Biology 231 Human Anatomy and Physiology Chapter 6 Lecture Outline

Skeletal System – body’s framework of bones and their associated cartilage & ligaments

Functions of skeletal system Support Protection – surround vital organs (eg. brain, heart and lungs) Levers for movement – muscles attached Mineral homeostasis – stores and releases minerals as needed (eg. calcium and phosphorus) Blood cell production – in red marrow Triglyceride (fat) storage – in yellow marrow

BONE HISTOLOGY – bone is a connective tissue

Matrix 1/3 fibers – collagen 2/3 ground substance – composed of 1/3 water and 2/3 mineral salts mainly hydroxyapatite (calcium salts) calcification – deposition of mineral salt crystals between and around collagen fibers hardness depends on amount of mineral salts resistance to deformation depends on collagen fibers

Cells osteoprogenitor cells – stem cells derived from mesenchyme; found on inner periosteum, endosteum and around blood vessel canals; differentiate into osteoblasts

osteoblasts – form new bone by secreting matrix around themselves

osteocytes – mature bone cells within lacunae; maintain daily metabolic processes of bone

osteoclasts – huge cells from fusion of up to 50 phagocytes; concentrated in endosteum ruffled border – deeply folded plasma membrane that releases enzymes and acids to digest bone matrix (resorption); involved in bone growth and repair

1 2 TYPES OF BONE – compact and spongy bone

1) Compact bone – bone arranged in osteons with little space between; external layer of all bones and major portion of long bones

osteon – 4 main components central canal – runs longitudinally; contains blood vessels and nerves concentric lamellae – rings of calcified matrix around central canal

lacunae – spaces between concentric lamellae which contain osteocytes canaliculi – tunnels running outward from osteocytes; contain processes of osteocytes surrounded by extracellular fluid; pathway for diffusion to blood vessels in central canal

perforating canals – run transversely through osteons; contain blood vessels and nerves; connect with central canals

circumferential lamellae – encircle inner and outer surfaces of bone (beneath periosteum and lining medullary cavity)

interstitial lamellae – fragments of old osteons lying between current osteons

osteons align along lines of stress to resist bending; bone is thicker where more stress occurs; remodel depending on physical demands

2) Spongy bone – no osteons; consists of an irregular network of small trabeculae (little beams) with large spaces between (filled with red marrow); found mainly in flat, short, and irregular bones and ends of long bones

trabeculae – composed of concentric layers of matrix with lacunae containing osteocytes, connected by canaliculi; osteocytes are all near surface so diffusion occurs directly into marrow space

red marrow – site of blood cell formation (hip bones, ribs, breastbone, vertebrae, and ends of long bones)

STRUCTURE OF LONG BONES

Diaphysis – shaft; mainly compact bone Epiphyses (sing. –sis) – proximal and distal ends; mainly spongy bone with an outer shell of compact bone spaces between trabeculae filled with red marrow

2 Metaphyses (sing. –sis) – transitional zone between diaphysis and epiphysis; epiphyseal plate (growth plate) – in immature bone; plate of hyaline cartilage where growth occurs

Articular cartilage – hyaline cartilage covering joint surfaces of epiphyses; reduces friction and absorbs shock

Periosteum – membrane covering bone (except at joint surfaces) outer fibrous layer – dense irregular connective tissue site of tendon, ligament, and joint capsule attachment inner cellular layer – osteoprogenitor cells for growth, remodeling and fracture repair perforating fibers – collagen fibers anchoring periosteum to bone

Medullary cavity (marrow cavity) – space in central diaphysis which contains yellow (fatty) marrow

Endosteum – thin cellular layer lining medullary cavity; made of osteoprogenitor cells and some osteoclasts

BLOOD AND NERVE SUPPLY OF BONE – rich blood supply

Periosteal arteries – run along outer periosteum of diaphysis; branches pass through periosteum and perforating canals; supply periosteum and outer compact bone

Nutrient artery – single or few large arteries; enter at mid-diaphysis through nutrient foramen (hole) into medullary cavity; proximal and distal branches supply inner compact bone and spongy bone to epiphyseal line

Metaphyseal arteries – pass into and supply metaphyses branches form epiphyseal arteries

veins and nerves accompany the arteries; periosteum is rich in sensory nerves which detect pain

BONE FORMATION (OSSIFICATION) embryonic“skeleton” is composed of loose fibrous connective tissue membranes and hyaline cartilage which serve as a template; ossification is replacement of these tissues with bone tissue

2 METHODS OF OSSIFICATION – intramembranous and endochondral

3 Intramembranous ossification – bone forms within loose fibrous connective tissue membranes; formation of flat bones of skull and clavicle

1) ossification center develops mesenchymal cells cluster where bone will develop; differentiate into osteoblasts and secrete organic matrix around themselves calcification – osteocytes trapped in lacunae deposit mineral salts

