resist compressive forces
• Hyaluronic acid, which can form huge disaccharide polymers, and uniquely is non-sulphated nor linked to proteins
• Heparin, which is a variably sulphated disaccharide polymer that adopts a helical arrangement and is more of a typical GAG, often being linked to proteins and being sulphated

Question 7

Describe the nature and properties of proteoglycans, using examples.

Answer 7

GAGs covalently bonded to a core protein, ranging from huge molecules such as aggrecan (which has 100 GAGs linked to one long core protein) to tiny molecules such as decorin (which has 1-10 GAGs attached to a shorter core protein

They have three functions:

• act as molecular sieves, such as in basal lamina of glomerulus
• resist compressive forces, as they form gels
• act as lubricant

Question 8

Give two examples of glycoproteins found in connective tissue, and describe their nature and properties.

Answer 8

Laminin:
-contains 5 binding domains for binding GAGs (such as heparin), cell-surface receptors and collagen IV

Fibronectin:

• contain RGD (arg-gly-aspg) domain which bind integrin, a cell membrane protein that binds to the cytoskeleton
• also have collagen binding sites to connect the cytoskeleton to the strong collagen fibres of the ECM

Question 9

Outline fibroblast structure and function

Answer 9

Resident cell type of the ECM

Secretory cells that secrete all the components of the ECM ground substance and fibres:

• collagen (in a specific orientation)
• elastin
• GAGs
• reticular fibres
• proteoglycans
• growth factors
• glycoproteins

Cytoplasm elongated along the line of the fibres they produce
Prominent nucleus, highly active for producing proteins for ECM
RER and Golgi are extensive, for producing and secreting high volume of molecules and performing many post translational modifications
Extensive cytoskeleton for transport of secretory vesicles

Question 10

Outline macrophage structure and function

Answer 10

Derived from blood monocytes, develop into macrophages when they enter tissues

Largest type of WBC
Contain multiple protrusions of the cytoplasm called pseudopodia
Many cytoplasmic granules, especially lysosomes for hydrolytic activity
Round nucleus

Function:

• phagocytosis
• initiate and downregulate inflammatory responses
• antigen presentation
• recruit polymorphonuclear leukocytes (such as neutrophils)
• have other specialised functions based on which tissue they are in
• degrade ground substance of ECM in order to maintain homeostasis of connective tissue

Question 11

Outline mast cell structure and function

Answer 11

Granulated cell containing histamine (heparin, prostaglandins. serotonin, TNF, proteases etc.) which are released on binding to IgE receptors, which coat their surface
Round nucleus

Release inflammatory mediators contained within granules when stimulated by allergen binding to IgE receptors
Can also phagocytose bacteria, which can also stimulate them to release granules
The granules they release can stimulate vasodilation, smooth muscle contractions, breakdown of ECM and increased capillary permeability

Question 12

Outline basophil structure and function

Answer 12

Basophils:
Two-lobed nucleus
Many granules in cytoplasm, containing similar molecules as mast cells (especially histamine and heparin, which is highly negatively charged hence baso-phil)
IgE receptors in plasma membrane

Involved in inflammatory responses, particularly allergic responses and parasitic infections

Question 13

Outline eosinophil/acidophil structure and function

Answer 13

Eosinophils:
Contains many granules with distinguishing cationic proteins
Multilobed nucleus
Lipid bodies and sombrero vesicles

Involved in inflammatory responses, allergic and vasoactive reactions and control mast cells

Question 14

Outline neutrophil structure and function

Answer 14

Neutrophils:
Most abundant granulocyte, but also smallest of all granulocytes
Contains primary (microbicidal) and secondary (lysosomal) granules
ER absent and Golgi rare
Multilobed nucleus, 3-5 lobes, lost with maturity

Main function is phagocytosis
Rapid turnover from bone marrow, die quickly
Produce antimicrobial proteins as well

Question 15

Outline white adipocyte structure and function

Answer 15

Single huge lipid droplet - unilocular adipose tissue

Precursor is fibroblast-like cell, which forms into a brown fat-like cell with multiple lipid droplets in the cytoplasm with a centrally located nucleus.
The lipid droplets then fuse and the nucleus and cytoplasm are pushed towards the periphery of the cell

They store triglycerides, which are a source of energy
They provide insulation and padding

Question 16

Outline brown adipocyte structure and function

Answer 16

Multiple smaller lipid droplets - multilocular adipose tissue
Contains numerous mitochondria which are specialised to generate heat via fatty acid metabolism instead of generate ATP
Well vascularised and receive autonomic innervation

Found only in new born humans and lost in childhood, and serve primarily to provide heat for the neonatal.

