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Ossification, Bone Growth, Remodeling and Fractures Cell and Tissue Biology John S. Halle, PhD, PT January 25, 2002

Bones Have Structural Strength

Question? When two groups of Leghorn roosters are compared, (one group that jogs regularly and one group that is content to lead the average life of a rooster), is there any difference in the development of bone?

Big Picture Overview Bone development occurs two ways: Intramembranous Endochondral Three basic types of bone exist in the adult Woven (immature) Cancellous (spongy) Compact Growth in length/diameter occurs by either: Appositional growth Interstitial growth Haversian system found in cancellous and compact bone Principal cells involved: Osteoprogenitor cells Osteoblasts Osteocytes Osteoclasts Components of a typical long bone Fracture healing and bone related clinical conditions

Ossification Ossification = the process of bone formation sometimes also referred to as osteogenesis Bone provides the chief supporting tissue of the body

Bone Contrasted with Cartilage Similarities: Both contain populations of living cells that become embedded in a matrix Both have a matrix that is reinforced by collagen Both occupy matrix spaces called lacunae Both possess fibrous connective tissue counterparts Both osteoblasts and chondroblast arise directy from mesenchyme Differences: Bone possesses more collagen and less water Bone has less amorphous substance Bone is more heavily mineralized Osteoblasts differentiate in the vicinity of blood capillaries Bone grows only by apposition Bone can live after mineralization occurs

Development of Bone Intramembranous formation of bone from mesenchyme tissue found in the flat bones of skull, face, mandible and clavicle Endochondral cartilage model serves as the precursor this is how most bones form - extremities, and weight bearing axial skeleton Note: similar final product

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Intramembranous  ossification Begins towards the second month of gestation Capillaries grow into mesenchyme Some cells differentiate into osteoblasts Secretes collagen and proteoglycans of bone matrix Osteoblasts remain attached via cytoplasmic processes Once surrounded with bone matrix, become osteocytes Calcified bone matrix -> spicules and trabeculae of bone are formed

Spicules and calcified cartilage

Intramembranous Ossification Further growth is due to trabeculae extending from the newly formed bone matrix in a radial pattern Osteogenic cells are found on the trabecular surfaces Vascularized - give rise to osteoblasts Non-vascularized - give rise to chondroblasts Osteogenic cells are bipotential & self-renewing Continued appositional growth and bone resorption

Osteoblasts

Osteoblasts and osteocytes

Intramembranous ossification Development of compact bone Compact bone has a higher proportion of bone matrix than soft tissue spaces Osteons are formed by filling in the soft tissue spaces The growing plate of bone undergoes remodeling With remodeling, plates convert from single plate to double plates of compact bone with cancellous bone between

Immature Bone Immature bone (woven bone – may also be called bundle bone) No lamellae Collagen and cells randomly arranged in matrix Matrix has lower mineral content Contains relatively more cells per unit area than mature bone More ground substance is found in immature bone Immature bone grows more rapidly Immature bone found: At fracture sites In aveolar sockets of the teeth – permits orthodontic remodeling Embryo of the newborn Metaphyseal region of growing bone Some tumors and disease states

Mature Bone Mature (lamellar) bone Lamellae - concentric circles (compact) sheets (cancellous) Collagen regularly arranged in lamellae parallel to one another within lamellae Perpendicular to fibers in adjacent lamellae Both compact and cancellous bone are “mature” bone types

Lamellae

Canaliculi

Endochondral ossification Mesenchyme differentiates into cartilage and forms a cartilage model of the developing bone The model is surrounded by a fibrous perichondrium The model increases in length by interstitial growth and width by appositional growth Interstitial growth occurs mostly at the two ends as the center starts to mature

Periosteum

Endochondral ossification Cartilage model assumes the general shape of the bone that will be formed Chondrocytes enlarge, become vacuolated and accumulate glycogen Lacunae enlarge at the expense of the cartilage matrix Remaining cartilage becomes calcified with calcium phosphate

Endochondral ossification Chondrocytes surrounded by calcified matrix die, leaving cavities inside Developing capillaries invade the perichondrium Cells in the inner layer of the perichondrium form osteoblasts and a thin layer of bone is laid down Note: developed from fibrous tissue - this membraneous in nature

Endochondral ossification One or more blood vessels penetrate the midshaft area Initially the periosteal bud Becomes the nutrient artery Cells gather on the remnants of the calcified cartilage and lay down bone on the surfaces - thus, appositional bone is formed The two ends continue to grow as cartilage models and extend each end away from the midshaft

Endochondral ossification Cells closest to the midshaft at each end go through the same processes of maturation, hypertrophy, secreting phosphatase, mineralization of the matrix, and death resulting in more cartilage remnants extending from each end toward the midshaft for more osteogenic cells to line-up and lay down bone

