Human Anatomy & Physiology
Chapter 06
Bones and Bone Tissue
PowerPoint® Lectures created by Suzanne Pundt, University of Texas at Tyler
Copyright © 2019, 2016 Pearson Education, Inc. All Rights Reserved
Second Edition
Module 6.1: Introduction to Bones as Organs 
Skeletal System
•	Skeletal system includes: 
•	Bones, joints, and other supporting tissues
•	Bones are main organs of this system:
•	Like any organ, they are composed of more than osseous tissue
•	Also composed of both dense regular and irregular collagenous connective tissue as well as bone marrow
Functions of the 
Skeletal System
•	Functions of skeletal system include:
•	Protection: certain bones, including skull, sternum (breastbone), ribs, and pelvis, protect underlying organs; example of Structure-Function Core Principle
Functions of the 
Skeletal System
•	Functions of skeletal system (continued):
•	Mineral storage and acid-base homeostasis: bone is most important storehouse in body for calcium, phosphorus, and magnesium salts; these minerals, also present in blood as electrolytes, acids, and bases; critical for electrolyte and acid-base maintenance/balance

Functions of the 
Skeletal System
•	Functions of skeletal system (continued):
•	Blood cell formation: bones house red bone marrow; special form of connective tissue, in this tissue the process of hematopoiesis (formation of blood cells) takes place
Functions of the 
Skeletal System
•	Functions of skeletal system (continued):
•	Fat storage: bones also contain yellow bone marrow; contains fat cells, or adipocytes, with stored triglycerides; fatty acids from breakdown of triglycerides can be used for fuel by cells
Functions of the 
Skeletal System
•	Functions of skeletal system (continued):
•	Movement: bones serve as sites of attachment for most skeletal muscles; when muscles contract, they pull on bones; generates movement at a joint
Functions of the 
Skeletal System
•	Functions of skeletal system (continued):
•	Support: skeleton supports weight of body and provides its structural framework
Functions of the 
Skeletal System
Bone Structure
•	Bone structure can be organized into 5 classes despite diversity of bone appearance; all 206 bones fit into one of following categories based on 
•	shape:
•	Long bones – named for overall 
shape; not their actual size; 
longer than they are wide; 
include most bones in arms 
and legs
Bone Structure
•	Bone categories based on shape (Figure 6.2):
•	Short bones – also named for shape rather than size; roughly cube-shaped or about as long as they are wide; include bones of wrist or carpals and ankle or tarsals 
Bone Structure
•	Bone categories based on shape (continued):
•	Flat bones – thin and broad bones; include ribs, clavicles, sternum (breastbone), and most bones in skull
Bone Structure
•	Bone categories based on shape (continued):
•	Irregular bones – include vertebrae and certain skull bones; do not fit into other classes because of irregular shapes
Bone Structure
•	Bone categories based on shape (continued):
•	Sesamoid bones – specialized bones located within tendons; generally small, relatively flat, and oval-shaped; give tendons a mechanical advantage, which gives muscles better leverage; patella (kneecap) is an example of this class of bones
Bone Structure
Bone Structure
•	Structure of a long bone:
•	Periosteum – membrane composed of dense irregular collagenous connective tissue; forms a covering, rich with blood vessels and nerves; surrounds outer surface of long bones
•	Perforating fibers (Sharpey’s 
fibers) – collagen fibers; anchors periosteum firmly to underlying bone by penetrating deep into bone matrix
Bone Structure
•	Structure of a long bone (continued):
•	Diaphysis – shaft of a long bone; each end is its epiphyses; epiphysis is covered with a thin layer of hyaline cartilage (articular cartilage) found within joints (articulations) between bones
•	Within diaphysis is a hollow cavity known as marrow cavity (or medullary cavity); houses either red or yellow bone marrow, depending on bone and age of individual
Bone Structure
•	Structure of a long bone (continued):
•	Compact bone – one of two bone textures; hard, dense outer region that allows bone to resist linear compression and twisting forces 
•	Spongy bone (cancellous bone) – 
second bone texture; inner, honeycomb-like 
bone; resists forces from 
many directions; provides a cavity 
for bone marrow
Bone Structure
•	Structure of a long bone (continued):
•	Bony struts of spongy bone and all inner surfaces of bone are lined with a membrane called endosteum; contains different types of bone cells involved in maintenance of bone homeostasis
•	Epiphyseal lines – lines running across the proximal and distal ends of the bone between the epiphysis and diaphysis. These are remnants of epiphyseal plates (growth plates), a line of hyaline cartilage from which a long bone grows in length in children and adolescents.
