BONES AND BONE TISSUES CHAPTER 6 9/16/07. Introduction One of the most remarkable tissues of the human body Far from inert and lifeless, bones are living,

BONES AND BONE TISSUES

CHAPTER 69/16/07

Introduction One of the most remarkable tissues of the human body

Far from inert and lifeless, bones are living, dynamic structures

Bones serve a wide variety of very diverse functions within us

Noted for their strength and resiliency during life, bones will remain long after we are gone

Chapter Outline

Skeletal cartilages Bones Disorders of bones The skeleton throughout life

Location and Basic Structure

Cartilages are found throughout the adult human body


Initially our skeleton is made up of fast growing cartilages and fibrous membranes

Gradually our skeletal cartilages are replaced by bone

Upon reaching adulthood the skeleton becomes almost fully ossified

Only a few cartilages remain in the adult skeleton


A typical cartilage is composed of connective tissue cartilage

It contains no nerves or blood vessels

It is surrounded by a layer of dense irregular connective tissue called the perichondrium which resists outward expansion of the tissue when subjected to pressure


Each type of cartilage contains a high proportion of water which makes them resilient after compression

Cartilage is 60-80% water The water allows nutrients to diffuse rapidly through a loose matrix

Basic structure, type & location

There are three types of cartilage tissue: hyaline, elastic, and fibrocartilage

Each type consists of chondrocytes living in an extracellular matrix

Each contains a matrix of jellylike ground substance of complex sugar molecules that attract and hold water that is laced with connective tissue fibers

Hyaline cartilages

The most prevalent type of cartilage

Its high proportion of collagen fibers give it flexibility and resilience while providing support

Upon examination the tissue appears white, frosted, and smooth

Hyaline cartilages

The chondrocytes appear spherical

Each chondrocyte occupies a cavity in the matrix called a lacuna

The only type of fiber in the matrix is a collagen unit fibril

Hyaline cartilage locations

Articular - covers the end of bones

Costal - connect ribs to breastbone

Laryngeal - skeleton of larynx Tracheal & bronchial - reinforce the respiratory passages

Fetal - forms the embryonic skeleton

Elastic cartilage Elastic cartilage is similar to hyaline cartilage but its matrix contains many more elastic fibers in addition to collagen fibers

Its elastic fibers enable it to withstand repeated bending

Found only in the external ear and the epiglottis

Fibrocartilage The tissue consists of parallel rows of thick collagen fibers alternating with rows of chondrocytes

Tissue is highly compressible and has great tensile strength

Found in thick pad-like structures like the menisci of the knee or the discs of the vertebral column

Growth of Cartilage A cartilage grows in two ways Appositional growth occurs when cells in the surrounding perichondrium secrete new matrix next to existing cartilage tissue (growth from the outside)

Interstitial growth occurs when the chondrocytes within the cartilage divide and secrete new matrix, expanding the cartilage (growth from within)

Growth of Cartilage Cartilage stops growing in the late teens when the skeleton itself stops growing

Chondrocytes stop dividing and growth stops

Cartilage regenerates poorly in adults with most of the “healing” reflecting the ability of the remaining chondrocytes to secrete additional extracellular matrix

BONES

SECTION II

Bones Bones of the skeleton are organs that contain several different tissues

Bones are dominated by bone tissue but also contain – Nervous tissue and nerves– Blood tissue and vessels– Cartilage in articular cartilages– Epithelial tissue lining the blood vessels

Function of Bones:

Bones perform several important functions:– Support– Movement – Protection– Mineral storage – Blood cell formation and energy storage

Function of Bones

Support Bones provide a hard framework that supports the body

Bones provide support for internal organs

Function of Bone

Movement Skeletal muscle attached to bones use the bones as levers to move the body

Arrangement of bones and joints determine the movements possible

Function of Bone

Protection Fused bones provide a brain case that protects this vital tissue

Spinal cord is surrounded by vertebrae

Rib cage protects vital organs

Function of Bones

Mineral Storage Bone serves as a mineral reservoir

Phosphate and calcium ions can be released into the blood steam for distribution

Deposition and removal are ongoing

Function of Bones

Blood cell formation

Hematopoiesis occurs within the red marrow cavities of the long bones

The yellow marrow cavities are involved in fat storage

CLASSIFICATION OF BONE

SECTION III

Classification of Bone:

Bones vary in shape and size The unique shape of each bone fulfills a particular need

