Bone Growth and Development

• many fibers aligned parallel to eachother
• strong, little flexibility
• relatively few cells, non-vascular

• few fibers, aligned randomly
• highly porous
• flexible
• relatively few cells, non-vascular

• strength and flexibility varies with type and abundance if fibers
• relatively few cells
• fibers aligned randomly
• non-vascular

• a connective tissue sheath made up of a double layer of cells surrounding the bone
• loosely adheared when growing, tightly adhered when no longer growing

• bridge between the bone and other connective tissues
• proliferation
• nutrition
• source of osteogenic precursor cells (very inside layer)
• some growth restriction properties

connective tissue sheath made up of a single layer of cells which lines the inner surface of the bone

• nutrition
• sources of osteogenic precursor cells

• highly specialized form of connective tissue
• very hard; somewhat flexible, able to withstand stress
• dynamic tissue -10% of bone is replaced every year
• exhibits regenerative powers

• support
• protection
• contains bone marrow
• maintains mineral homeostasis
• locomotion

• physical activity
• dietary calcium
• heredity
• oral contreaceptives
• age

• 20% water
• 45-50% inorganic matterial
• 30-35% organic matter; 90-95% collagen

• aka cortical
• covers the surface of all bones
• dense
• very strong
• no cavities
• Haversian system
• collagen fibers arranged in parallel to eachother in helical formation but perpendicular to the next lamella; gives strength to bone

• central canal where blood vessels and nerves are contained
• lined with endosteum
• younger canals are larger (due to inward growth)

• aka spongy, trabecular
• consists of a network of fine, interlacing trabeculae
• has areas with numerous interconnecting cavities
• typically surrounded by cortical bone
• bone marrow is located in the spaces between the trabeculae
• organization is dependent on the mechanical stress put on the bone
• able to withstand less stress then cortical bone

• Lamelia: layers of calcium deposits surrounding osteocytes
• Osteocytes: bone cells
• Lacuna: space where osteocytes reside
• Canaliculus: small canals connecting lacuna and osteocytes

• aka porus bone
• characterised by low bone mass and structural deterioration
• bone fragility and increased susceptibility to fractures

• Osteoprogenitor cells: source of new cells
• Osteoblasts: builders of bone
• Osteocytes: housekeepers of bone (in lacunea)
• Bone lining cells: surface protection
• Osteoclasts: bone demolition

• source of new bone cells
• located in periosteum and endosteum
• proliferation and renewal (numbers decrease with age)
• differentiate sequentially into osteoblasts, osteocytes, and bone lining cells

• responsible for the formation of new bone: 1. synthesize the organic matrix of the bone 2. regulate the mineralization of bone
• secrete osteoid in one direction (toward an existing hard surface)
• cuboidal cell
• one-cell thick layer loacted exclusively at the surface of bone (interior and exterior)
• don't proliferate
• Communicates with other osteoblasts, osteocytes, osteoclasts, and non-bone cells
• respond to growth factors and hormones from other cells
• capable of synthesizing growth factors

• house keepers
• osteoblasts that have become trapped in the mineralized matrix of bone (differentiate into osteoclast)
• most abundant type of cell in mature bone
• maintain mineralized matrix
• involved in the exchange of mineral ions between matrix and extracellular fluid
• detection of strain (through changes in electrical current)

• not a part of the periosteum (just underneath)
• thin, flat cells covering most bone surfaces in adult bone (surfaces not undergoing formaion or reabsorption)
• serve as a selective barrier
• When remodeling is required: secrete collagenase, which removes unmineralized collagen, allowing for osteoclast attachment

• Within the lacunae
• in contact with each other through canaliculi

• strain magnitude
• duration
• frequency
• history type (i.e. compression, tension, shear)
• distribution

• demolition cells
• arise from a different type of cell than the rest of the bone cells
• travel to bone via the bloodstream
• differentiate and fuse to form osteoclasts
• large, multinucleated cells w/a ruffled border
• must sit on a mineralized matrix
• reabsorb the mineralized matrix of the bone
• erode bone with enzymes and acid
• create pits which lead to a new Haversian canal

• secreted when blood Ca levels are high
• acts directly on osteoclasts -inhibiting bone reaborption
• increases renal excretion of Ca and P