Question 17

What is the structure of a tendon? Outline also how it joins to bone and muscle respectively

Answer 17

Dense regular connective tissue

Made up mostly of collagen aligned along the long axis of the tendon

Hierarchical: Type 1 collagen molecules crosslinked–>collagen fibrils crosslinked–>collagen fibres crosslinked–>fascicles bound by areolar connective tissue endotenon–>tendon sheathed by areolar connective tissue epitenon

Areolar connective tissue made of ECM ground substance- outer sheath that is superficial to epitenon known as paratenon, and promotes gliding

Specialised fibroblast cells found loosely within collagen fibrils and more densely within the areolar tissue

Insert on bone at specialised interface called enthesis, which is much stiffer than the tendon and is fibrous/fibrocartilaginous in nature

Muscle inserts via myotendinous junction, where finger-like projections of sarcolemma interdigitate with collagen fibres of tendon

Question 18

What are the mechanical properties and functions of tendons?

Answer 18

Collagen 1 arrangement gives high tensile strength in resisting unidirectional forces

Energy storing tendons (such as Achilles or patellar), where reticular fibres are present instead, have a certain degree of elasticity, extensibility and fatigue resistance

Transfer muscle generated force to the skeleton

Question 19

What is the structure of a ligament?

Answer 19

Dense regular connective tissue

Same hierarchical arrangement as tendons, with similar arrangement of ECM

Collagen fibres arranged parallel to long axis but also in other directions

White ligaments made predominantly of collagen for rigidity and yellow ligaments made predominantly of elastic fibres for elasticity

Mainly type 1, and quite insignificant ECM

Question 20

What are the mechanical properties and functions of ligaments?

Answer 20

Parallel arrangement of collagen, but in more than one direction, gives it tensile strength in many directions
Buckle under compression

Prevent excessive motion in any direction
Guide joint movements
Resist external load

Question 21

What is an aponeurosis, and what is its function? Give examples of aponeuroses that you’ve studied thus far (hint: they are in the limbs)

Answer 21

Dense regular connective tissue

A variant of deep fascia that is a sheet of fibrous tissue that insert sheet-like muscles with large attachment surface areas to bone or the body part they act upon

Histologically similar to tendons

Also functions like fascia to bind muscle groups together

Example is the plantar aponeurosis running from the calcaneus tuberosity to the heads of the metatarsals, or the palmar aponeurosis which is a triangular structure in the centre of the palm that invests the muscles of the palm, continuous with the tendon of palmaris longus and then the flexors of digits II-V

Question 22

What is the structure of fascia? What are its mechanical properties and functions?

Answer 22

Dense regular connective tissue

Collagen parallel to direction of pull, in a wavy pattern that can be pulled taught

Also contains gel-like ground substance

Fibroblasts located within the fascia

Embedded with nerve endings and mechanoreceptors

Question 23

What are the mechanical properties and functions of fascia?

Answer 23

Surround muscles, groups of muscles, blood vessels and nerves, bind them together and to reduce friction so that they can slide over each other

Important for proprioception as movement will activate mechanoreceptors to send signals to CNS

Shock absorption and sliding functions

Question 24

What are the key molecules and cells that make up hyaline cartilage? Where is it found?

Answer 24

Chondrocytes, which produce these molecules:
Collagen type II
Proteoglycans and GAGs- proteoglycans sometimes attached to hyaluronic acid by linker proteins
Very hydrated matrix

e.g. Articular surfaces, rib cartilage

Question 25

What are the key molecules and cells that make up fibrocartilage? Where is it found?

Answer 25

Chondrocytes, which produce these molecules:
Collagen type I
Ground substance, but much less significant

e.g. Intervertebral discs

Question 26

What are the key molecules and cells that make up elastic cartilage? Where is it found?

Answer 26

Chondrocytes, which produce these molecules:
Lots of elastic fibres/elastin-fibrillin complex
Ground substance

e.g. external ear, epiglottis

Question 27

Relate the mechanical properties and functions of hyaline cartilage to its function

Answer 27

Very hydrated matrix allows it to resist compressive forces

Forms a layer on both articular surfaces of a joint

Question 28

Relate the mechanical properties and functions of fibrocartilage to its function

Answer 28

Collagen I and lack of ground substance makes the tissue more rigid, and thus have greater tensile strength- helps to prevent leakage of the gel-like fluid within the nucleus pulposus which applies a lot of pressure on the surrounding annulus fibrosus when compressed

Question 29

Relate the mechanical properties and functions of elastic cartilage to its function

Answer 29

Elastic fibres give the tissue an additional ability to maintain their shape as they can stretch and then recoil

Question 30

What are the causes and symptoms of Marfan’s syndrome?

Answer 30

Gene defect in FBN1 gene, which makes fibrillin (glycoprotein component of connective tissue)
The molecular scaffold for elastin of elastic tissue is thus defective, and so elastic tissues are specifically effected.