Bones are classified by their shape as long, short, flat, or irregular bone

Bones differ in the distribution of compact and spongy osseous tissues

Classification of Bones

Classification:Long Bone

Long bones have a long shaft and two distinct ends

Classification is based on shape not size

Compact bone on exterior w/ spongy on the interior

Classification:Short Bones

Short bones are roughly cubelike

Thin compact bone layer surrounding spongy bone mass

Short bones are often carpal, tarsal and sesamoid bones

Classification:

Flat Bones Flat bones are thin, flattened and usually curved

Parallel layer of compact bone with spongy bone layer between

Skull, sternum and ribs are examples

Classification:

Irregular Bone

Irregular bones don’t fit into the previous categories

Complicated shapes

Consist of spongy bone with a thin layer of compact

Examples are hip bones & vertebrae

Gross Anatomy of Bones

SECTION IV

Gross Anatomy Landmarks

– Diaphysis – Proximal epiphysis

– Distal epiphysis

Membranes– Periosteum

– Endosteum

Diaphysis Long tubular diaphysis is the shaft of the bone

Collar of compact bone surrounds a central medullary or marrow cavity

In adults, cavity contains fat

Epiphysis The epiphyses are the ends of the bone

The joint surface of the epiphysis is covered with articular cartilage

Epiphyseal line separate diaphysis and epiphysis

Blood Vessels Unlike cartilage bone is well vascularized

Nutrient arteries serve the diaphysis

The nutrient artery runs inward to supply the bone marrow and the spongy bony

Medullary cavity The interior of all bones consists largely of spongy bone

The very center of the bone is an open or marrow cavity

The cavity is filled with yellow bone marrow

Membranes Periosteum covers outer bone surfaces except the ends of the epiphysis

The membrane has two sublayers– Superficial layer

– Osteogenic layer

Membranes The superficial layer consists of dense irregular connective tissue which resists tension placed on a bone during bending

The osteogenic layer abuts the compact bone and contains bone-depositing cells called osteoblasts and osteoclasts that are responsible for bone remodeling

Membranes During periods of bone growth or deposition the osteogenic cells differentiate into osteoblasts

Osteoblasts produce the bone tissue that forms the circumferential lamellae that encircle the perimeter of the bone

Membranes Periosteum is richly supplied with nerves and blood vessels

The periosteum is supplied by branches of the nutrient artery and epiphyseal vessels

Membranes The periosteum is secured to the underlying bone by perforating fibers (Sharpey’s fibers)

Thick bundles of collagen fibers run from the periosteum into the bone matrix

Membranes Internal bone structures are covered by a thinner connective tissue membrane the endosteum

It also contains the osteoclasts and osteoblasts necessary for bone remodeling

Membranes The endosteum covers the trabeculae of spongy bone and lines the central canals of osteons

Short, Irregular and Flat Bones

Bones consist of thin layers of compact bones over spongy bone

No shaft, epiphysis or marrow cavity

Spongy area between is a diploe

Flat sandwich of bone is common in bones of skull

Bone Design and Stress The internal anatomy of each bone reflects the stresses most commonly placed upon it

Bones are subjected to compressive forces in weight bearing and tension forces when muscle pulls upon them

Often weight bearing loads are applied off center which threatened to bend the bone

Bone Design and Stress Bending compresses the bone on one side and compresses it on the other

Compression and tension are greatest at the external surfaces of the bone

Bone Design and Stress Compact bone occurs at the external surfaces to resist these tension and compression forces

Internal bone structures are not subjected to these forces and thus spongy bone is sufficient

Bone Design and Stress As there are only limited tension and compression forces at the bone’s center the hollow medullary cavity does not impact a long bone’s weight bearing capacity

Bone Design and Stress Spongy bone is not a random network of bone fragments

The trabeculae align along stress lines in an organized patterns of tiny struts that provide internal support for the bone

Bone Markings Bones are shaped by the tissues that act upon and around them

Bones display bulges, depressions and holes which serve as sites of muscle, ligament and tendon attachment, points of articulation, or as conduits for blood vessels and nerves

Projections from the bone surface include heads, trochanters, spines, and others

Depressions include fossae, sinuses, foramina, and grooves

Bone Markings Tuberosity - a large rounded projection which may be roughened– tibial tuberosity

Bone Markings

Crest - A narrow ridge of bone; usually prominent– Crest of the ilium

Bone Markings Trochanter - A very large, blunt, irregularly shaped process– Greater trochanter of femur

