Bone 1

Question 1

What are bones?

Answer 1

• specialized connective tissue composed of cells and calcified extracellular matrix (ECM)
• The cells and calcified ECM form a rigid organ

Question 2

What is the function of bones?

Answer 2

• Hematopoiesis= the formation of blood cells from hematopoietic stem cells found in the bone marrow
• Lipid & mineral storage= bone is a reservoir holding adipose tissue within the bone marrow and calcium within the hydroxyapatite crystals
• Support= bone forms scaffolding for skeletal muscle and other soft tissues, it creates a framework that gives the general shape of the body
• Protection= particularly the axial skeleton which surrounds the major organs of the body

Question 3

What are components of bone?

Answer 3

1. Extracellular matrix

2. Cells of bone tissues

Question 4

Describe the extracellular matrix bone

Answer 4

Refers to the molecules that provide biochemical and structural support to the cells that populate bone

Question 5

Describe the organic portion of ECM in bone

Answer 5

-Organic portion is predominantly (90% of total weight of matrix proteins) composed of collagen one proteins
-Mainly type 1 collagen (also type V, III, XI, XIII)
-Non-collagenous proteins (about 10%) constitute the ground substance and include proteoglycans, glycoproteins, bone specific proteins, growth factors and cytokines
Essential to bone development, growth, remodel8ng, and repair

Both collagen and ground substance become mineralized to form bone tissue

Question 6

Describe the inorganic portion of ECM in bone

Answer 6

Represents about 50% of the dry weight of the matrix and is composed mainly of calcium phosphate in the form of hydroxyapatite crystals
-Contains trace minerals such as bicarbonate, citrate, magnesium, potassium and sodium

Question 7

Describe the cells that are components of bone

Answer 7

-Osteoprogenitor(osteogenetic): the stem cells of bone, they are derived from primitive mesenchymal cells
Forms a population of stem cells that can differentiate into osteoblasts which secrete the organic unmineralized matrix or osteoid

• Osteoblasts: synthesize uncalcified/unmineralized extracellular matrix called osteoid. This will later become calcified/mineralized to form bone.
• Osteocytes: As the osteoid mineralizes, the osteoblasts become entombed between lamellae in lacunae where they mature into osteocytes. They then monitor the minerals and proteins to regulate/maintain bone mass

-Osteoclasts: Multinucleated giant cells derived from monocyte progenitor cells responsible for resorbing and remodeling bone by releasing H+ ions and lysosomalenzymes
The balance of osteoblast to osteoclast activity is crucial in the maintenance of the tissue’s structural integrity. It also plays a role in conditions such as osteoporosis

Bone-lining cells: found on the inner and outer surface of bone; the endosteum & periosteum

Question 8

What is the function of osteoclasts?

Answer 8

Functions in resorption, the breakdown of bone matrix

Question 9

Describe osteocytes formation

Answer 9

Osteoggenic cell develops into an osteoblast

Osteoblast forms bone matrix and develops into osteocytes

Osteocyte maintains bone tissue

Question 10

What are osteoblasts?

Answer 10

Cuboidal cells located on the periphery of bone. Displays euchromatic nuclei with a basophilic cytoplasm

Question 11

What are osteocytes?

Answer 11

Mature osteocytes: mature osteoblasts entrapped in bones matrix

Question 12

What are osteoclasts?

Answer 12

Large, multinucleated macrophages located along the endosteum

Question 13

What are the shape and location of osteogenic cells?

Answer 13

• Are spindle shaped connective tissue cells (derived from embryonic mesenchyme stem cells) in the periosteum and endosteum that can differentiate into osteoblasts
• In mature bone in which there is no active new bone formation or remodeling, the osteoprogenitor cells become flattened spindle shaped cells closely applied to the bone surface, when they are sometimes called “inactive osteoblasts”

Question 14

What are osteoblasts derived from?

Answer 14

Osteoprogenator cells; are basophilic

Question 15

What are the shapes of osteoblasts?

