BIO 304 · Human Anatomy & Physiology
Bone Tissue & Bone Growth
Skeletal System · Module 5
A reference for the Bone Tissue video. Bone is a living, organ-system-active tissue. It builds, breaks down, and rebuilds throughout your life; it stores calcium and houses the marrow.
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By the end
- Name the four bone cell types and the role of each in bone turnover.
- Distinguish compact from spongy bone by structure and function.
- Contrast intramembranous and endochondral ossification, and locate the epiphyseal plate.
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Bone Cells & Matrix
Four bone cells
- Osteogenic (osteoprogenitor)stem cells in periosteum & endosteum; divide into osteoblasts
- Osteoblastbone builder; secretes osteoid (collagen + ground substance)
- Osteocytemature bone cell trapped in lacuna; maintains matrix
- Osteoclastmultinucleate giant cell; dissolves bone (resorption)
Matrix composition
- Organic (~33%)osteoid: collagen fibers + ground substance · tensile strength
- Inorganic (~67%)hydroxyapatite (calcium phosphate crystals) · compression strength
Compact bone
- Osteon (Haversian system)cylindrical unit of compact bone
- Central canalruns lengthwise; blood vessels and nerves
- Lamellaeconcentric rings of matrix
- Lacunaesmall spaces housing osteocytes
- Canaliculitiny channels connecting lacunae; nutrient diffusion
- Perforating (Volkmann) canalsrun perpendicular; connect central canals
Functions of the skeleton
- Support & shapeframework that holds up the body and cradles soft organs
- Protectionskull guards the brain; ribs guard heart and lungs; vertebrae guard the spinal cord
- Movement (leverage)bones act as levers that muscles pull on across joints
- Mineral storagereservoir for calcium and phosphate, released or stored on demand
- Blood cell formationred marrow makes red and white blood cells and platelets (hematopoiesis)
- Energy storageyellow marrow stores fat (triglycerides)
Bone shapes (classification)
- Longlonger than wide; humerus, femur, phalanges
- Shortcube-like; carpals, tarsals
- Flatthin, often curved; sternum, skull bones, ribs, scapula
- Irregularcomplex shape; vertebrae, hip bones
- Sesamoidforms within a tendon; patella
Long bone gross anatomy
- Diaphysisthe shaft; compact bone wall around the medullary cavity
- Epiphysiseach end; spongy bone under a thin compact shell
- Metaphysisregion between diaphysis and epiphysis; holds the growth plate
- Medullary cavityhollow center of the shaft; yellow marrow in adults
- Articular cartilagehyaline cartilage on the epiphysis tips; cushions joints
- Periosteumtough outer membrane; anchors tendons, carries vessels
- Endosteumthin membrane lining the medullary cavity
- Nutrient foramenopening where blood vessels enter the bone
Growth & Remodeling
Spongy (cancellous) bone
- Trabeculaelattice of bony struts; aligned along stress lines
- No osteonsosteocytes nourished by diffusion from marrow spaces
- Houses red marrowhematopoiesis — blood cell production
Ossification
- Intramembranousbone forms directly from mesenchyme · flat skull bones, clavicle
- Endochondralcartilage model is replaced by bone · most bones
- Primary ossification centerin diaphysis (shaft); forms first
- Secondary ossification centerin each epiphysis (end)
Growth in length & width
- Epiphyseal (growth) platecartilage between epiphysis and diaphysis; longitudinal growth
- Epiphyseal lineclosed growth plate — growth stopped (late teens/early 20s)
- Appositional growthosteoblasts add bone under periosteum; bone grows wider
Calcium homeostasis
- Parathyroid hormone (PTH)low blood Ca²⁺ → PTH ↑ → osteoclasts active → Ca²⁺ up
- Calcitoninhigh blood Ca²⁺ → calcitonin → deposits Ca²⁺ into bone
- Vitamin Denables Ca²⁺ absorption from the gut
Endochondral ossification, step by step
- 1. Cartilage modela hyaline cartilage version of the future bone forms
- 2. Bone collarosteoblasts build a bony collar around the shaft
- 3. Primary centercartilage in the shaft calcifies; osteoblasts lay down bone
- 4. Medullary cavityosteoclasts hollow out the center of the shaft
- 5. Secondary centersossification begins in each epiphysis
- 6. Cartilage remainsonly at articular surfaces and the epiphyseal plate
Epiphyseal plate: five zones
- Resting (reserve)anchors the plate to the epiphysis; quiet cartilage
- Proliferationchondrocytes divide and stack; the plate lengthens
- Hypertrophychondrocytes enlarge
- Calcificationmatrix calcifies; chondrocytes die
- Ossificationosteoblasts replace cartilage with bone
What controls bone growth
- Growth hormonemain driver of childhood length growth
- Thyroid hormonelets growth proceed at a normal pace
- Sex hormones (puberty)trigger the growth spurt, then close the plates
- Calcium & vitamin Dthe raw material and the means to absorb it
- Vitamin Cneeded to build the collagen in the matrix
- Mechanical stress (Wolff’s law)bone thickens where it is loaded, thins where it is not
Dr. Sharilyn Rennie
BIO 304 · Module 5 · Bone Tissue
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