Histology of Bone
Biphasic tissue composed of a mineralized organic matrix (largely collagenous) and at least 3 specialized cells.
provides ideal hardness, flexibility and tensile strength
is a living tissue capable of:
- Framework that gives body shape
- Attachment site upon which muscles act (making movement possible)
- Protect internal organs and viscera
- Mineral Homeostasis and storage: 99% of bodyís Ca,; 85% of phosphorous; 65% of bodyís Na and Mg
- Hematopoesis: houses bone marrow.
- modeling: formation and resorption that reshapes bone in response to physical or biological stimuli.
- growth: modeling predominated by bone formation or endochondral ossification at growth plate.
- remodeling: continual breakdown and replacement of new bone matrix (osteoid) at maturity.
- axial Skeleton = vertebra, skull, ribs, sternum hyoid, acral (extreme peripheral axial)
- peripheral skeleton (appendicular) = limbs, pelvis clavicle, scapula
- Tubular = long and short bones (i.e. tibia, femur, humerus)
- Flat = skull, ileum, scapula, mandible
Gross and Hystopathologic Structures:
Epiphyseal plate = growth plate
Metaphysis: zone of endochondral ossification
Periosteum: important in the appositional (layering) growth and repair medullary cavity
Endostium: has a higher level of remodeling than the periostium and is important in bone turnover
- Endochondral (typical of long bones): formation preceded by cartilaginous analge (mesenchymal) which is dissolved then removed by osteoclasts, allowing for the in growth of blood vessels and osteoprogenitor cells.
- Primary center of ossification: the periostium produces osteoprogenitor differentiation into oseoblasts that deposit the cortex of the midshaft of long bone.
- Secondary center of ossification: the expansion of primary ossification creates 2 centers of endochondral ossification in both epiphysis of long bones: the secondary centers of ossification
- Growth Plate: the plate of cartilage that becomes trapped between the 2 centers of ossification.
- generates cancelous bone and elongation of long bone.
- Intramembranous: (typical of flat bones) : bone formation directly from a fibrous layer of tissue that was derived from mesenchyme. In more mature bone, this also takes place along the periostium of the diaphysis of long bone.
Medullary cavity: contains bone marrow
Material Properties: Type I collagen Þ tensile strength Mineral Þ compressive strength therefore lack of Vit D or Sun Þ Rickets
Þ strength (lifts axillary load); predominates in long bone; 80% of skeletal mass
- lines the inner surface of cortex and cancellous bone
Cancellous Bone: (trabecular or spongy bone) :vertebrae and pelvis (axial skeleton); 20% of skeletal mass
- Has Haversian system (aka cortical osteon) made of lamellae arranged in concentric layers along a cental (haversian) canal containing blood vessels
- cortical osteon is a Bone Structural Unit (BSU) that represents the end result of cyclic remodeling
- Volkmanís Canals radially connect haversian canals.
- Lamellae; Concentric = column of osteon; Interstitial = hemi osteons in between osteon; Circumferential = outer border
- Lacuna: space that osteocyte resides in. Lacunae are connected to one another by canaliculi, allowing neighboring cells to extend cytoplasmic processes and communicate by gap junctions
Patterns of collagen/bone arrangement:
Lamallar bone: the mature pattern of bone (both compact and cancellous)
- Metabolically more active than compact bone
- Honeycomb pattern ensures maximal adaptation to a variable pattern of stress.
- No Haversian system
Woven bone: the immature pattern of primary bone.
- plywood like arrangement: fibrils in alternating orientation
- provide mechanical strength.
- disorganized fibril arrangement
- physiological in embryo and growth plate (replaced by cancellous bone in growth plate)
- pathological in fracture callus, codmanís triangle, tumor bone or states of high turn over
- is normally resorbed and resynthesized as lamellar
Cells: osteoprogenitor cells (pluripotent mesenchymal stem cells) Þ osteblstast Þ (lay down matrix) Þ osteocytes
Extracellular organic matrix Ė glycogen ground substance and collagen fibers (mostly type I)
- Osteoclasts (resorptive cells) derived from hematopoietic progenitor cells of the same lineage as monocytes and macrophages
Inorganic salts: calcium hydroxyapatite.
- Osteoid is unmineralized bone.