Bone Structure
Bone Structure
•	Structure of short, flat, irregular, and sesamoid bones: these bones do not have diaphyses, epiphyses, medullary cavities, epiphyseal lines, or epiphyseal plates:
•	Covered by periosteum with associated perforating fibers, well supplied with blood vessels and nerves 
•	Have an outer layer of compact bone that surrounds inner spongy bone
•	Internal structure is fairly simple: a secton through one of these bones resembles a “sandwich” made up of 2 thin layers of compact bone and a middle layer of spongy bone housing bone marrow. In flat bones, the spongy bone is called diploe.
•	Some flat and irregular bones of skull contain hollow, air-filled spaces called sinuses, which reduce bone weight
Bone Structure
Bone Structure
•	Blood and nerve supply to bone – bones are well supplied with blood vessels and sensory nerve fibers:
•	Blood supply to short, flat, irregular, and sesamoid bones is provided mostly by vessels in periosteum that penetrate bone 
•	Long bones get a about a third of their blood supply from periosteum; mostly supplies compact bone
Bone Structure
•	Blood and nerve supply to bone (continued):
•	Greatest source of blood comes from one or two nutrient arteries; enter bone through a small hole in diaphysis called nutrient foramen
•	Nutrient artery passes through compact bone to supply internal structures of long bone
•	Epiphyses receive some blood supply from nutrient arteries; majority comes from small blood vessels that enter and exit through small holes in their compact bone
Bone Structure
•	Red bone marrow – consists of loose connective tissue that supports islands of blood-forming hematopoietic cells
•	Amount of red marrow decreases as a person ages
•	Red marrow in adult is found only in pelvis, proximal femur and humerus, vertebrae, ribs, sternum, clavicles, scapulae, and small amount in certain bones of skull
•	In Infants and young children, most bone marrow is red because their rapid rate of growth requires a constant supply of new blood cells.
Bone Structure
•	Yellow bone marrow – Stores triglycerides, consists mostly of blood vessels and adipocytes.
Bone Marrow Transplantation
•	Patients with diseases of blood (such as leukemia, sickle-cell anemia, aplastic anemia) have improperly functioning hematopoietic cells; many can therefore benefit from bone marrow transplantation in which hematopoietic cells from red bone marrow are removed from a matching donor and given to a recipient
•	Needle is inserted into pelvic bone of matching donor and red marrow is withdrawn; repeated until up to 2 quarts (about 2% of total) is removed
•	Recipient’s marrow is destroyed and donor marrow is given intravenously; cells travel to recipient’s marrow cavities; and if all goes well will begin producing new blood cells in about 24 weeks 
•	Complications – flu-like symptoms, infection or transplant rejection
•	If transplant “takes” many recipients can return to a healthy life
Module 6.2: Microscopic Structure of Bone Tissue 
Microscopic Structure
•	Bone or osseous tissue – primary tissue found in bone; composed mostly of extracellular matrix with a small numbers of cells scattered throughout
•	Extracellular matrix of bone is unique:
•	Inorganic matrix – consisting of minerals makes up about 65% of bones total weight
•	Organic matrix – makes up remaining 35%; consists of usual ECM components including collagen fibers 
Extracellular Matrix
•	Inorganic matrix – made up predominantly of calcium salts; bone stores around 85% of total calcium ions in body as well as a good amount of phosphorus:
•	Most calcium and phosphorus salts exist as large molecules of a mineral called hydroxyapatite crystals [Ca10(PO4)6(OH)2]
•	Makes bone one of hardest substances in body; makes it strong and resistant to compression
•	Allows bone to be both protective and supportive;  demonstrates Structure-Function Core Principle
•	Bicarbonate, potassium, magnesium, and sodium are also found in inorganic matrix
Extracellular Matrix
•	Organic matrix – known as osteoid; consists of protein fibers, proteoglycans, glycosaminoglycans, glycoproteins, and bone-specific proteins
•	Collagen – predominant protein fiber; forms cross-links with one another; helps bone resist torsion (twisting) and tensile (pulling or stretching) forces
•	Collagen fibers also align themselves with hydroxyapatite crystals; enhances hardness of bone
Extracellular Matrix
•	Osteoid (continued):
•	Glycosaminoglycans and proteoglycans create a gradient that draws water OUT of the blood vessels and cells by osmosis which traps water within the ECM and helps tissue resist compression
•	Glycoproteins bind different components together
Extracellular Matrix
Bone Cells
•	Bone is a dynamic tissue; new bone is continually being formed as older bone is broken down; three cell types responsible for bone’s dynamic nature:
•	Osteoblasts
•	Osteocytes
•	Osteoclasts
Bone Cells
•	Osteoblasts – “bone builders”, found in inner periosteum and endosteum:
•	Osteogenic cells – flattened cells that differentiate into osteoblasts when stimulated by specific chemical signals
•	Osteoblasts are bone-building 
cells that perform bone deposition
•	Bone deposition –build up bone ECM,                       process where osteoblasts secrete organic 
matrix and assist in 
formation of inorganic matrix 
Bone Cells
•	Osteocytes
•	Osteoblasts become surrounded and eventually trapped by secreted bone matrix in a small cavity known as lacuna. At this point, these cells are called osteocytes. They are mostly “retired” from their jobs and are relatively inactive, they do secrete chemicals that are required for maintaining ECM. Also appear to recruit osteoblasts to areas under tension.
