Skeletal System Overview: Bone Growth and Ossification

TOPIC IX: SKELETAL SYSTEM The skeletal system does far more than just provide a structural support for the body — it actively helps to maintain homeostasis and supports essential physiological functions. It helps to regulate calcium balance which impacts several important physiological processes ra...

TOPIC IX: SKELETAL SYSTEM The skeletal system does far more than just provide a structural support for the body — it actively helps to maintain homeostasis and supports essential physiological functions. It helps to regulate calcium balance which impacts several important physiological processes ranging from nerve signaling (recall synaptic transmission requires calcium!), muscle contraction, and blood clotting. Through bone marrow, it produces red and white blood cells, crucial for oxygen transport and immunity. Understanding the skeletal system is important to our understanding of human physiology, as it directly impacts movement, blood circulation, and overall health. A) Skeletal System Overview: — Functions: 1) structural support 2) protection of vital organs. 3) attaches muscles and allows movement 4) red marrow produces ALL blood cells 5) stores Ca++ B) Bone Ossification: — before week 8 skeleton of embryo constructed of fibrous membranes and hyaline cartilage — bone formation begins at ~8th week ⇒ 20 years — types: 1) Intramembranous ossification — flat bones e.g. some skull bones, mandible, clavicle — develop from fibrous connective tissue membrane — process: o osteoblasts produce bone along connective tissue fibres o cells at the periphery form the periosteum and the osteoblasts under it lay down compact bone — fontanels = unossified membranes in the skull that remain at births 2) Endochondral ossification (most bones) — hyaline cartilage used as a “model” (similar shape) for bone construction – then ossifies to form spongy bone — periosteum forms and the osteoblasts beneath lay down compact bone — articular cartilage and epiphyseal plates = cartilage that has not ossified C) Bone Growth (postnatal): 1) Growth in Length — = endochondral growth - at epiphyseal plates — process: o cartilage grows ⇒ matrix near diaphysis becomes bone o growth in length stops when epiphyseal plate ossifies into epiphyseal line (closure)  2) Growth in Diameter — = appositional growth — process: o osteoblasts beneath periosteum secrete bone matrix (external bone surface) o osteoclasts resorb bone on the endosteal surface ∴ size of cavity keeps pace with growing bone 3) Factors Affecting Bone Growth/Remodeling: a) Mechanical Stress — typically due to muscle action — examples: i. moderate exercise: increases osteoblast activity in areas of stress (more building of bone than breakdown) ii. injury that restricts movement: decreases osteoblast activity and no change in osteoclast activity causes a reduction in bone mass iii. weights or electric currents: may speed healing by increasing osteoblast activity b) Nutrition — Ca2+, PO4- - required for bone matrix production — vitamin C - required for collagen production — vitamin D – increases absorption of Ca2+ from intestine o If lacking vitamin D – can cause Rickets (soft bones) c) Hormones — growth hormone (GH), thyroid ⇒ stimulate bone growth — Estrogen and Testosterone — Increases osteoblast activity — estrogen (in all people) causes ossification of epiphyseal plate (converts it to epiphyseal line) — Calcitonin — inhibits osteoclast activity — increases Ca2+ movement from blood into bone; promotes bone growth — Parathyroid Hormone (PTH) — increases osteoclast activity and decreases osteoblast activity. Breakdown of bone releases Ca2+ into the blood (increases concentration of Ca2+ in the blood) D) Osteoporosis: — a bone disease characterized by a reduction in bone density and mass that leads to more fragile bones that are more like to fracture. — Risk factors: 1) increasing age (decreased production of sex hormones) 2) post-menopause – decreased estrogen production 3) inadequate diet (decreased vitamin D, Ca2+) 4) illness, excess parathyroid hormone 5) too little/excessive exercise (stress - cortisol)  6) drugs e.g. cortisone, alcohol - reduce osteoblast activity, smoking - decreases estrogen production E) Bone Repair: — process: 1) begins with formation of a blood clot 2) replaced by a callus - consists of fibrous network and fibrocartilage islets 3) callus ossifies ⇒ intramembranous and endochondral ossification 4) takes 4 - 6 weeks ⇒ cast required since movement can re-fracture new matrix