2) formation of trabeculae – matrix fuses to form trabecular network of spongy bone blood vessels grow between trabeculae and associated connective tissue differentiates into red marrow

3) development of periosteum and endosteum – osteoprogenitor cells and connective tissue around spongy bone condense to form periosteum and endosteum thin layer of compact bone forms under periosteum

Endochondral ossification – bone forms within hyaline cartilage; most bones formed this way

1) cartilage model develops mesenchymal cells cluster in shape of future bone differentiate into chondroblasts which secrete matrix of hyaline cartilage mesenchyme and connective tissue at surface condense to form perichondrium

cartilage model grows by 2 methods:

interstitial growth – chondrocytes of model divide and secrete matrix between themselves; model grows in length appositional growth – perichondrium produces new chondroblasts which deposit matrix; model grows in thickness

cartilage cells enlarge and die

2) primary ossification center develops – stimulated by blood vessels

periosteal arteries develop at diaphysis cells in perichondrium differentiate into osteoblasts and secrete matrix under perichondrium (now called periosteum) nutrient artery grows into middle of cartilage model

4 fibroblasts in middle differentiate into osteoblasts & secrete matrix ossification center grows towards ends of bone 3) osteoclasts form medullary cavity remodeling replaces spongy bone with compact bone

4) secondary ossification centers develop – metaphyseal arteries induce formation of secondary ossification centers (around time of birth or later); ossification proceeds outward from center

5) epiphysis fills with spongy bone articular cartilage – cap of hyaline cartilage on joint surface epiphyseal plate (growth plate)– remaining cartilage at metaphysis

BONE GROWTH – depends on nutrient availability minerals – especially calcium and phosphorus vitamins – especially C and A

Growth in Length – interstitial growth only at epiphyseal plates plate closes at age 18-25 injury to the plate can cause early closure epiphyseal line – bony line in mature bone where growth plate was

Growth in Thickness – appositional growth osteoprogenitor cells in periosteum differentiate into osteoblasts which secrete matrix around themselves, becoming osteocytes layers (lamellae) of matrix and osteocytes develop concentric lamellae surround periosteal blood vessels forming osteons osteoblasts in periosteum deposit outer circumferential lamellae osteoclasts in endosteum enlarge medullary cavity

Hormone Regulation: human growth hormone(hGH) – from pituitary gland; promotes growth of bone; too much causes giantism, too little causes dwarfism sex steroids (estrogen and androgen) – increase at puberty stimulates growth spurt, high levels eventually close growth plates

BONES AND HOMEOSTASIS

Remodeling – constant process in which osteoclasts remove bone tissue and osteoblasts replace it; renews aging bone tissue, repairs damaged tissue redistributes bone along lines of mechanical stress

bone resorption – breakdown of bone matrix by osteoclast secretions enzymes - digest collagen acid – dissolves mineral salts by-products of resorption enter bloodstream

5 Homeostasis – resorption = ossification

exercise – weight bearing exercise places stress on bones which stimulates ossification; exercise increases bone mass and strength osteoporosis – porous bone; resorption greater than ossification

Calcium homeostasis – bone stores 99% of body’s calcium calcium aids in nerve and muscle function, blood clotting, and enzyme function blood calcium level is regulated within a narrow range

Hormonal regulation of blood calcium

Parathyroid Hormone(PTH) negative feedback loop

low calcium detected by receptors parathyroid gland increases production and secretion of PTH PTH effectors osteoclasts stimulated to resorb bone Ca from matrix enters bloodstream kidneys – excrete less calcium produce more calcitriol (needed to absorb dietary calcium) blood calcium increases

Calcitonin negative feedback loop

high calcium detected by receptors thyroid gland increases calcitonin production and secretion calcitonin effectors osteoclasts inhibited calcification of bone increases kidneys – excrete more calcium blood calcium decreases

Fracture repair – break in bone

Types of Fractures: Closed (simple) fracture – skin not broken Open (compound) fracture – skin broken, bone protrudes Comminuted fracture – splintered at fracture site Green-stick fracture – partial break, occurs in children Stress fracture – microfractures of bone tissue (eg. shin splints)

6 Steps of bone repair:

1) fracture hematoma (hours) – torn blood vessels bleed into area and form a clot phagocytic cells and osteoclasts remove damaged tissue 2) cartilage callus formation (weeks)– fibroblasts enter and become chondroblasts which produce hyaline cartilage osteoprogenitor cells in periosteum and endosteum produce spongy bone at margins 3) bony callus formation (months)– beginning near healthy bone osteoblasts replace cartilage of callus with spongy bone bridges fracture site and stabilizes it 4) bone remodeling (months-years) – bony callus replaced with “normal” bone

reduction – aligning ends of a fractured bone fixation – holding fracture still so a bony callus can form (eg. cast, pins, plates)

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