Bone Cells 
.
Bone Cells
•	Osteoclasts 
•	Look like “jellyfish”, are large multinucleated derived from the fusion of cells formed in the bone marrow. Responsible for bone resorption (during which they break down the bone ECM); secretes hydrogen ions and enzymes 
•	Hydrogen ions create an acidic environment that dissolves the inorganic matrix and enzymes break down the organic matrix. The liberated minerals enter the osteoclast and eventually are delivered to the blood for reuse in the body or excreted as waste.

Bone Cells
Histology of Bone
•	Histology of bone tissue is quite different between hard outermost compact bone and porous inner spongy bone 
•	Both gross and histological 
differences can be attributed to 
different functions each region 
performs; Structure-Function 
Core Principle
Histology of Bone
•	Structure of compact bone-as an outer shell, it is subject to a great amount of stress, or forces that tend to strain or deform an object, it must be able to withstand these forces or suffer damage: 
•	Compact bone, in cross section, resembles forest of tightly 
packed trees where each tree 
is a unit called an osteon or 
a Haversian system
•	Rings of trees are made 
up of thin layers of bone 
called lamellae
Histology of Bone
•	Osteon structure consists of following components:
•	Lamellae-Most osteons contain from 4 to 20 lamellae, sometimes called concentric lamellae, the lamellar structure of compact bone greatly enhances strength. 
•	Collagen fibers of neighboring lamellae 
run in opposite directions; resist twisting 
and bending forces in more than one direction
Histology of Bone
•	Osteon structure (continued):
•	Central canal – endosteum-lined hole found in center of each osteon, contains blood vessels and nerves
that supply cells of the osteon
•	Osteocytes reside in lacunae – small 
cavities found between lamellae; filled 
with extracellular fluid
Histology of Bone
•	Osteon structure (continued):
•	lacunae are connected to one another by tiny canals called canaliculi; osteocytes have long thin arms, cytoplasmic 
extensions that extend through the canaliculi to               contact the arms of other osteocytes.  
•	At places of contact are gap junctions that allow small molecules to pass from cell to cell. 
Histology of Bone
•	Overall compact bone structure:
•	Osteons are not permanent structures; they are regularly resorbed by osteoclasts and rebuilt by osteoblasts. 
•	Interstitial lamellae –                                         remnants of resorbed osteons 

Histology of Bone
•	Overall compact bone structure (continued):
•	Circumferential lamellae – outer and inner rings of lamellae just inside periosteum and outside            spongy bone; add strength to bone as a whole
•	The central canals of neighboring osteons are connected by perforating canals (Volkmann’s canals) which run perpendicular to the osteons                      
Histology of Bone
Histology of Bone
•	Structure of spongy bone: 
•	Spongy bone – usually not weight-bearing part of a bone, much less densely packed
•	Resists forces from many directions 
•	Forms a protective framework for bone marrow

Histology of Bone
•	Structure of spongy bone (continued):
•	Branching “ribs” of bone called trabeculae; project into marrow cavity, covered with endosteum and usually do not contain osteons. 
•	However, they do have concentric lamellae, within which we find canaliculi and lacunae housing osteocytes. 