These cause symptoms such as:

• Growth to abnormally large height and disproportionally long limbs
• lens dislocation in eyes
• cardiovascular issues causing shortness of breath, undue fatigue etc.

Question 31

Which stem cells differentiate to form the cartilage cells that secrete and degrade the ECM in cartilage?

Answer 31

Chondroblasts

Question 32

What cells are present in bone, and what makes up the ECM of bone?

Answer 32

Cells: Osteoblasts, osteoclasts and osteocytes
ECM: collagen type I, hydroxyapatite, proteoglycans

Question 33

What is the organic, uncalcified portion of the ECM of bone known as?

Answer 33

Osteoid

Question 34

What is the inorganic component of bone ECM made of, and how is it arranged with the organic component? What aspects of strength do each component contribute?

Answer 34

Hydroxyapatite, which is a phosphate and calcium containing mineral of the apatite group

It associates in orthogonal rods with the lamellar bone (collagen type I fibres arranged in different layers or lamellae in different orientations) to form a crystal lattice

The hydroxyapatite gives the bone compressive strength and rigidity, whereas the collagen gives tensile strength- work together, otherwise hydroxyapatite on its own would be too brittle, and collagen on its own would be too bendy

Question 35

Outline osteoblast derivation, location, structure and function

Answer 35

Derivation: fibroblast lineage, induced by bone morphogenic proteins (BMPs)

Location: Present in outer portion

Structure: Cuboidal, form single layers, prominent Golgi, extensive RER and many secretory vesicles

Function:

• lay down osteoid
• uniquely secrete osteocalcin (involved in binding calcium and mineral ions, as well as being pro-osteoblastic)
• help in osteopontin, osteonectin and bone sialoprotein secretions, which are involved in mineral bindind. Osteopontin also contains RGD sequences perhaps for anchoring osteoclasts to the matrix
• control osteoclast differentiation and action

Question 36

Outline osteoclast derivation, location, structure and function

Answer 36

Derivation: hematopoietic stem cells –> monocytes –> osteoclasts

Location: On the surface of bone which are undergoing resorption, in pits known as resorption bays

Structure:

• Polarised cell, with the cytoplasm exhibiting finger-like processes/deep invaginations/ruffled border at the border of the resorption bay, and the other end being a secretory domain
• Periphery of ruffled border surrounded by ring-like cytoplasm
• multinucleate
• cytoplasm contains many secretory vesicles, an extensive Golgi but sparse ER

Function:

• mobilise calcium stores within inorganic portion of ECM
• enables tissue remodelling by degrading the minerals and organic matrix so it can be rebuilt

Question 37

Explain the mechanism by which the mineral phase of bone is broken down

Answer 37

Osteoclast seals of region of bone via binding of cell surface integrins (part of the podosome) to RGD sequences found within the proteins of the matrix

Demineralisation occurs via the secretion of acids, which dissolve the hydroxyapatite

• ->acids obtained by pumping out protons though a proton pump
• ->protons come from the activity of carbonic anhydrase II catalysing the reaction of CO2 + H2O to make carbonic acid, which dissociates to release protons
• ->the dissociation of carbonic acid also releases bicarbonate ions which makes the cytoplasm more negatively charged and alkaline
• ->the HCO3- ions are then exchanged in a HCO3-/Cl- exchanger
• ->the pumping out of protons thus generates a charge accumulation within the cell, which is compensated for by the subsequent pumping out of Cl- ions into the resorption pit through chloride channels, to form HCl in the pit

Question 38

Explain the mechanism by which the organic matrix of bone is broken down

Answer 38

Osteoclast seals of region of bone via binding of cell surface integrins (part of the podosome) to RGD sequences found within the proteins of the matrix

Certain enzymes to digest the matrix proteins (collagen, glycoproteins, proteoglycans) are released:

• tartrate resistant acid phosphatases
• matrix metalloproteinases
• cysteine proteases

These are then transcytosed out through the secretory domain

Question 39

Outline osteocyte derivation, location, structure and function

Answer 39

Derivation: Osteoblasts that have been completed surrounded by their own bone matrix

Location: in the calcified bone matrix, occupying lacunae and their interconnecting canaliculi

Structure: Gap junctions between osteoblasts and other osteocytes, found in the cell membranes of finger-like processes of cytoplasm emanating in all directions from the cell called filopodia. Uninucleate

Function:

• Provide mechanosensory information in order to induce bone remodelling. Mechanical stress causes osteocytes to send biochemical signals through its cytoplasmic processes and gap junctions to other osteocytes, osteoblasts and osteoclasts. Osteoclasts will begin process of bone remodelling and osteoblasts will continue it by placing down new bone, until it’s surrounded in the matrix, at which point it becomes an osteocyte
• Maintain bone mass

Question 40

What is a lacuna (in bone)?