Answer 15

Morphologically May appear as a cuboidal layer(high secreting activity) or flattened (declining secretions)

Cytoplasmic processes contact them with other osteoblasts and osteocytes

Question 16

Describe the significant chemical composition and secretions of osteoblasts

Answer 16

• During synthesis osteoblasts have well developed rER and Golgi apparatus
• PAS positive granules in the cytoplasm are most likely the precursors of glycosaminoglycans (of the osteoid matrix)
• Eventually matrix secretion entraps the osteoblast in a lacunae with it’s cytoplasmic processes extending into canaliculi
• Can secrete osteoclast stimulating factor
  
  • Osteoclast eventually ceases its secretory function, undergoes changes to its morphology and becomes an osteocyte

Question 17

Explain the structure-function of osteocytes

Answer 17

• Mature bone cells, derived from osteoblasts
• Housed in their own lacunae, maintain communication and exchange nutrients and metabolites via gap junctions between their narrow cytoplasmic processes (filopodia) extending through the canaculi
• Increased amounts of condensed nuclear chromatin, reduced amounts of rER, and a smaller Golgi apparatus vs osteoblasts

Question 18

Describe ground bone preparations

Answer 18

• Bone samples are ground to near translucency using mechanical attrition
• No organic components of bone remain after grinding
• The tissue is then impregnated with Indian ink

Question 19

Describe the structure function of osteoclasts

Answer 19

Multinucleated giant cells (up to 50 nuclei) resulting from the fusion of hematopoietic monocyte progenitor cells

• Howship’s lacunae (resorption bay) are depressions on the bone surface that house osteoclasts
• These lacunae result from the osteolytic activities of osteoclasts

Question 20

What are the 3 distinct regions of osteoclasts?

Answer 20

1. Ruffled border
2. Clear zone
3. Basal region

Question 21

What are the ruffle border for osteoclasts?

Answer 21

Finger-like evaginations along Howship’s lacunae depict sites of active bone resorption

Question 22

What is the clear zone of osteoclasts?

Answer 22

Region of cytoplasm that surrounds the ruffled border

-Microfilaments, anchor osteoclast to the bone surface and help isolate osteoclastic activity

Question 23

What is the. Basal region of the osteoclasts?

Answer 23

Houses the organelles and numerous nuclei of the cell

Question 24

How do osteoclasts undertake bone resorption?

Answer 24

Lysosomal enzymes(From Golgi complex) and hydrogen ions released into the confined space betweeen bone matrix and the osteoclast’s peripheral clear zone
   -Acidic environment provides optimal pH for activity of lysosomal hydrolases and facilitates the dissolution of calcium phosphate from bone

Decalcalcified bone matrix is then broken down by acid hydrolases, collagenous and proteolytic enzymes and the cell resorbs the organic and inorganic

Question 25

What are the bone-lining cells?

Answer 25

- Osteoblast derived flat cells with little cytoplasm and scant organelles - Found on the outer (periosteal cells) and inner (endosteal cells) surface of non-remodeling bone - Function in maintenance and nutritional support of underlying cells - Regulate movement of phosphate and calcium into and out of bone

Question 26

What is the significance of the periosteum and endosteum ?

Answer 26

-Nom-calcified connective tissue layers covering external & internal surfaces of bone, except over Articular surfaces

Question 27

Where is the periosteum?

Answer 27

-Attached to the outer circumferential lamellae by Sharpey’s fibers (type 1 collagen)

Question 28

Describe the layers of the periosteum

Answer 28

- Outer fibrous layers: consists mostly of a dense irregular connective tissue with fibroblasts. Contains blood vessels and nociceptors - Inner cellular/ osteogenic layer: contains osteoprogenitor cells and their derivatives (periosteal cells) - cells actively secreting matrix increase the width of bone (appositional growth)

Question 29

Where is endosteum contained?