•	No central or perforating canals 
are present in trabeculae; cells
obtain oxygen and nutrients                                             from the blood vessels in bone marrow. 
Histology of Bone
Osteopetrosis
•	Primary defect in osteopetrosis (“marble bone disease”) is defective osteoclasts; do not properly degrade bone; causes bone to overgrow and bone mass to increase, while paradoxically bones become weakened and brittle. 
•	Main forms:
•	Infantile – predominately inherited, more severe form; the openings of the skull fail to enlarge as the infant grows, the skull may trap nerves causing blindness and deafness. The marrow cavities in all bones fail to enlarge which decreases the amount of red bone marrow, a condition that can be fatal.
•	Adult – also inherited; develops during adolescence or later; symptoms may include bone pain, recurrent fractures, nerve trapping, joint pain; treated symptomatically only
 6.3: Bone Formation: Ossification 
Ossification
•	Process of bone formation is called ossification or osteogenesis; begins in embryonic period and continues through childhood with majority of bones completing the process by age 7:
•	Two types of ossification:
•	1. intramembranous-built on a model made of a membrane of embryonic connective tissue.
•	2. endochondral- (chondr=“cartilage”) built on a model made of hyaline cartilage.
Ossification
•	Ossification or osteogenesis (continued):
First bone formed by both types of ossification is immature bone called primary bone (or woven bone), consists of irregularly arranged collagen bundles, abundant osteocytes, and little inorganic matrix.
Most body locations, primary bone is resorbed by osteoclasts and replaced by mature secondary bone (lamellar bone). Has fully formed lamellae with regularly arranged, parallel, collagen bundles (makes it stronger than primary bone), contains higher percentage of inorganic matrix which contributes to its strength.
Please note: bones formed by either type of ossification are histologically identical once they mature.
Intramembranous Ossification
•	Intramembranous ossification – forms many flat bones, including bones of skull and clavicles, during fetal development:
•	Bones formed in this way are primary bone, which is eventually resorbed and replaced with secondary bone.
•	This type of ossification occurs within a mesenchymal membrane composed of a sheet of embryonic connective tissue; richly supplied with blood and populated with mesenchymal cells
•	As you consider the events of intramembranous ossification , remember the structure of flat bones, they are like a sandwich with two layers of compact bone as the “bread” and the spongy bone as the “meat”.
Intramembranous Ossification
•	Intramembranous ossification (continued):
•	During intramembranous ossification, the inner spongy bone forms before the outer compact bone.
•	Beginning at a place called the primary ossification center.
Intramembranous Ossification 
Intramembranous Ossification
•	Intramembranous ossification (continued):
•	Early spongy bone is formed as osteoblasts continue to lay down new bone to form trabeculae; smaller trabeculae can merge forming larger structures
•	Some mesenchymal cells differentiate and form periosteum; some of vascular tissue in early spongy bone will become bone marrow
•	Early compact bone is formed-spongy bone deep to periosteum becomes heavily calcified and its structure is rearranged to form immature compact bone
Intramembranous Ossification 
Intramembranous Ossification
•	Intramembranous ossification (continued):
•	Larger bones have more than one primary ossification center
•	Leads to pieces of bone that must fuse to one another over time
•	An example of early incomplete ossification is fontanels (soft spots) in skulls of newborn babies
Intramembranous Ossification
Endochondral Ossification
•	Endochondral ossification:
•	Bone development for all bones below head except clavicles
•	Begins in fetal stage of development for most bones; some bones (wrist and ankle) ossify much later
•	Many bones complete ossification by age 7
Endochondral Ossification
•	Endochondral ossification occurs from within a model of hyaline cartilage; serves as a scaffold for the developing bone
•	The hyaline cartilage model consists of chondrocytes and cartilage ECM, and is surrounded by a connective tissue membrane called the perichondrium and immature cartilage cells called chondroblasts.
•	Also begins at primary ossification centers and the bone laid down is primary bone that is eventually replaced with secondary bone. 
•	However, long bones also contain secondary ossification centers within their epiphyses that ossify by a similar process.
Endochondral Ossification
•	After the cartilage model has formed, endochondral ossification occurs in following steps:
•	Chondroblasts in perichondrium differentiate first into osteogenic cells then osteoblasts, when this happens the perichondrium is now called the periosteum.
