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HISTOLOGY & EMBRIOLOGY II > LECTURE 4 (Bones Part I) > Flashcards

LECTURE 4 (Bones Part I) Flashcards

1
Q

What is Bone?

A

A specialised connective tissue composed of calcified extracellular material

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2
Q

What are the different functions of bone?

A
  • Provides solid support for the body
  • Protects vital organs
  • Encloses internal cavities containing bone marrow (where blood cells are formed)
  • Reservoir for calcium, phosphate & other ions that can be released & stored to maintain CONSTANT CONCENTRATIONS in bodily fluids
  • Form a system of levers (multiply forces generated during skeletal muscle contraction and transform them into bodily movements)
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3
Q

What are the three major cell types found in bone?

A
  • Osteocytes
  • Osteoblasts
  • Osteoclasts
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4
Q

What are Osteocytes?

A

Found in cavities (LACUNA) between bone matrix layers with cytoplasmic processes in small canaliculi that extend into the matrix and assist with nutrition of the bone

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5
Q

What are Osteoblasts?

A

Growing cells which synthesise and secrete the organic components of the matrix and are responsible for synthesising and mediating the mineralisation of the bone matrix

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6
Q

What are Osteoclasts?

A

Giant, multinucleated cells involved in removing calcified bone matrix and remodelling bone tissue

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7
Q

What do the exchanges between osteocytes and blood capillaries depend on?

A

Communication through the very thin, cylindrical spaces of the CANALICULI

[this is because metabolites are unable to diffuse through the calcified matrix of bone]

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8
Q

All bones are lined on their internal and external surfaces by layers of connective tissue containing what cells?

A

Osteogenic cells

ENDOSTEUM = on the internal surface surrounding the marrow cavity

PERIOSTEUM = on the external surface

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9
Q

How is bone sectioned?

A

Bone matrix is softened by IMMERSION in a DECALCIFYING SOLUTION before paraffin embedding or embedded in PLASTIC after FIXATION and sectioned with a SPECIALISED MICROTONE

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10
Q

Where do Osteoblasts originate from?

A

Mesenchymal stem cells

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11
Q

Where are active osteoblasts located?

A

At the surfaces of bone matrix where they are bound by INTEGRINS typically forming a single layer of cuboidal cells joined by ADHERENT and GAP JUNCTIONS

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12
Q

What happens to osteoblasts once synthetic activity is completed?

A
  • Differentiate as osteocytes entrapped in matrix-bound lacunae
  • Flatten and cover the matrix surface as BONE LINING CELLS
  • Apoptosis
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13
Q

What are Osteoblasts during the process of matrix synthesis and calcification?

A

Polarised cells with ultrastructural features denoting active protein synthesis and secretion

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14
Q

What happens during matrix synthesis?

A

Matrix components are secreted at the cell surface in contact with existing bone matrix producing a layer of unique collagen-rich material called OSTEOID between the osteoblast layer and the pre-existing bone surface

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15
Q

What is prominent among the non-collagen proteins secreted by osteoblasts?

A

Vitamin K-dependent polypeptide OSTEOCALCIN which together with various glycoproteins binds Ca2+ ions and concentrates this mineral locally

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16
Q

What do Osteoblasts secrete?

A
  • Osteocalcin
  • Membrane-enclosed matrix vesicles rich in alkaline phosphatase
  • Other enzymes whose activity raises the local conc of phosphate ions
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17
Q

Describe the process of matrix mineralisation

A

Microenvironment with high concentrations of CALCIUM and PHOSPHATE IONS -> Matrix vesicles serve as foci for the formation of HYDROXYAPATITE CRYSTALS -> Crystals grow rapidly by increase of mineral -> Produce a CONFLUENT MASS of CALCIFIED MATERIAL embedding the collagen fibers and proteoglycans

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18
Q

Where can the cancer Osteosarcoma arise in?

A

Osteoprogenitor cells

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19
Q

The skeleton is often the site of which type of tumours?