• secreted when blood Ca levels are low
• increases intestinal Ca aborption
• Mobilizes Ca and P from bone
• increases active vitamin D in kidney to decrease P
• maxamizes reaborpion of Ca by kidney
• increase number and activity of osteoclasts

intramembraneous and endochondral

• Transformation of soft connective tissue into bone
• Mesenchymal cells become closely packed within a soft connective tissue layer
• Cells differentiate into osteoblasts at a number of different sites simltaneously
• Osteoblasts at each site make new bone
• Ossification centers (sites of new bone) merge together
• Areas of assification spread out toward each other and eventualy meet

replacement of cartilage with bone

• aka soft spots
• regions where the connective tissue has not been ossified

• replacement of cartilage with bone
• bone collar growth (bone collar formation = intramembranous)

specilized form of connective tissue

• Chondrocytes: house keeping cells -located within the cartilage matrix
• Chondroblasts: builders of cartilage
• Chondroclasts: demolition of cartilage

• on the surface by the recruitment of chondroblasts
• interstitially by proliferation of the chondrocytes which separate and continue to synthesize matrix

connective tissue sheath which surrounds cartilage

• hypertrophy and calcification of cartilage model
• osteogenic bud penetration into spaces left by degenerating cartilage model bring in osteoblasts to form bone

• mesenchymal cells condense/secrete substances forming a cartilage model
• perichondrium (connective tissue sheath around the cartilage model) is formed
• intramembranous ossification of the inner layer of perichondrium occurs forming a bone collar around thediaphysis shaft of the cartilage model; (should completion of the ossification fail to take place, cartilage growsbetween the defect to produce an exostosis; the perichondrium of the shaft develops osteogenic potential andassumes the role of the periosteum.
• by definition the perichondrium is now a periosteum
• diffusion of nutrients are blocked by the bone collar – resulting in starvation/death of the chondrocytes
• the chondrocytes of the cartilage model begin to enlarge greatly: they swell through a process of fluid intake;they reabsorb their surrounding matrix, causing an enlargement of the lacunas; The intercellular substancebetween the swollen cells then decreases, leaving only slender perforated trabeculae of cartilage matrix; thechondrocytes lose their ability to maintain the surrounding matrix, calcium deposits form and the cartilage thatis left becomes calcified.
• the chondrocytes degenerate and leave large interconnecting spaces
• osteoclasts from the periosteum on the bone collar erode holes in the bone collar, through which bloodvessels and osteogenic precursor cells (from the periosteum; the periosteum is highly heterogeneous andcontains cells in all transitional stages of osteogenic differentiation,) arrive (forming an osteogenic bud)
• osteogenic precursor cells penetrate the calcified cartilage matrix, proliferate and give rise to osteoblasts andbone marrow cells
• chondroclasts resorb calcified cartilage
• osteoblasts deposit osteoid on the calcified cartilage remnants; osteoid becomes ossified: primary bonedevelops
• osteoclasts and osteoblasts remodel the interior of the bone

• occurs at the end of the bone (radial expansion)
• found in epiphysis (has no perichondrium)
• osteogenesis begins at specific sites and coalexce into an ossification center

• primary ossification center occurs first
• secondary ossification center occurs second
• distal portion of the long bone contributes 60% of the total length whereas proximal portion contributes 40%

• Resting: or reserve zone -chondrocyte stem cells
• Proliferative zone: chondrocytes increase in number in stacks
• Hypertrophic zone: chondrocytes increase in size, surrounding cartilage becones calcified
• Degenerative zone: chondrocytes undergo apoptosis, the cartilage becomes calcified and forms the scaffolding for osteoblasts
• Ossification zone: endochondral bone tissue is formed by osteoblasts brought by the encroaching diaphyseal marrow tissue

• A) resting
• B) proliferative
• C) proliferative
• D) hypertrophic
• E) bone formation

• the rate of division of the chondrocytes
• the size of the proliferation zone
• the degree of cellular hypertrophy

• Systemic factors: genetics, nutrition, hormones, etc
• Local factors: physical stress, blood supply, mineralization of osteoid

• increased osteoblast proliferation
• increase osteoblast differentiation

• immature
• collagen is randomly oriented
• less mineral
• fewer osteocytes
• temporary (replaced by secondary bone in adult or after repair)

• mature bone
• lamellar configuration
• trabeculae
• Haversian
• more mineral
• more osteocytes

• reduction (bring bone pieces together)
• fixation
• use

• Inflammatory: immune system respince to injury
• Reparitive: repairs typically exceed original size
• Remodeling: functional adaptation

• blood vessels associated with the Haversian system are damaged
• periosteum is often torn
• soft tissue is often damaged
• a hematoma forms within the bone and beneath the periosteum
• osteocytes die

• callus forms: mass of new bone forming around the periphery of fracture
• no callus formed under condition of compression and rigid-fixation

• fracture fragments
• movement between fragments

• removal of bone from a site
• addition of bone to a site
• facilitated by motion of adjacent joints and use of extremity
• final bone will closely resemble the original structure