Answer 40

Space occupied by an osteocyte and its surrounding matrix

Question 41

What are the two types of bone, and where are they located in relation to one another?
How do they create the perfect balance of strength and lightness?

Answer 41

Compact and cancellous, with cancellous covered by compact
Compact bone is dense with collagen and mineral content in order to provide maximum strength to the points of higher tension (the outer portions of the bone)- but, in order to maintain lightness, the inside of the bone is less compact as it is under less tension containing trabeculae efficiently laid down along lines of tension to reduce overall bone mass without sacrificing strength

Question 42

What are the branches in the spongey type of bone known as?
What tissue is found between them?
How are they arranged?

Answer 42

Trabeculae
Bone marrow
Not randomly, but along lines of stress to make them as efficient as possible

Question 43

Describe the Haversian System of compact bone

Answer 43

Concentric layers of bone matrix, each layer produced by osteocytes trapped in their individual lacunae, surrounding a central/Haversian canal through which travel blood vessels. There are also intercalated lateral/ Volkmann’s canals, and all these structures together form the Haversian system

Question 44

What is an osteon?

Answer 44

A single Haversian canal with its associated lamellae of bone

Question 45

What are the regions between osteons called? What is the outer surface encompassing all these structures called?

Answer 45

Interstitial lamellae

Circumferential lamellae

Question 46

Why do Haversian systems not form in cancellous bone?

Answer 46

Trabeculae are thin enough for the enclosed osteocytes to be in close contact with the bone marrow (which can provide it with an adequate blood and nutrient supply)

Question 47

What are canaliculi?

Answer 47

Microscopic canals that link the lacunae of osteocytes so their can be an exchange of molecules. The filopodia of the osteocytes protrude into these canals and link with adjacent osteocytes via gap junctions

Question 48

What two molecules does bone act as a storage tissue for?

Answer 48

Calcium and phosphate

Question 49

What are the two ways that bone can develop?

Answer 49

Intramembranous ossification and endochondral ossification

Question 50

Name and describe the process by which the cartilage model becomes calcified in a long bone

Answer 50

Endochondral ossification:

Primary ossification center

First, a cartilage model of the bone is formed; mesenchymal cells condense and differentiate into chondrocytes, forming the hyaline cartilage model. The chondrocytes hypertrophy, and then secrete the alkaline phosphatase (hydroxyapatite) which precipitates the calcium salts in the matrix.

Bone matrix secreted by the hypertrophied chondrocytes prevents nutrients from reaching them, causing them to degenerate.

Blood vessels simultaneously invade the center of the cartilage model and cause the perichondrium to differentiate into periosteum. As this occurs, chondrogenic cells convert to osteoprogenitor cells.

Osteoprogenitor cells then convert to osteoblasts. Bone matrix secreted by the osteoblasts forms a bone/periosteal collar. The bone collar ALSO prevents nutrients from reaching the hypertrophied chondrocytes, causing them to degenerate.

Osteoclasts, cells that break down bone, arrive and form holes in the bone collar allowing the passage of periosteal buds. Periosteal buds consist of blood vessels, osteoprogenitor cells, and hemopoietic cells.

Osteoprogenitor cells brought to the developing bone through the periosteal buds divide, forming more osteoprogenitor cells. Some of these cells will differentiate into osteoblasts that will continue to form bone matrix on the surface of the calcified cartilage.

As the bone matrix calcifies, it forms the calcified cartilage-calcified bone complex

The bone collar continues to grow in either direction towards the epiphyses and osteoclasts resorb the calcified cartilage-calcified bone complex to widen the marrow cavity.

Secondary ossification center

Secondary ossification centers are found at the epiphyses of long bones. This process is similar to that of the primary center of ossification, but occurs without a bone collar. Instead, osteoprogenitor cells enter the epiphyseal cartilage, differentiate into osteoblasts, and secrete matrix on the cartilage framework. Long bones increase in length at the secondary ossification centers.

Question 51

What function does appositional growth have in a long bone? Outline the steps/zones in appositional growth

Answer 51

It allows for the growth of a long bone, from the epiphyseal plate

1. Zone of resting cartilage found in epiphyseal plates. Here there are a few chondrocytes which make the ECM
2. Zone of proliferation found just below the plate, in the direction towards the diaphysis. Chondrocytes divide and form stacks of cells, and secrete new fibres and ECM on the epiphyseal side of the epiphyseal plate
3. Zone of hypertrophy where chondrocytes enlarge and mature, again towards the diaphysis
4. Zone of calcification where chondrocytes die and matrix is calcified
5. Zone of ossification where calcified matrix replaced by bone. Here is where osteoblasts and osteoclasts congregate to make and model the bone

Question 52

What are the two functions of marrow cavities?

Answer 52

Fat storage

Haematopoiesis (formation of blood cells)