L14: Bone

Question 1

Characteristics of bone

Answer 1

• Specialized CT
• Mineralized living tissue – has cells, vessels and nerve fibers
• Continuously being remodeled
• Serves as storage depot for minerals
• Contains blood cell-forming tissue – bone marrow
• Supports and protects structures
• Pressure to bone results in resorption of bone (breakdown)
• Tension on bone results in deposition of bone (addition)

Question 2

Gross morphologic characteristics of long bones

Answer 2

• Epiphyses refers to bulbous ends of long bone – covered by hyaline cartilage
• Epiphyseal plates are present adjacent to the epiphyses, consist of hyaline cartilage and allow bone to grow. Disappears around 18-20 years of age.
• Metaphysis is angulation bw epiphyseal plate and diaphysis
• Diaphysis is long cylindrical shaft between two epiphyses
• Core of long shaft = medullary (marrow) cavity
• Outer dense bone referred to as compact bone
• Inner sparse bone referred to as cancellous/spongy bone

Question 3

Structure and function of membranes of bone

Answer 3

1. ) Periosteum = outer CT membrane of bone. Covers outer surface of bone except articular surfaces and where tendons attach to bone.
   - Inner layer = cellular layer
   a. ) In active growing bone, this layer contains osteoprogenitor cells which differentiate into osteoblasts to deposit bone on bone surface
   b. ) In bone not actively growing, this layer contains periosteal cells. These may differentiate into osteoblasts if necessary to repair bone
   - Outer layer = fibrous layer
   a. ) Consists of dense collagenous CT (contain fibroblasts). Contains blood vessels, lymphatic vessels and nerves. Blood vessels and nerves perforate and supply bone. Outer layer is anchored into bone via bundles of collagenous fibers (known as Sharpey’s fibers)
2. ) Endosteum = inner CT membrane of bone. Covers bony spicules and trabeculae of spongy bones, lines bone marrow cavity, lines Haversian and Volkman canals
   - Consists of thin CT layer: collagen, fibroblasts with VAN bundle and inner single row of osteoprogenitor cells. These differentiate into osteoblasts or endosteal cells (bone-lining cells)

Question 4

Organic and inorganic components of ECM of bone

Answer 4

1. ) Organic component: fibers (type I collagen) and ground substance (proteoglycans, growth factors: BMP and glycoproteins: osteonectin, sialoproteins, osteopontin, osteocalcin)
2. ) Inorganic component (aka minerals): hydroxyapatite crystals, bicarb, citrate, mag, sod, potas

Question 5

Origin, structure and function of osteoprogenitor cells, osteoblasts, bone-lining cells and osteocytes

Answer 5

*Mesenchymal stem cells – osteoprogenitor cells – osteoblasts – bone line cells and osteocytes

1. ) Osteoprogenitor cells: located in inner layer of mature periosteum (called periosteal cells here) and in endosteal cells, star-shaped, give rise to osteoblasts or chondrogenic cells (in low o2 tension environment), active during bone growth
2. ) Osteoblasts: cuboidal, columnar shaped, basophilic (rER), synthesize and secrete organic component of bone matrix known as osteoid (pre-bone – as non-mineralized), become enclosed within matrix and then are called osteocytes, use gap junctions to communicate with other cells of bone
3. ) Bone-lining cells: cover bone surface, protecting it from osteoclasts, nutritionally support osteocytes
4. ) Osteocytes: grown-up osteoblasts, maintain bone matrix, enclosed in lacuna, their cytoplasmic processes are enclosed in canaliculi, which radiate from lacuna, surrounded by osteoid which they synthesize (pre-bone)

Question 6

Origin, structure, zones and function of osteoclasts

Answer 6

• Osteoclasts arise from mononuclear hemopoietic progenitor cells in the bone marrow
• Structure: multinucleated, motile, acidophilic (d/t lysosomes containing acid phosphatase), very large cells
• Function: resorb, remodel bone
• Lie in enzymatically depressed areas called Howship’s lacuna on the bone surface
• Osteoclast in process of breaking down bone has 3 distinct regions:
  a. ) Ruffled border: where bone resorption is occurring, cell membrane has thickened infoldings that increase surface area, site of exocytosis of hydrolytic enzymes and protons to dissolve inorganic part of matrix, site of endocytosis of broken down bone and other products
  b. ) Clear zone: lacks organelles, actin microfilaments and ring, is sealing zone that isolates acidic corrosive resorption compartment from surrounding area
  c. ) Basolateral region: contains organelles, multiple nuclei, GA, some rER and mitochondria, functions in exocytosis of broken-down matrix

Question 7

Structure of spongy bone and compact bone

Answer 7

1. ) Spongy/cancellous bone: bony trabeculae and spicules protrude from inner surface of compact bone toward marrow cavity, in skull cap forms diploë, lamella (layers of bone) present, no Haversian systems in bone spicules, but Haversian systems present in more prominent bone trabeculae
2. ) Compact bone: shell around exterior of long bones, outer and inner table with diploë between in skull, contains Haversian systems (aka Osteons)

Question 8

Microscopic structure of primary and secondary bone

Answer 8

1. ) Primary (aka immature, woven, bundle) bone:
   - forms during fetal development and during bone repair
   - irregular/interlacing collagen bundles
   - less mineralized than mature, so weaker
   - temporary tissue, resorbed by osteoclasts and then replaced by mature
2. ) Secondary (aka mature, lamellar) bone:
   - consists of parallel, concentric lamellae or wafers
   - collagen fibers are parallel within a lamella
   - more mineral content than primary, so stronger
   - exists as spongy or compact

Question 9

Describe lamellar systems of compact bone, including structure of Haversian canal system

Answer 9

1. ) Outer lamellae
   - deep to periosteum, surrounds bone like growth rings of tree, contains Sharpe’s fibers, which anchor periosteum to underlying compact bone
2. ) Interstitial lamellae
   - lamellar fragments, remnants of former osteons
3. ) Haversian canal systems (aka osteons)
   a. ) Haversian canal: vascular space encloses VAN, no lymphatics – only present in periosteum
   b. ) Consists of concentric lamellae containing collagen fibers helically-arranged
   c. ) Lamellae contain osteocytes in lacunae, osteocytes within system extract nutrients via canaliculi from their respective Haversian canal
   d. ) Volkmann canals are obliquely oriented canals that connect osteons that are next to each other (these do not have concentric lamellae)
   e. ) Note that bone grows toward and around an existing vessel, ie. from outside inward. Blood vessels cannot penetrate/perforate bone once mineralized

4.) Inner circumferential lamellae: present around marrow cavity

Question 10

Describe ankylosis

Answer 10

• Condition in which two articulating bones fuse obliterating joints so there is no movement

Question 11

Describe RA

Answer 11

• autoimmune disease that attacks synovial joints, damaging articular cartilages, producing disfigurement of the joints and severe pain

Question 12

Describe gouty arthritis

Answer 12

• accumulation of uric acid crystals in joints of fingers and toes (especially), crystals cause excruciating pain

Question 13

Describe rickets

Answer 13

• disorder resulting from calcium deficiency or inadequate vitamin D (necessary for calcium absorption), osteoid does not mineralize
• in adults, this deficiency = osteomalacia or bone softening

Question 14

Describe osteoporosis

Answer 14

• reduction in bone mass both in organic and inorganic components
• breakdown of bone exceeds reformation, resulting in porous bones that fracture and break easily