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
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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
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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
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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
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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
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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)
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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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