•	The bone begins to ossify from the outside
•	Osteoblasts build the bone collar on external surface of bone
•	At same time bone collar forms, internal cartilage begins to calcify and chondrocytes die-as bone collar calcifies, ECM surrounding internal chondrocytes calcifies, cuts off the blood supply to chondrocytes (causing them to eventually die). Their death leaves cavities surrounded by calcified cartilage.
Endochondral Ossification
Endochondral Ossification
•	Endochondral ossification steps (continued):
•	In primary ossification center, osteoblasts replace calcified cartilage with early spongy bone; secondary ossification centers and medullary cavity develop. 
•	As medullary cavity enlarges, remaining cartilage is replaced by bone; epiphyses finish ossifying
Endochondral Ossification 
Endochondral Ossification
•	Endochondral ossification steps (continued):
•	Medullary cavity is filled with bone marrow
•	Cartilage only persists in two places; epiphyseal plates and articular surfaces where bones 
interact at a joint (called 
articular cartilage)
•	Articular cartilage persists 
into adulthood while 
epiphyseal plates are 
eventually filled in, once 
bone is finished growing in 
length
Osteoporosis and 
Healthy Bones
•	Most common bone disease in United States; bones become weak and brittle due to inadequate inorganic matrix; increases risk of fractures
•	Diagnosed by measuring bone mass (amount of inorganic matrix)
•	Causes – dietary (calcium and/or 
vitamin D deficiency), female gender, 
advanced age, lack of exercise,
hormonal (lack of estrogen in 
postmenopausal women), genetic 
factors, and other diseases
Osteoporosis and 
Healthy Bones
•	Prevention – balanced diet, with supplementation as needed, weight-bearing exercise, and estrogen replacement if appropriate
•	Treatment – drugs that inhibit osteoclasts or stimulate osteoblasts
Module 6.4: Bone Growth in Length and Width 
Growth in Length
•	Long bones lengthen by a process called longitudinal growth; involves division of chondrocytes (not osteocytes or osteoblasts) in epiphyseal plate
•	Bone growth takes place at epiphysis on side closest to diaphysis
Growth in Length
•	Epiphyseal plate, composed of hyaline cartilage that did not ossify during endochondral ossification
•	Epiphyseal plate contains 5 zones, each with a distinct appearance:
•	Zone of reserve cartilage – (found closest to epiphysis) contains cells that are not directly involved in bone growth but can be called upon to divide if needed.  
•	Zone of proliferation (next region-closer to diaphysis) consists of actively dividing chondrocytes in lacunae
Growth in Length
•	Epiphyseal plate zones (continued):
•	Zone of hypertrophy and maturation (next region) contains mature chondrocytes
•	Zone of calcification (second to last region) contains dead chondrocytes, some of which have been calcified
•	Zone of ossification (last region) consists of calcified chondrocytes and osteoblasts
Growth in Length
Growth in Length
•	Longitudinal grown sequence of events:
•	Chondrocytes divide in zone of proliferation forcing cells ahead of them into next zones, moving toward diaphysis
•	Chondrocytes that reach zone of hypertrophy and maturation enlarge and stop dividing
Growth in Length
•	Process of longitudinal growth (continued):
•	Chondrocytes that reach zone of calcification die and their matrix calcifies
•	Calcified cartilage is replaced with bone in zone of ossification; osteoblasts invade calcified cartilage and begin to lay down bone
•	Eventually calcified cartilage and primary bone is resorbed by osteoclasts and completely replaced with mature bone
Growth in Length
•	Longitudinal growth continues at epiphyseal plate as long as mitosis continues in zone of proliferation:
•	Mitotic rate slows around ages of 1215 years old while ossification continues; causes epiphyseal plates to shrink as zone of proliferation is overtaken by zone of calcification and ossification
•	Between ages of 1821, zone of proliferation is completely ossified, longitudinal growth stops, and epiphyseal plate is considered closed
•	Epiphyseal line is a calcified remnant of epiphyseal plate
Growth in Length
Growth in Length 
Growth in Width
•	Bones not only grow in length, they also grow in width; process called appositional growth
•	Osteoblasts, found in between periosteum and bone surface, lay down new bone
•	Appositional growth does not initially result in formation of new osteons; instead, new circumferential lamellae are formed
Growth in Width
•	Appositional growth (continued):