Answer 29

-Lines any space or cavity in bone | (Marrow spaces, Haversian canals, volkmann’s canal, lacunae and canaculi)

Question 30

What is contained in the endosteum?

Answer 30

-It contains mesenchymal stem cells which have the potential to differentiate into osteogenic progenitor cells and differentiated osteoblasts, fibroblasts, blood vessels, and sympathetic nerves Contains osteoclasts (bone remodeling occurs from within)

Question 31

Summarize bone growth and remodeling

Answer 31

Elongation of bones is a result of interstitial growth of cartilage at epiphyseal growth plate -Interstitial (internal)growth doesn’t occur in bone Bone increases in width by apossitional growth (osteprogenitor cells in the periosteum) - Remodeling of bone is a coordinated action between osteoblasts and osteoclasts

Question 32

What are the classifications of bones?

Answer 32

1. Immature (primary or woven) bone 2. Mature (secondary or lamellar) bone 3.

Question 33

Describe immature (primary or woven) bone

Answer 33

- First compact bone elaborated; contains numerous osteocytes and irregular arranged collagen - low mineral content - remodeled and replaced by mature bone except in certain places such as in tooth sockets, near suture lines in the bones of the skull, and at insertion site of tendons

Question 34

Describe mature bones (secondary or lamellar) bone

Answer 34

- Replaces primary bone and made up to Haversian systems, or osteons - Calcified matrix is arranged in regular concentric layers of lamellae (3 to 7 nm thick) surrounding a Haversian canal that transmits blood vessels, nerves and some loose connective tissue - Osteocytes in lacunae are located between the lamellae and occasionally within them

Question 35

Contrast gross structure of mature and immature bone

Answer 35

Immature- non-lamellar, no Haversian systems Mature- lamellar, Haversian systems compact outside, spongy inside

Question 36

Contrast collagen fiber arrangement of immature & mature bone

Answer 36

Immature- random/disorganized Mature- parallel, stress oriented (organized)

Question 37

Contrast osteocyte population density of immature and mature bone

Answer 37

Immature- high (many) Mature- relatively low (few)

Question 38

Contrast constitution of immature and mature bone

Answer 38

Immature- low mineral density: high water content Mature- high mineral density: low water content

Question 39

Contrast location of immature and mature bone

Answer 39

Immature- embryo, newborn, fracture callus, growth plate Mature- develops from 1-month post natal

Question 40

What are the types of secondary (mature) bone?

Answer 40

1. Compact bone- very dense and heavy 2. Spongy (cancellous) bone - filled with spaces that are interconnected - In long space filled with red (containing hemopoietic cells) or yellow (containing primarily fat) bone marrow

Question 41

What are the characteristics of Haversian canal?

Answer 41

Functional unit= Haversian system or osteon -cylindrical and composed of 4 to 20 concentric lamellae Because bone is constantly being remodeled, the Haversian systems are NOT all uniform and in parallel array - Have numerous canaliculi that permit communication between lacunae and with the Haversian canals - Each lamellae consists of multiple parallel arrays of collagen fibers - adjacent lamellae, have alternating directions of collagen fibers providing great strength despite its light weight

Question 42

Describe the outer & inner circumferential lamellae of the compact bone

Answer 42

- deep to the periosteum the lamellae are arranged in concentric rings- outer circumferential lamellae - Similar arrangement of lamellae can be found next to the endosteum-inner circumferential lamellae - interstitial lamellae are remnants of old remodeled osteons between newly laid down osteons

Question 43

Describe volksmann canal of the compact bone

Answer 43

Connect adjacent Haversian canals to each other and to endosteum at the marrow cavity on the inside and periosteum on the outside Allow for passage of neurovascular urel through compact bone

Question 44

Where are inner and outer circumferential lamellae found?

Answer 44

These follow the entire inner and outer circumferences of the shaft of long bones Both perforated by blood vessels and nerves as they access Volkmann’s canals and Haversian canals OCL lined by periosteum ICL lined by Endostium