Endochondral ossification Terms: Ossification (osteogenesis): Process of bone formation Calcification: The deposition of insoluble calcium salts in a tissue Ossification can occur without calcification (Rickets), and calcification can occur without ossification (heterotrophic bone formation) Need calcification to clearly see x-ray image (osteoid is difficult to recognize) As osteoblasts are forming bone tissue within the model, the periosteum continues to add more bone to the periphery, making the peripheral cortex stronger.  As a result, the original central bone is no longer necessary and it is resorbed leaving a marrow cavity for hematopoiesis to develop within. This results in the formation of a primary center of ossification The diaphysis

Endochondral ossification Development described thus far occurs within the first 8 - 10 weeks of fetal life Simply continues throughout fetal life, adding length and girth Epiphyseal cartilage Zone of reserve cartilage Zone of proliferation Zone of hypertrophy Zone of calcified cartilage Zone of resorption

Endochondral ossification Secondary centers of ossification occur after birth Some very near birth (distal femur and proximal tibia occur just before birth) Others delayed for years Most long bones have two, but some have just one secondary center

Endochondral ossification Once the secondary center starts, it develops much like the primary center, except: Cartilagenous ring is left at the end of the long bone = articular cartilage Cartilagenous plate is left between the primary and secondary center = physis or growth plate The physis is responsible for permitting growth in length of the bone

Endochondral ossification Description of bone at this point in time: articular cartilage bony epiphysis cartilagenous growth plate = physis bony metaphysis diaphysis metaphysis physis bony epiphysis articular cartilage

Long bone growth As long as physis persists, bone continues to grow: Zone of reserve cartilage Zone of proliferation Zone of hypertrophy Zone of calcified cartilage Zone of resorption This is occurring at both ends, although one end may proceed much more rapidly than the other The new bone forming at the metaphysis does not become longer, due to resorption

Physis

Physis (II)

Physis (III)

Physis (IV)

Diameter growth of bone Increase in diameter is due to appositional growth at the periphery of the diaphysis Osteoblasts arise from osteogenic cells of the periosteum and contribute successive lamellae Compensatory widening of the medullary cavity is also required

Bone diameter growth from new osteons Occurs at a later stage Process Formation of a groove Bone matrix is deposited to extend the longitudinal ridges towards one another The ridges meet and create a tunnel The tunnel is lined by periosteum and thus has osteogenic cells Osteon is formed

Internal reorganization of compact bone

Cessation of Growth in Length In the late teens or early twenties: formation rate is greatly exceeded by resorptive rate proliferation of new cartilage slows resorption from the metaphysis reduces the growth plate width until the diaphysis bone fuses with the epiphysis bone thus, growth ceases in a longitudinal direction

Other important elements associated with bone Periosteum (2 layers): Outer fibrous layer – irregularly arranged dense connective tissue Inner osteogenic layer – contains osteoprogenitor cells Endosteum contains osteoprogenitor cells Osteoprogenitor cells are also found in the region of the Haversian canal Sharpey’s fibers connect tendon to periosteum, then periosteum to bone

Remodeling Bone tissue constantly adapts to prevailing stresses Compact bone is always being resorbed to some extent by osteoclasts Resorbed bone is replaced with new compact bone Lack of time for adaptation = March fracture Bone formation is normally coupled directly to previous bone resorption Aging issue - osteoporosis More bone resorption than deposition Fracture predisposition of hip, wrist and vertebrae Prolonged bedrest can also accelerate bone loss

Other clinical issues Rickets Osteomalacia Heterotrophic bone formation Osteomyelitis Scurvey Radiation Fracture healing

Fracture repair of a long bone Fx creates two “fragments” Torn blood vessels hemorrhage with clot torn Haversian vessels death of osteocytes near fracture site acute inflammatory reaction within 1 or 2 days macrophages clean-up site of injury fibroblasts and capillaries proliferate and grow into the site of injury internal and external callus develops

Fracture repair of long bone Cartilage in callus becomes replaced with bone Bony callus is remodeled Initial trabeculae were cemented to dead bone tissue as well as living tissue Dead bone is removed Callus is a spindle shaped mass of cancellous bone at this point in time The trabeculae growing from each fragment interconnect Internal callus forms a secure bridge of cancellous bone blood supply must be adequate Subsequent remodeling converts the newly formed cancellous bone into dense cortical bone Trabeculae in the periphery of the callus undergo resorption

A four stage system for summarizing fracture repair Stage I: Hemorrhage, cell death (bone necrosis), inflammation Stage II: Soft callus formation – bony fragments are united by fibrous or cartilaginous tissue Stage III: Hard callus formation – conversion of the callus to woven bone (both endochondral and intramembranous bone formation occurs) Stage IV: Remodeling – woven bone is converted to lamellar bone and the medullary canal is reconstituted

Fractures and the epiphysis Salter Harris system Type I: Fracture through the physis Type II: physis and metaphyseal fragment (most common) Type III: Intraarticular Type IV: Intraarticular with metaphyseal fragment (alignment is crucial) Type V: Crushed resting zone of physis (poor prognosis)

Leghorn roosters and running Began running at 3 wks Ran for either 5 or 9 wks Results (exercise group): Suppression of circumferential growth Enhanced middiaphysial cortical thickening Bones were more compliant The strenuous exercise appeared to retard long bone maturation

Questions?