•	As new lamellae are added, older deeper circumferential lamellae are either removed or incorporated into osteons
•	Bones may continue to increase in width even after bone growth in length ceases
Achondroplasia
•	Most common cause of dwarfism; gene defect inherited from a parent or caused by new mutation
•	Defective gene produces an abnormal growth factor receptor on cartilage; interferes with hyaline cartilage model used in endochondral ossification; also articular and epiphyseal cartilage
•	Bones form and grow abnormally; results in short limbs, a disproportionately long trunk and facial abnormalities
•	Long-term problems include joint disorders, respiratory difficulties, and spinal cord compression; may be managed with medications
Role of Hormones 
in Bone Growth
•	Multiple factors play a role in how much cell division occurs in epiphyseal plate and how long process remains active:
•	One of main factors affecting bone growth is a group of chemicals called hormones
•	Hormones are secreted by cells of endocrine glands; example of Cell-Cell Communication Core Principle
Role of Hormones 
in Bone Growth
•	Growth hormone – secreted by anterior pituitary gland; enhances protein synthesis and cell division in nearly all tissues, including bone
•	Has following effects on both longitudinal and appositional growth:
•	It increases rate of mitosis of chondrocytes in epiphyseal plate
•	It increases activity of the osteogenic cells, including their activity in zone of ossification
•	It directly stimulates osteoblasts in periosteum; triggers appositional growth
Role of Hormones 
in Bone Growth
•	Male sex hormone testosterone has a pronounced effect on bone growth:
•	Increases appositional growth causing bones in males to become thicker with more calcium salt deposition than in females
•	Increases rate of mitosis in epiphyseal plate; leads to “growth spurts” in teenage years
•	Accelerates closure of epiphyseal plate
Role of Hormones 
in Bone Growth
•	Female sex hormone estrogen also plays a role in bone growth:
•	Increases rate of longitudinal bone growth and inhibits osteoclast activity
•	When estrogen levels spike in teen years an accompanying “growth spurt” occurs in females
•	Accelerates closure of epiphyseal plate with a much more potent effect,  partly why women are generally shorter than men.
Gigantism and 
Acromegaly
•	Excess growth hormone can produce two conditions, depending on when in life it develops; both generally caused by a tumor that secretes hormone; treated by tumor removal
•	Childhood – condition is gigantism; epiphyseal growth plates have yet to close; individuals get very tall due to excessive longitudinal and appositional bone growth
•	Adulthood – condition is acromegaly; epiphyseal growth plates have closed; no increase in height, but enlargement of bone, cartilage, and soft tissue
•	Skull, bones of face, hands, feet, and tongue affected
•	Can cause heart and kidney malfunction; associated with development of diabetes
Module 6.5: Bone Remodeling and Repair 
Bone Remodeling
•	Once bone has finished growing in length it is far from inactive; undergoes a continuous process of formation and loss called bone remodeling; new bone is formed by bone deposition and old bone is removed by bone resorption; cycle occurs for following reasons:
•	Maintenance of calcium ion homeostasis
•	Replacement of primary bone with secondary bone
•	Bone repair
•	Replacement of old brittle bone with newer bone
•	Adaptation to tension and stress
Bone Remodeling
•	Bone remodeling:
•	In healthy bone, process of formation and loss occur simultaneously; bone breakdown by osteoclasts matches bone formation by osteoblasts
•	In children, deposition far outweighs resorption, but once epiphyseal plates close and longitudinal growth ceases, bone deposition approximately equals deposition                                                              
Bone Remodeling
•	Bone deposition:
•	Carried out by osteoblasts
•	Found in both periosteum and endosteum;
make organic matrix and facilitate 
formation of inorganic matrix
•	Secrete proteoglycans and glycoproteins 
that bind to calcium ions
•	Secrete vesicles containing calcium ions, ATP, and enzymes; bind to collagen fibers; calcium ions eventually crystallize, rupturing vesicle and beginning calcification process
Bone Remodeling
•	Bone resorption: 
•	Osteoclasts secrete hydrogen ions on bone ECM 
•	H+ make the pH more acidic, which breaks down the pH-sensitive hydroxyapatite crystals in the inorganic matrix. 

