Bone tissue

The musculoskeletal system is comprised of bones and connective tissue structures, such as cartilage, ligaments, and tendons. These structures are brought into motion by skeletal muscles. To withstand resultant forces, bone tissue resists pressure and tension and is minimally elastic. Bone tissue mainly consists of bone cells (osteoblasts, osteocytes, and osteoclasts) and a mineralized extracellular matrix that is primarily made up of collagen fibrils and hydroxyapatite crystals. Ossification, or bone formation, begins with a framework that consists of either mesenchymal connective tissue (intramembranous ossification) or cartilage (endochondral ossification). Woven bone is formed, which is replaced by the more solid and layered lamellar bone. The outer cortical layers can be macroscopically differentiated from the branched center of the trabeculae.

Function of bone

• Supportive function
• Protective function
• Storage (calcium and phosphorus reservoir)
• Hematopoiesis

Types of bone

• Long bones: include the femur, humerus, ulna, radius, fibula, tibia, metacarpals, and phalanges
  • Epiphysis
    • Consists of compact bone that surrounds trabecular bone
    • Proximal or distal end of a long bone with its trabeculae aligned along the lines of stress
    • Linear growth of long bones in children and adults occurs in the epiphysis (epiphyseal plate).
    • Contains the articular surface of bones
  • Metaphysis: area between the epiphysis and the diaphysis
  • Diaphysis
    • Shaft of a long bone (central region)
    • Consists of compact bone and the medullary cavity where the bone marrow is stored
  • Apophysis: large bony projections where ligaments and tendons attach
• Short bones: include tarsal and carpal bones
• Flat bones
  • Include the scapulae, sternum, ribs, and most bones of the skull
  • Composed of two layers of compact bone that surrounds trabecular bone and bone marrow
• Sesamoid bones
  • Bones that develop within tendons (e.g., the patella)
  • Function to reduce friction of the tendon and to reduce excessive wear
• Irregular bones
  • Include the sacrum, coccyx, vertebrae, mandible, palatine, hyoid, and temporal bones
  • Have several functions including:
    • Attachment for the tendon of muscles
    • Protection (e.g., vertebrae protect the spinal cord)

References:^[1][2]

Overview

The exact composition or organization of individual bone components differs in the various bones types and maturation stages. All human bones are composed of the same basic elements:

• Bone cells: build and remodel bones
• Bone matrix: composed of organic and inorganic components
• Bone membranes: cover the inner and outer surface of the bone
• Bone marrow: either actively involved in hematopoiesis (red bone marrow) or primarily replaced by adipose cells (yellow bone marrow)

Bone cells

Bone matrix

Composed of organic and inorganic material:

• Organic
  • Type 1 collagen
  • Ground substances: chondroitin sulfate and keratan sulfate
  • Glycoproteins (e.g., osteocalcin, osteopontin) and proteoglycans: bind to hydroxyapatite and to integrin molecules on osteoclasts and osteoblasts
• Inorganic
  • Hydroxyapatite: hydroxylated calcium phosphate salts that adhere to collagen fibrils in a crystallized form and increase resistance to pressure
  • Various ions (e.g., calcium, phosphate, magnesium, bicarbonate, citrate, potassium, and fluoride)

Bone membranes

• Periosteum: a membrane of connective tissue that covers the outer surface of the bone in two distinct layers
  • Cambium layer: inner vascular and innervated layer (very sensitive to pain) that is lined with bone cells
  • Fibrous layer: outer layer consisting of connective tissue from which Sharpey fibers extend into the bone
• Endosteum
  • A membrane that lines the inner surface of the bone (e.g., trabecular bone or Haversian canal)
  • Composition: thin (nonmineralized) layer of collagen lined with bone cells

The periosteum and the endosteum consist of the same type of bone cells.

Bone marrow

• Red bone marrow (hematopoietic): primarily in the short and flat bones and partly in the epiphyses of the long bones in adults
• Yellow bone marrow (fat storage): primarily fills the medullary cavities of the long bones in adults
• For more details on bone marrow, see “Basics of hematology”.

Development of bone

• Bone is derived from mesoderm.
  • Axial skeleton arises from paraxial mesoderm.
  • Appendicular skeleton arises from the lateral plate mesoderm.

Ossification

The skull undergoes both processes: some bones (e.g., frontal, parietal bones) are derived from the neural crest and undergo membranous ossification, whereas other bones (e.g., sphenoid, occipital bones) are derived from the paraxial mesoderm and undergo endochondral ossification.

Stages of bone maturity

Bones are arranged into woven bone (primary bone) during embryonic development or bone healing. The structure of woven bone is disorganized and transformed into organized tissue of lamellar bone (secondary bone) through continuous remodeling.

The direction of collagen fibers of the bone extracellular matrix is an important distinguishing characteristic between immature woven bone and mature lamellar bone.