Bone Markings Line - Narrow ridge of bone; less prominent than a crest– Intertrochanteric line

Bone Markings Tubercle - Small rounded projection or process– adductor tubercle

Bone Markings Epicondyle - raised area on or above a condyle– medial epicondyle of the humerous

Bone Markings Spine - A sharp, slender, often pointed projection– Spinous process of vertebrae

Bone Markings Head - Bony expansion carried on a narrow neck– head of the humerus

Bone Markings Facet - Smooth, nearly flat articular surface

– facet on transverse process of thoracic vertebrae

Facet

Bone Markings Condyle - Rounded articular projection– lateral condyle of femur

Bone Markings Ramus - Armlike bar of bone– ramus of the pubis

Bone Markings

Meatus - canal-like passageway– External auditory meatus

Bone Markings Sinus - Cavity within a bone, filled with air and lined with mucous membrane– nasal sinus

Bone Markings Fossa - Shallow, basinlike depression in a bone often serving as an articular surface– Olecranon fossa

Bone Markings Groove - a narrow furrow in the surface of the bone– radial groove

Bone Markings Fissure - Narrow, slitlike opening

Bone Markings Foramen - Round or oval opeing through a bone– Foramen magnum

Compact Bone Compact bone appears very dense It actually contains canals and passageways that provide access for nerves, blood vessels, and lymphatic ducts

The structural unit of compact bone is the osteon or Haversian system

Each osteon is an elongated cylinder running parallel to the long axis of the bone

Functinally each osteon represents a weight bearing pillar

Compact bone

Compact Bone Structurally, an osteon is a group of concentric rings of bone tissue surrounding a central canal

Each of the concentric rings called lamella is a layer of bone matrix in which the collagen fibers and mineral crystals align and run in a single direction

Fibers of adjacent lamella run in roughly opposite direction

Compact bone

Compact bone

An Osteon Each osteon is a group of hollow tubes of bone matrix

Each matrix tube contain lamella

Collagen fibers in each layer run in opposite directions

Orientation resists torsion stresses

Compact Bone The alternating pattern of lamella orientation is optimal for withstanding torsion, stresses

The lamella of bone also inhibit crack propagation

When a crack reaches the edge of a lamella, the forces causing the crack are dispersed around lamellar boundaries, thus preventing the crack from progressing into deeper parts of the bone and causing fracture

An Osteon Running through the core of each osteon is the central or Haversian canal

The canal contains small blood vessels that supply the cells of the osteon

Central or Haversian Canal

The canal is lined by endosteum

The canal contains the blood supply for the osteon

Perforating (Volkmann’s) Canal

Canals lie at right angles to long axis of bone

Connect the vascular supply of the periosteum to those of the central canal and medullary cavity

Compact Bone Osteocytes are the mature bone cells occupying the small spaces in the solid matrix called lacuna

Thin tubes called canaliculi run through the matrix connecting

Compact Bone Osteocytes occupy small cavities or lacuna at the junctions of lamella

Fine canals called canaliculi connect the lacuna to each other and to the central canal

Canaliculi tie all the osteocytes in an osteon together

Compact Bone Canaliculi run through the matrix connecting neighboring lacunae to one another and to the nearest capillaries such as those in the central canal

Within the canaliculi, the extensions of neighboring osteocytes touch each other and form gap junctions

Compact Bone Gap junctions allow nutrients diffusing from the capillaries to cross these junctions

Nutrients are then passed from one osteocyte to the next

Compact Bone The passage of nutrients through gap junctions occurs throughout an entire osteon

This direct transfer from cell to cell is the only way to supply osteocytes with nutrients as the intervening bone matrix is too solid and impermeable to act as a diffusion medium

Compact Bone Osteocytes remain in the matrix they have secreted

Live cells appear to be needed to maintain the matrix

Loss of osteocytes from the matrix results resorbtion of the matrix

Compact Bone Not all lamellae in compact bone occur in osteons

Interstitial lamellae are incomplete lamellae lying between the cylindrical osteons

These represent remnants old osteons cut by bone remodeling

Spongy Bone Spongy bone occurs at the ends of long bones and surrounding the medullary cavity

It is less dense and complex than compact bone

Spongy Bone Trabeculae are the dominate feature

Trabeculae contain irregularly arranged lamallae and osteo-cytes interconnected by canaliculi

