Bone Tissue

Chapter 6

Skeletal Cartilage• Chondrocytes in lacunae in gel-like

ground substance– Ground substance of water & protein

(collagen) resiliency • Avascular & no innervations• Perichondrium (2 layers)– Outer = DICT – Inner = chondroblasts

• 3 types– Hyaline cartilage– Elastic cartilage– Fibrocartilage

Cartilage Growth• Appositional growth– Growth from outside (inner perichondrium layer)– Mitosis produces chondroblasts which secrete matrix

• Form lacunae around selves• Become chondrocytes

• Interstitial growth– Growth from inside (existing matrix)– Mitotic chondrocytes produce daughter cells

• Daughter cells (chondroblasts) produce more matrix• Calcified cartilage when Ca2+ salts deposited ≠

bone

• Axial– Protection and support

• Appendicular– Locomotion and

manipulation

• Storage and hematopoiesis too

Classifying Bones

Bone Shape

• 206 bones (adult)– Long– Short– Flat– Irregular

• Sesamoid and wormian– Not part of 206– Individual variation

Bone Structure• Markings (table 6.1)

– Projections• Grow from bone• Attachment for muscles or form joints

– Tuberosity, crest, trochanter, line, tubercle, epicondyle, spine, process, head, facet, condyle, ramus

– Depressions• Conduits for nerves and blood vessels• Groove, fissure, foramen, notch, meatus, sinus, fossa

– Helps for lab too• Types

– Compact (osteon)– Spongy (trabeculae)

• Red (blood) and yellow (fat) marrow

Long Bone Anatomy• Diaphysis

– (Yellow) marrow cavities• Epiphyses

– Articular surface absorb stress hyaline– Epiphyseal plate hyaline

• Membranes– Periosteum (2 layers)

• Outer = DICT• Inner = osseous cells• Nutrient foramina in diaphysis• Sharpey’s fibers (collagen)

– Endosteum• Covers trabeculae and line marrow cavites• CT with osseous cells

Other Bone Anatomy

• Membranes same as long bones• Diploë• No dia- or epiphysis• (Red) bone marrow, but no cavity

Composition of Osseous Tissue

• Cell types– Osteoprogenitor cells (mesenchyme)– Osteoblasts sectete matrix lacunae– Osteocytes– Osteoclasts

• Matrix– Osteoid (organic)

• Ground substance and collagen fibers• Structure & strength resist stretching and

twisting– Hydroxyapatites (mineral salts)

• Calcium phosphates• Hardness resist compression, but not

stretching and twisting

Bone Physiology• Ossification replaces other tissues with bone– Methods

• Intramembranous: from mesenchyme or fibrous CT (flat bones & jaw)

• Endochondral: replaces hyaline cartilage template (all other bones)

– Forms• Osteogenesis, bone formation embryos, about 6 weeks• Bone growth (length and width) to early adulthood

– Hormonal control (hGH, T3, T4, sex hormones)• Bone remodeling and repair adults

• Calcification deposits Ca2+ salts in a tissue– Possible in any tissue

Intramembranous Ossification• Mesenchymal cells cluster = 1° ossification center– Vacularization signals transformation to osteoblasts– Osteoid secreted, trapping some osteoblasts = osteoclasts– Calcification of osteoid (hydroxyapatite )

• Trabeculae grow from 1° ossification center – Above activities repeated (continuous)– Join around blood vessels (increased rate)

• Spongy bone is remodeled to form diploë– Outer mesenchyme becomes periosteum– Outer trabeculae thicken compact bone– Red marrow fills in trabecular space

Endochondral Ossification• Perichondrium becomes vascularized = perisosteum

– Mesenchyme = osteoblasts– Compact bone formed around cartilage

• Inner chondrocytes enlarge = signals calcification of cartilage– Matrix deteriorates as chondrocytes die = cavities

• Periosteal bud enters cavitites (1° ossification center)– Osteoclasts erode calcified cartilage– Osteoblasts secrete osteoid on remains = trabeculae (spongy)– Osteoclast activity breaks down new spongy bone = medullary cavity

• Chondrocyte growth still occurring – Endochondral ossification follows = increase in length– Ossification chases calcification to lengthen bones

• 2° ossification center after birth– Almost same process repeats but in center of epiphysis– No medullary cavity formed

• Leaves hyaline at epiphyseal plates and articular cartilage

Endochondral Ossification (cont.)

Epiphyseal (Growth) Plates• Resting zone

– No bone growth, looks like hyaline• Proliferative zone

– Tall stacks of chondrocytes– Mitotic division bone lengthens

• Hypertrophic zone– Chondrocytes enlarge = calcification signal

• Calcification/ossification zone– Endochondral ossification occurs– No perisoteal bud, osteoblasts/clasts from

medullary cavity• With age, ossification speeds up and

proliferation of cartilage slows– Growth plates narrow disappears

Bone Growth• Endochondral ossification at

epiphyseal plates increases length• Appositional growth increases width– Perisoteal osteoblasts

– Osteoid added to surface (lamella) – Once surrounded osteocyte

– Endosteal osteoclasts – Break down, resorb, inner surface– Medullary cavity growth

• Growth more than break down– Stronger, thicker, but less heavy bone

Bone Remodeling • Antagonistic processes on both surfaces• Continuously occurring

– Same rates = healthy– Disrupted homeostasis = breaks

• Ca2+ from bone when low– Osteoclasts

• Ca2+ added/redeposited as needed– Osteoblasts

• Bone replacement– Spongy: 3 – 4 years– Compact: about 10 years

Homeostatic Control of Calcium• Blood Ca2+ levels signal• Parathyroid hormone (PTH)– Osteoclast resorption

• Only osteoid is safe no salts• Mechanical stresses influence

– Ca2+ release from bone matrix

• Calcitonin (negligible in adult)– Inhibits resorption– Ca2+ salts deposited into bone

matrix by osteoblasts

• Are these positive or negative feedback examples?

Bone Repair

• Torn vessles cause hematoma = kills cells– Inflammatory response

• Fibrocartilaginous callus splints bone– Phagocytes & osteoclasts clean debris– Fibroblasts, chondrocytes, &

osteoblasts lay down collagen, cartilage, & osteoid to connect ends (endochondral ossification)

• Bony callus forms to solidify bone union– Trabeculae as braces for 2 months

• Remodeling– Excess bone on exterior and in cavity

removed– Compact bone recreates shaft walls

Fractures• Treated by reduction– Closed – physician sets– Open –pins/wires

• Cast immobilizes• Types– Closed (simple)– Open (compound)– Comminuted– Compression– Depressed

– Spiral– Epiphyseal– Transverse– Greenstick

Bone Disorders

• Osteomalacia (rickets): inadequate mineralization of bone• Osteoporosis: bone resporption faster than deposit• Spina bifida: incomplete closure of vertebrae• Achondroplasia: dwarfism; defective cartilage growth and

improper endochondral ossification• Acromegaly: gigantism; growth plate slow to ossify• Scoliosis: lateral curve of spine• Kyphosis: hunckback spine; exaggerated thoracic curvature• Lordosis: swayback; exaggerated lumbar curvature• Cleft palate: palatine bones don’t fuse• Carpal tunnel syndrome: nerve impairment in wrist