A

SECONDARY METASTATIC TUMORS

[arise when cancer cells move into bones via small blood or lymphatic vessels from malignancies in other organs]

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20
Q

What happens during the transition from osteoblasts to osteocytes?

A

The cells extend many long dendritic processes which also become surrounded by calcifying matrix

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21
Q

Where does the diffusion of metabolites between osteocytes and blood vessels occur?

A

Through the small amount of interstitial fluid in the CANALICULI between the bone matrix and the osteocytes and their processes

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22
Q

How do osteocytes communicate with one another and with nearby osteoblasts and bone lining cells?

A

Via GAP JUNCTIONS at the ends of their processes

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23
Q

What are the properties of Osteocytes?

A
  • Most abundant cells in bone
  • Exhibit less ER, smaller Golgi complexes and more condensed nuclear chromatin than osteoblasts
  • Maintain calcified matrix
  • Death is followed by rapid matrix resorption
  • Express paracrine and endocrine effects that help regulate bone remodelling
24
Q

What is the Mechanostat?

A

The extensive network of osteocyte dendritic processes and other bone cells

25
Q

What are the functions of Mechanostat?

A
  • Monitor mechanical loads within bones and signalling cells to adjust ions levels
  • Maintain the adjacent bone matrix accordingly
26
Q

What does lack of exercise cause?

A

Decreased bone density due to lack of mechanical stimulation of the bone cells

27
Q

What are the properties of Osteoclasts?

A
  • Very large, motile cells with multiple nuclei
  • Essential for matrix resorption during bone growth and remodelling
28
Q

Which polypeptides are required for Osteoclasts development?

A

MACROPHAGE-COLONY-STIMULATING FACTOR & RECEPTOR ACTIVATOR OF NUCLEAR FACTOR-XB LIGAND (RANKL)

29
Q

In areas of bone undergoing resorption, where as osteoclasts found?

A

Osteoclasts on the bone surface lie within enzymatically etched depressions or cavities in the matrix known as RESORPTION LACUNAE/HOWSHIP LACUNAE

30
Q

What is Osteoclast activity controlled by?

A

Local signalling factors from other bone cells

31
Q

Describe Osteopetrosis

A

A genetic disease where patients have defective osteoclasts due to mutations in genes for the cells’ proton-ATPase pumps or chloride channels

SYMPTOMS:
- Dense, heavy bones
- Osteoclasts lack ruffled borders
- Bone resorption is defective
- Overgrowth and thickening of bones
- Obliteration of marrow cavities -> depresses blood cell formation -> anaemia and loss of white blood cells

32
Q

Why is the surface of hydroxyapatite crystals hydrated?

A

To facilitate the exchange of ions between the mineral and body fluids

33
Q

What are Inorganic materials found in the bone matrix?

A
  • Calcium hydroxyapatite (most abundant)
  • Bicarbonate
  • Citrate
  • Magnesium
  • Potassium
  • Sodium
  • Noncrystalline calcium phosphate
34
Q

What are the two types of bone?

A
  • COMPACT (CORTICAL) BONE = Dense portion closer to the surface
  • CANCELLOUS (TRABECULAR) BONE = Deeper portion with interconnecting cavities
35
Q

Microscopically what are the two types of bone?

A
  • LAMELLAR BONE = arranged in sheets (80% within compact bone & 20% within trabecular bone)
  • WOVEN BONE = arranged randomly & found in developing and growing bones
36
Q

What is the Organic matter found in the bone matrix?

A
  • Collagen
  • Proteoglycans
  • Glycoproteins
37
Q

What provides bone with hardness and resistance?

A

Association of minerals with collagen fibers during calcification

38
Q

External and internal surfaces of bones are covered by which connective tissue?