There are no osteons present

Osteocytes receive nutrients from capillaries in endosteum

Spongy Bone Trabeculae align along lines of stress

Function as struts of bone

Chemical Composition of Bone

The organic components of bone are:– Osteoblasts (bud cells) – Osteocytes (mature cells) – Osteoclasts (large cells which resorb matrix)

– Osteoid (organic part of the matrix)•Osteoid makes up 1/3 of the matrix•Includes proteogylcans, glycoproteins, & collagen

•These components, particularly collagen contribute to the flexibility and tensile strength of bone to resist stretching and twisting

Chemical Composition of Bone The inorganic components of bone

(65% by mass) consist of hydroxyapatites or mineral salts, largely calcium phosphate

Tiny crystals of calcium salts are deposited in and around the collagen fibers of the extracellular matrix

The crystals are exceptionally hard and resist compression

Organic and inorganic components of matrix allows a bone to be strong but not brittle

Bone Development Osteogenesis and ossification refer to the process of bone formation

Osteogeneis begins in the embryo and continues until adulthood

Remodeling is bone resorption and deposition in response to stress and repair of bone

Bone Development Before week 8 the skeleton of the human embryo is made entirely from hyaline cartilage and mesenchyme membranes

At 8 weeks bone begins to appear and eventually replaces most cartilage and mesenchymal membranes

Bone Development Bones that develop from mesenchymal membranes are called membrane bones

Membrane bones develop from a fibrous membrane in a process called intra-membranous ossification

Other bones develop as hyaline cartilage initially, which is replaced through a process called endochondrial ossification

These are referred to as endochondrial bones or cartilage replacement bones

Intramembranous Ossification

Membrane bones form directly from mesenchyme without being modeled in cartilage

All bones of the skull (except a few at the base of skull) are membrane bones

The clavicles are also membrane bones

Note that most of these bones are flat bones





Endochondral Ossification

Most bones (except clavicles and most skull bones) form by the process of endochondral ossification

The bones are first modeled in hyaline cartilage, which is then gradually replaced by bone tissue

This process uses hyaline cartilage “bones” as the pattern for bone construction


Endochondral ossification begins late in the second month of development and continues into early adulthood when the skeleton is fully ossified

In endochondral ossification the cartilage model of the bone is replaced by bone

Growing endochondral bones increase in length and in width


Cartilage bones are surrounded by a perichondrium

At the 8th week of development, the perichondrium (fibrous connective tissue layer) becomes infiltrated by blood vessels converting it to a vascularized, bone forming periosteum

The increase in nutrition enables the mesenchyme cells to differentiate into osteoblasts that form a collar of bone

Endochondrial Ossification


Formation of a bone collar around diaphysis of cartilage model

Osteoblasts of the new periosteum secrete osteoid against the hyaline cartilage along the length of the diaphysis

Endochondral Ossification Cartilage in the

center of the diaphysis calcifies

Calcification of cartilage blocks nutrients and chondrocytes die

Matrix deteriorates and cavities develop

Bone is stabilized by collar; while new cartilage adds to bone growth


Invasion of the internal cavities by the periosteal bud

Bud contains nutrient artery & vein, lymphatics, nerve fibers, red marrow elements, osteoblasts and osteoclasts

Spongy bone forms


Formation of the medullary cavity as ossification continues

Secondary ossification centers form in epiphyses

Cartilage in epiphyses calcifies and deteriorates opening cavities for entry of periosteal bud


Ossification of the epiphyses

Hyaline cartilage remains only at epiphyseal plates

Epiphyseal plates promote growth along long axis

Ossification chases cartilage formation along length of shaft


After the secondary ossification sites have appeared and epiphyses have largely ossified, hyaline cartilage remains on– Epiphyseal surfaces where it forms articular cartilages

– Between the diaphysis and the epiphysis where it forms the epiphyseal plates

– The epiphyseal plates or growth plates are responsible for lengthening of bones during the two decades following birth

Long Bone Growth Cells in the epiphyseal plate undergo rapid cell mitosis pushing epiphysis away from diaphysis

Older cells enlarge, matrix becomes calcified

Chondrocytes die and their matrix deteriorates

Calcified cartilage is covered by bone matrix secreted by osteoblasts to form spongy bone

Epiphyseal Growth Areas

In the epiphysis of the fetus and the epiphyseal plates are organized to allow bones to grow quickly & efficiently

The cartilage cells nearest the epiphysis (quiescent zone) are relatively inactive