Trabecular bone (spongy or cancellous bone)

• Definition: thin lattice-shaped (trabeculae) units inside the lamellar bone
• Characteristics
  • Trabeculae are aligned along large compressive and tensile forces (trajectory direction).
  • The bone marrow is located in the intermediate space of the trabecular bone.

Cortical bone (compact bone)

• Definition: homogeneous and dense cortical layer of lamellar bone
• Main components
  • Osteon
    • Definition: concentric bony lamellae with a central Haversian canal
    • Direction: several concentric lamellae whose collagen fibers change direction from one lamella to the other
    • Boundary: cement lines
  • Interstitial lamellae
    • Remnants of partially destroyed osteons
    • Separate osteons from one another
  • Circumferential lamellae: provide an outer and inner boundary for the cortical bone through at least one lamellar layer

Lamellar bones vascularization and canals

• Bone canals and associated vessels
  • Haversian canal: a canal in the center of the osteon that contains Haversian vessels, which supply the bone with blood
  • Volkmann canal (perforating channels): canals radiating from the periosteum that are at a right angle to Haversian vessels and are supplied by Volkmann vessels

The course of lamellar bone vessels is strictly defined by the Haversian and Volkmann canal structures. In contrast, woven bone vessels are disorganized.

Development of long bones

• Primary ossification center: diaphysis
  1. Bone collar formation
  2. Endochondral ossification
• Secondary ossification center: epiphysis
  • Time period: shortly before or after birth
  • Process: endochondral ossification, similar to diaphyseal
  • Nonossifying segments
    • Joint surfaces
    • Epiphyseal plate
• Longitudinal growth of the long bones
  1. Chondrocyte proliferate at the epiphyseal plates → longitudinal growth with the epiphysis pushed away from the diaphysis
  2. Cartilage tissue is degraded and remodeled in bone tissue from the medullary cavity.
  3. The proliferation zone progresses with the same velocity as the ossification zone.
  4. Chondrocytes cease proliferating and bone replaces cartilage (ossification zone) in a process known as epiphyseal fusion.

Epiphyseal plate

The part of the bone where longitudinal growth takes place. Layers include (from epiphysis to diaphysis):

1. Zone of resting cartilage: deposits undifferentiated precursor chondrocytes that provide the proliferation zone with new chondrocytes
2. Proliferation zone: mitosis of chondrocytes
   • Proliferation process
     • Isogenic chondrocytes are vertically stacked on top of one another (each chondrocyte lies in a separate lacuna).
     • In the column, the chondrocytes mature in the direction of growth.
     • Chondrocytes enlarge and produce an extracellular matrix.
   • Chondrocyte separation
     • Vertical (from an isogenic neighbor): transverse septum
     • Horizontal (from nonisogenic neighbors): longitudinal septum
3. Zone of hypertrophy: enlarges cartilage through chondrocyte hypertrophy, which leads to collagen (type X) production by hypertrophic chondrocytes and mineralization of the longitudinal septa
4. Zone of calcification
   • Functions
     • Opening of the cartilaginous lacunae by removal of the transverse septa
     • Apoptosis of hypertrophic chondrocytes
   • Process
     • Secretion of VEGF and matrix metalloproteinases by chondrocytes
     • Blood vessels and macrophages migrate → transverse septa erode
   • Effects
     • Mineralized columns remain (ossified longitudinal septa).
     • Approx. ⅓ of the longitudinal septa is retained, and the remaining septa is degraded by chondroclasts.
5. Zone of ossification: colonization of the mineralized longitudinal septae by osteoblasts → osteoid formation → mineralization

References:^[5]

The human skeleton is in a continuous dynamic state of remodeling. Not only does this apply to the replacement of immature woven bone by lamellar bone, but also for adaptation of adult bones to their individual load.

Bone remodeling ^[6]

• Cells involved
  • Osteoclasts: degrade bone tissue by secreting collagenase and H⁺
  • Osteoblasts
    • Build bone tissue by secreting type I collagen
    • Activity assessed by an increase in bone ALP, osteocalcin, and type I procollagen propeptides
• Duration: usually longer than the lifespan of the cells involved (continual replacement of involved cells)

Blasts Build, Clasts Crumble.