•	Calcium ions and other liberated 
minerals can be reused 
elsewhere in body
Bone Remodeling
•	Bone resorption (continued): 
•	Osteoclasts secrete enzymes 
•	Degrade organic matrix, including: proteoglycans, glycosaminoglycans, and glycoproteins
•	Breakdown products of these 
molecules are taken up by 
osteoclast for recycling
Bone Remodeling
•	Bone remodeling in response to tension and stress: One of the primary influences on bone remodeling is the stress placed on bones because bone deposition occurs in proportion to it. The heavier the load a bone must carry, the more bone tissue is deposited in that bone. 
•	Load does not necessarily refer to weight, but amount of compression (act of squeezing or pressing together)
•	Other factors that function in bone remodeling:
•	Tension – stretching force; bone deposition occurs in regions of bone exposed to tension
•	Pressure – continuous downward force; bone resorption is stimulated in regions of bone exposed to continuous pressure
Bone Remodeling
•	Other factors influencing bone remodeling:
•	Hormones – Testosterone promotes bone deposition while estrogen inhibits osteoclast activity
•	Age – As individual ages growth hormone and sex hormones decline; decreases protein synthesis in bone
•	Calcium ion intake from diet must be adequate to support bone deposition
•	Vitamin D intake must be adequate to promote calcium ion absorption from gut and prevents calcium ion loss in urine
Bone Remodeling
•	Other factors influencing bone remodeling (continued):
•	Vitamin C intake from diet must be adequate for synthesis of collagen
•	Vitamin K intake-thought to aide in synthesis of calcium ion-binding glycoproteins by osteoblasts
•	Protein intake from diet must be adequate for osteoblasts to synthesize collagen fibers found in organic matrix
Bone Remodeling
•	Bone remodeling and calcium ion homeostasis:
•	Bone stores most of calcium ions in body
•	Stored calcium ions are not only used for bone deposition and remodeling; used throughout body for several critical processes such as muscle contraction
•	A negative feedback loop maintains calcium ion homeostasis in blood; example of Feedback Loops Core Principle

Bone Remodeling
•	Bone remodeling and calcium ion homeostasis (continued):
•	Negative feedback loop:
•	Calcium ion levels in blood are closely monitored;
both high and low levels of calcium ions can lead 
to major disruptions in homeostasis and even death
•	Stimulus and receptor: when calcium ion level 
drops in blood it is detected by parathyroid cells
•	Control center and effector: parathyroid cells 
act as control center and secrete parathyroid 
hormone (PTH)
Bone Remodeling
•	Bone remodeling and calcium ion homeostasis (continued):
•	Negative feedback loop (continued):
•	Effect/response: PTH stimulates effects that increase blood calcium ion levels
•	Increases osteoclast activity; breaks down bone releasing calcium ions into blood
•	Increases absorption of calcium 
from gut 
•	Inhibits calcium loss in urine
Bone Remodeling
•	Bone remodeling and calcium ion homeostasis (continued):
•	Negative feedback loop (continued):
•	Homeostasis and negative feedback: As calcium ion levels return to normal in blood, change is detected by parathyroid cells and they reduce secretion of PTH, closing feedback loop
Bone Remodeling
Bone Remodeling
•	Bone remodeling and calcium ion homeostasis (continued): 
•	Negative feedback loop (continued):
•	An increase in blood calcium levels triggers a different negative feedback loop; first response is a drop in PTH secretion by parathyroid gland
•	Calcitonin is secreted by thyroid gland and has basically opposite effects as PTH; leads to bone deposition; pulls calcium ions out of blood and deposits them in bone; calcitonin is most critical during times of active bone remodeling, like longitudinal bone growth
•	Vitamin D is important for calcium ion homeostasis due to its effects on the absorption of calcium ions from the gut
Bone Remodeling
•	Factors influencing bone remodeling are summarized:
Bone Repair
•	Bones are commonly injured while performing their protective and supportive functions 
•	Most dramatic bone injury is a fracture (broken bone):
•	Simple fractures (closed fractures)– skin and tissue around fracture remain intact
•	Compound fractures (open fractures) – skin and tissues around fracture are damaged
Bone Repair
Bone Repair
•	General process of fracture healing involves:
•	Hematoma (blood clot) fills in gap between bone fragments; mass of blood cells and proteins form in an injury due to ruptured blood vessels
•	Fibroblasts and chondroblasts infiltrate hematoma and form a soft callus
Bone Repair
•	General process of fracture healing (continued):
•	Osteoblasts build a bone callus (hard callus)
•	Bone callus is remodeled and primary bone is replaced with secondary bone
Bone Repair