Chapter 6:  Skeletal System:  Bones and Bone Tissue


Skeletal System Functions

•      Support

•      Protection

•      Movement

•      Storage

•      Blood cell production

Function of Bones

•      Support – form the framework that supports the body and cradles soft organs

•      Protection – provide a protective case for the brain, spinal cord, and vital organs

•      Movement – provide levers for muscles

•      Mineral storage – reservoir for minerals, especially calcium and phosphorus

•      Blood cell formation – hematopoiesis occurs within the marrow cavities of bones

Skeletal System

•      Skeleton = “dried-up body” or “mummy”

•      think of sky-scraper analogy - strong, light, and perfectly adapted for protection, locomotion, and manipulation

•      20% of body mass (ie. 30 lbs of a 160 lb person)

•      Like other connective tissues, contains fibers, ground substance, and cells (ECM is calcified)

Skeletal Cartilage

•      Contains no blood vessels or nerves

•      Surrounded by the perichondrium (dense irregular CT) that resists outward expansion; also source of blood and nutrients

Hyaline Cartilage

•      Consists of specialized cells that produce matrix

•      Cells

•     Chondroblasts

•     Chondrocytes

•      Perichondrium

•      Articular cartilage

•      Growth

•     Appositional

•     Interstitial

Growth of Cartilage

•      Appositional (“from the outside”) – cells in the perichondrium secrete matrix against the external face of existing cartilage (mainly fibroblast mediated)

•      Interstitial (“from the inside”) – lacunae-bound chondrocytes inside the cartilage divide and secrete new matrix, expanding the cartilage from within

Classification of Bones

•      Axial skeleton (depicted in green) – bones of the skull, vertebral column, and rib cage

•      Appendicular skeleton (depicted in gold) – bones of the upper and lower limbs, shoulder, and hip

Bone Shapes

•      Long

•     Upper and lower limbs

•      Short

•     Carpals and tarsals

•      Flat

•     Ribs, sternum, skull, scapulae

•      Irregular

•     Vertebrae, facial

Classification of Bones: By Shape

•      Long bones – longer than they are wide
(e.g., humerus)

•      all bones of limbs (except patella and ankle)

•      Short bones

•    Cube-shaped bones of the wrist and ankle

•    Sesamoid bones form within tendons (e.g., patella)

•      Flat bones – thin, flattened, and a bit curved (e.g., sternum, and most skull bones)

•      Irregular bones – bones with complicated shapes (e.g., vertebrae and hip bones)

Structure of Long Bone

•      Diaphysis

•    Tubular shaft that forms the axis of long bones

•    Composed of compact bone that surrounds the medullary cavity

•    Yellow bone marrow (contains fat) is contained in the medullary cavity

Structure of Long Bone

•      Epiphyses

•    Expanded ends of long bones

•    Exterior is compact bone, and the interior is spongy bone

•    Joint surface is covered with articular (hyaline) cartilage

•    Epiphyseal plate/line (hyaline cartilage that grows during childhood) line separates the diaphysis from the epiphyses

Long Bone Structure

•      Medullary cavity

•     Red marrow

•     Yellow marrow

•      Periosteum

•     Outer bone surface

•      Sharpey’s fibers

•     Attachment

•      Endosteum

•     Lines bone cavities

Structure of Short, Irregular, and Flat Bones

•      Thin plates of periosteum-covered compact bone on the outside with endosteum-covered spongy bone (diploλ in flat bones) on the inside

•      Have no diaphysis or epiphyses

•      Contain bone marrow between the trabeculae

Gross Anatomy of Bones

•      Organs - contain nervous, cartilage, muscle, epithelial, and connective tissues

•      Compact bone – dense outer layer

•      Spongy bone (cancellous) – honeycomb of trabeculae (little beams) filled with yellow bone marrow

•      Why?

Bone Membranes

•      Periosteum (around bone) – double-layered protective membrane

•    Outer fibrous layer is dense regular CT

•    Inner osteogenic layer is composed of osteoblasts (germinators) and osteoclasts (breakers)

•    Richly supplied with nerve fibers, blood, and lymphatic vessels, which enter the bone via nutrient foramina

•    Secured to underlying bone by Sharpey’s fibers (anchor points for tendons and ligaments)

•      Endosteum (within bone) – delicate membrane covering internal surfaces of bone such as trabeculae and canals; also contains osteo-cells

Location of Hematopoietic Tissue (Red Marrow)

•      In infants

•    Found in the medullary cavity and all areas of spongy bone

•      In adults

•    Found in the diploλ of flat bones, and the head of the femur and humerus

•    Also found in some irregular bones such as the hip bone (routinely used for red marrow sampling)