The cartilage cells form tall columns in the proliferation zone

The rapid division of chondroblasts push the epiphysis away from the diaphysis

The growth here lengthens the entire long bong


Older cartilage cells that are deeper in the column in the (hypertrophic zone) enlarge and signal the surrounding matrix to calcify

In the calcification or osteogenic zone the matrix becomes calcified and the chondrocytes die


The process of ossification leaves long spicules (trabeculae) of calcified cartilage on the diaphysis side

The spicules are then covered with bone tissue by osteoblasts

Osteoclasts complete the remodeling of the bone

Postnatal Bone Growth During childhood and adolescence bone growth occurs entirely by growth at the epiphyseal plates

In growing bones cartilage is replaced with bone tissue on the diaphysis side about as quickly as it grows

The epiphyseal plate remains a constant thickness while the overall length of the bone increases

Postnatal Bone Growth As the end adolescence approaches, the chondroblasts in the epiphyseal plate divide less often

The epiphyseal plates become thinner, eventually exhausting their supply of mitotically active catilage cells

The cartilage stops growing and is replaced by bone tissue

The epiphyseal plate fuses and growth is done (18 female, 21 male)

Postnatal Bone Growth Bones grow in width by appositional growth

Osteoblasts in the periosteum add bone tissue to the external surface of the diaphysis

Osteoclasts in the endosteum remove bone from the internal surface of the diaphysis wall

These two processes occur at roughly the same rate

Postnatal Bone Growth Other types of endochondial bones grow in slightly different patterns

Bone growth is regulated by several hormones– Pituitary simulates growth at plates

– Thyroid regulates growth to ensure that the skeleton retains proper proportions

– Sex hormones influence growth at adolescent growth spurts

Growth and Remodeling

Bone Remodeling Bone is dynamic and active tissue Long bone growth is accompanied by almost continuous remodeling in order to maintain proper proportions

Large amounts of bone matrix and thousands of osteocytes are being continually removed and replaced

The small scale architecture of bones changes constantly

Bone Remodeling The spongy bone of the skeleton is replaced every 3 years

The compact bone is replaced every 10 years

The remodeling process is not uniform as some parts experiencing more stress are replaced at a faster rate (every 5-6 months) while other areas change more slowly

Bone Remodeling Bone remodeling involves both bone formation and resorption

Remodeling occurs at the periosteal and endosteal sufaces

Bone formation is done by osteoblasts and bone resorption is done by osteoclasts

Bone Remodeling

Bone remodeling is coordinated by cohorts of adjacent osteoclasts called remodeling units

Osteoclasts crawl along the bone surfaces digging pits as they break down bone surfaces

Bone Remodeling Osteoclasts are large cells with many nuclei

Their plasma membrane is highly folded or ruffled

Bone Remodeling The ruffled plasma membrane forms a tight seal against the bone and HCL dissolves the mineral portion of the matrix

Bone Remodeling Osteoclasts release calcium ions (Ca2+) and phosphate ions (PO4

3-) that enters the tissue fluid and the bloodstream

Lysosomal enzymes are also released by the osteoclasts and digest the organic part of the bone matrix

Finally, osteoclasts take up collagen and dead osteocytes by phagocytosis

Bone Remodeling

Bone deposition is accomplished by osteoblasts

Osteoblasts lay organic osteoid on bone surfaces and calcium salts crystalize within this osteoid

Bone Remodeling Bone forming osteoblasts form from mesenchyme-like stem cells located in the periosteum, endosteum, and the connective tissue of nearby bone marrow

Osteoclasts form in bone marrow from immature blood cells called hematopoietic stem cells

Many of these stem cells fuse together to form each osteoclast, thus their multinucleate structure

Bone Remodeling Bone of the skeleton are continually remodeled for 2 reasons– Bone remodeling helps maintain constant concentrations of Ca2+ and PO4

3- in bodily fluids– Bones are remodeled in response to the mechanical stress it experiences•Osteons of compact bone and the trabeculae of spongy bone are constantly replaced by new osteons and trabeculae that are more precisely aligned with newly experienced compressive and tensile forces

Bone Anatomy and Stress Wolff’s law holds that a bone grows or remodels in response to the forces which act upon it

Changes in bone density in response to exercise

Tension and compression forces must balance

Healing of a Bone Fracture

Common Types of Fractures



Documents

BONES AND BONE TISSUES CHAPTER 6 9/16/07. Introduction One of the most remarkable tissues of the human body Far from inert and lifeless, bones are living,