A

Periosteum & Endosteum

39
Q

Describe the Periosteum

A
  • Outer fibrous layer of dense connective tissue containing TYPE I COLLAGEN, FIBROBLASTS & BLOOD VESSELS
  • PERFORATING (SHARPEY) FIBERS penetrate the bone matrix and bind the periosteum to the bone
  • PERIOSTEAL BLOOD VESSELS branch and penetrate the bone carrying metabolites to and from bone cells
40
Q

Describe the Periosteum’s inner layer

A
  • More cellular
  • Includes osteoblasts, bone lining cells and mesenchymal stem cells
41
Q

Describe the Endosteum

A
  • Covers small trabecular of bony matrix that project into the marrow cavities
  • Contains osteoprogenitor cells, osteoblasts and bone lining cells within a matrix of collagen fibers
42
Q

Describe Osteoporosis

A

An imbalance in skeletal turnover so that bone resorption exceeds bone formation. This leads to calcium loss from bones and reduced BONE MINERAL DENSITY (BMD).

RISK FACTORS:
- Immobilised patients
- Postmenopausal women

TESTING:
- Individuals at risk for osteoporosis are routinely tested for BMD by DUAL-ENERGY X-RAY ABSORPTIOMETRY (DEXA Scans)

43
Q

Describe Long bones

A
  • EPIPHYSES (bulbous ends) = composed of CANCELLOUS (TRABECULAR) BONE covered by a thin layer of compact CORTICAL BONE
  • DIAPHYSIS (cylindrical part) = almost totally dense compact bone with a thin region of cancellous bone on the inner surface around the central MARROW CAVITY
44
Q

Describe Short bones

A

Cores of cancellous bone surrounded completely by compact bone

(e.g wrist & ankle)

45
Q

Describe Flat bones

A

Flat bones that form the CALVARIA (skullcap) have two layers of compact bone called PLATES separated by a thicker layer of cancellous bone called the DIPLOE

46
Q

What is an Osteon/Haversian system?

A

The complex of concentric lamellae surrounding a central canal that contains small blood vessels, nerves and endosteum

[each osteon is a long, sometimes bifurcated cylinder generally parallel to the long axis of the diaphysis]

47
Q

How do Osteonic canals communicate with each other?

A

Through transverse perforating canals that have few concentric lamellae

[all central osteonic canals and perforating canals form when matrix is laid down around areas with preexisting blood vessels]

48
Q

What is scattered among the intact osteons?

A

Numerous irregularly shaped groups of parallel lamellae called INTERSTITIAL LAMELLAE

49
Q

Describe the parallel lamellae found in Compact bone

A

Organised as multiple EXTERNAL CIRCUMFERENTIAL LAMELLAE immediately beneath the periosteum and fewer INNER CIRCUMFERENTIAL LAMELLAE around the narrow cavity

50
Q

What is woven bone?

A

Nonlamellar and characterised by RANDOM DISPOSITION of TYPE I COLLAGEN FIBERS and is the first bone tissue to appear in embryonic development and in fracture repair

51
Q

What are the properties of Woven bone?

A
  • Temporary and is replaced in adults by lamellar bone
  • Lower mineral content (easily penetrated by x-rays)
  • Higher proportion of osteocytes than mature lamellar bone
  • Forms more quickly but has less strength than lamellar bone
52
Q

Describe Woven bone

A

HISTOLOGICAL FEATURES: Irregular & random arrangement of cells and collagen; lightly calcified
MAJOR LOCATIONS: Developing & Growing bones; hard callus of bone fractures
SYNONYMS: Immature bone; Primary bone; Bundle bone

53
Q

Describe Lamellar bone

A

HISTOLOGICAL FEATURES: Parallel bundles of collagen in thin layers with regularly spaced cells between; heavily calcified
MAJOR LOCATIONS: Adult bone
SYNONYMS: Mature bone; Secondary bone

54
Q

Describe Compact bone

A

HISTOLOGICAL FEATURES: Parallel lamellae or densely packed osteons with interstitial lamellae
MAJOR LOCATIONS: Thick, outer region of bones
SYNONYMS: Cortical bone

55
Q

Describe Cancellous bone

A

HISTOLOGICAL FEATURES: Interconnected thin spicules or trabeculae covered by endosteum
MAJOR LOCATIONS: Inner region of bones adjacent to marrow cavities
SYNONYMS: Spongey bone; trabecular bone; medullary bone

HISTOLOGY & EMBRIOLOGY II (13 decks)

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