Bone remodeling in cortical bone

• Degradation
  • Osteoclasts organize in a basic multicellular unit (BMU) and excavate a tunnel in the cortical bone.
  • Connective tissue vessels and unmyelinated nerves grow in the tunnel.
• Formation
  • Osteoclasts are followed by osteoblasts → deposition of the first osteoid layer in the tunnel
  • Additional osteoblasts follow and deposit osteoid onto the first osteoid layer → osteoblasts of the first layer are walled in → osteoblasts become osteocytes
  • The deposition process is repeated until the tunnel is almost full → central Haversian canal remains open
  • The innermost (i.e., last) generation of osteoblasts is no longer walled in → cells return to their resting state and form the endosteum
• Mineralization: occurs successively
  • Osteoblasts secrete collagen and vesicles into the extracellular matrix.
  • Vesicles contain enzymes (e.g., alkaline phosphatase), which increase local phosphate levels (e.g., by cleavage of pyrophosphate).
  • Calcium-binding molecules in the vesicles most likely serve as a focal point.
  • Initial formation of hydroxyapatite crystals around the focal point in the vesicles
  • Independent growth of the crystals until penetration of the vesicle membrane
  • Release of crystals in the extracellular matrix
  • Growth of crystals in the extracellular matrix and accumulation of collagen fibrils

Bone remodeling in trabecular bone

• Degradation
  • Osteoclasts organize in Howship lacunae (small depressions on the trabecular bone surface). They move to resorb trabecular bone and form a tight seal around the resorption area.
  • Osteoclasts produce protons via the enzyme carbonic anhydrase.
  • Secretion of chloride ions (passive) and protons (active, via ATPase) in Howship lacunae, with the formation of an acidic environment (∼ pH 4.5) → dissolution of inorganic bone elements
  • Secretion of lysosomal enzymes (especially cathepsin K and matrix metalloproteinases) → degradation of organic bone elements
  • Endocytosis/transcytosis of the bone elements
• Formation
  • Filling of the gaps with groups of osteoblasts → walled in under each lamella (then termed osteocytes)
  • The last group of osteoblasts remains as a part of the endosteum on the bone surface.

Regulation of bone remodeling ^[7]

• RANK (receptor activator of nuclear factor κB): receptor on osteoclasts and osteoclast precursors, for interaction with osteoblasts
• RANKL (receptor activator of nuclear factor κB ligand)
  • Membrane-bound protein of osteoblasts that stimulates osteoclasts by interacting with RANK
  • Ensures fusion and differentiation into activated osteoclasts and prevents their apoptosis
• Osteoprotegerin (OPG)
  • A regulatory protein secreted by osteoblasts that binds RANKL
  • Inhibits RANK-RANKL interaction, leading to decreased osteoclast activity
• M-CSF (macrophage colony-stimulating factor)
  • Secreted by osteoblasts
  • Promotes the proliferation of osteoclast precursor cells
  • Simultaneous binding and fusion of M-CSF and RANKL on the surface of precursor osteoclasts on the synovium → differentiation of precursor osteoclasts into osteoclasts
• Mechanical load
  • Load on the bone leads to increased bone mass.
  • Absence of load (e.g., due to being confined in bed) results in decreased bone mass.
• Negative feedback by osteoclasts: growth factors are embedded in the bone matrix and are released during degradation by osteoclasts → osteoblasts stimulation
• Hormones
  • PTH effects
    • At low levels: increased bone formation and increased apoptosis of osteoclasts → decreased bone resorption (anabolic effects)
    • At high levels (e.g., primary hyperparathyroidism): activation of osteoclasts → increased bone resorption (catabolic effects) → osteoporosis or osteitis fibrosa cystica
  • Estrogen effects
    • Inhibits apoptosis of osteoblasts, leading to increased bone formation
    • Stimulates apoptosis of osteoclasts, leading to decreased bone resorption
    • Stimulates closure of the epiphyseal plate in puberty
    • Estrogen deficiency (e.g., postmenopausal or after bilateral oophorectomy) leads to increased bone resorption, which can result in osteoporosis.
• Vitamin D
  • Stimulates osteoblasts and osteoclast differentiation
  • Activation of osteoblasts → increased bone formation and mineralization
• Medication
  • Bisphosphonates inhibit osteoclasts → used to treat osteoporosis
  • Glucocorticoids increase osteoclast lifespan → increased bone resorption

Estrogen has a positive effect on bone balance because it inhibits osteoclast formation and activation and increases OPG formation.

Bone healing ^[8]

Fractures occur when bones are strained beyond their maximum load. Fractures can heal in two different ways.

• Primary bone healing
  • Occurs when the broken ends of the bone are very close together (interval < 1 mm), e.g., when surgically fixed
  • Lamellar bone can be directly formed in the fracture gap (gap healing).
• Secondary bone healing
  • Occurs when the distance of the fracture ends is larger
  • Initial bridging of the fracture gap is formed with connective tissue or cartilage (fibrocartilage callus).
  • Conversion to woven bone (bony callus) by endochondral ossification
  • Over the course of months, woven bone is slowly remodeled into resilient lamellar bone.
• Pseudoarthrosis: can occur if the healing process is permanently disturbed by motion of the fracture ends (e.g., through insufficient immobilization)

During childhood or adolescence, a fracture near a joint may damage the unclosed epiphyseal plate, which can result in growth disorders (e.g., asymmetry, inhibition, acceleration) during healing.

References:^[9][10]

• Osteoporosis
• Osteomalacia and rickets
• Paget disease
• Fractures