Bone Histology

•      Bone matrix

•    Organic: Collagen and proteoglycans (35%)

•    Inorganic: Hydroxapatite (65%)

•      Bone cells

•    Osteoblasts

•    Osteocytes

•    Osteoclasts

•    Stem cells or osteochondral progenitor cells

•      Woven bone: Collagen fibers randomly oriented

•      Lamellar bone: Mature bone in sheets

•      Cancellous bone: Trabeculae / Compact bone: Dense

Chemical Composition of Bone: Inorganic

•      Hydroxyapatites, or mineral salts

•    65% percent of bone by mass

•    Mainly calcium phosphates

•    Responsible for bone hardness and its resistance to compression

Chemical Composition of Bone: Organic

•      Osteoblasts – bone-forming cells

•      Osteocytes – mature bone cells

•      Osteoclasts – large cells that resorb or break down bone matrix

•      Osteoid – unmineralized bone matrix composed of proteoglycans, glycoproteins, and collagen (35% by mass; mostly collagen)

Bone Cells

•      Osteoblasts

•     Formation of bone through ossification or osteogenesis

•      Osteocytes

•     Mature bone cells

•     Lacunae: Spaces

•     Canaliculi: Canals

•      Osteoclasts

•     Responsible for bone resorption

Spongy Bone

•      Branching trabeculae that are a few cell layers thick

•      contains lacunae and canaliculi

•      functions to lighten bone and serve as struts

Microscopic Structure of Bone: Compact Bone

•      Haversian system, or osteon – the structural unit of compact bone

•    Lamella – weight-bearing, column-like matrix tubes composed mainly of collagen

•    Haversian, or central canal – central channel containing blood vessels and nerves

•    Volkmann’s canals – channels lying perpendicular to the central canal, connecting blood and nerve supply of the periosteum to that of the Haversian canal

Microscopic Structure of Bone: Compact Bone

•    Osteocytes – mature bone cells

•    Lacunae – small cavities in bone that contain osteocytes

•    Canaliculi – hairlike canals that connect lacunae to each other and the central canal - formed following hardening of the matrix around tentacle-like projections of osteoblasts that occur during bone formation

Bone Markings (use as landmarks to learn bones)

•      Bulges, depressions, and holes that serve as:

•    Sites of attachment for muscles, ligaments, and tendons

•    Joint surfaces

•    Conduits for blood vessels and nerves

Cancellous Bone

•      Consists of trabeculae

•     Oriented along lines of stress

Compact Bone

•      Central or haversian canals: Parallel to long axis

•      Lamellae: Concentric, circumferential, interstitial

•      Osteon or haversian system: Central canal, contents, associated concentric lamellae and osteocytes

•      Perforating or Volkmann’s canal: Perpendicular to long axis

Bone Development

•      Begins at week 8 of embryo development

•      Intramembranous ossification

•    Takes place in connective tissue membrane - bone develops from a fibrous membrane

•      Endochondral ossification

•    Takes place in cartilage - bone forms by replacing hyaline cartilage

•      Both methods of ossification

•    Produce woven bone that is then remodeled

•    After remodeling, formation cannot be distinguished as one or other

Intramembranous Ossification

•      Formation of most of the flat bones of the skull and the clavicles

•      Fibrous connective tissue membranes are formed by mesenchymal cells

Stages of Intramembranous Ossification

•      An ossification center appears in the fibrous CT membrane

•      Bone matrix is secreted within the fibrous membrane

•      Woven bone and periosteum form

•      Bone collar of compact bone forms, and red marrow appears

Endochondral Ossification

•      Essentially, base of skull down (except clavicles)

•      Begins in the second month of development

•      Uses hyaline cartilage “bones” as models (patterns) for bone construction

•      Requires breakdown of hyaline cartilage prior to ossification

Stages of Endochondral Ossification

•      Formation of bone collar (periosteum)

•      Cavitation of the hyaline cartilage

•      Invasion of internal cavities by the periosteal bud, and spongy bone formation

•      Formation of the medullary cavity; appearance of secondary ossification centers in the epiphyses

•      Ossification of the epiphyses, with hyaline cartilage remaining only in the epiphyseal plates

Postnatal Bone Growth

•      Growth in length of long bones

•    Cartilage on the side of the epiphyseal plate closest to the epiphysis is relatively inactive

•    Cartilage abutting the shaft of the bone organizes into a pattern that allows fast, efficient growth

•    Cells of the epiphyseal plate proximal to the resting cartilage form functionally different zones: growth, transformation, and osteogenic

Functional Zones in Long Bone Growth

•      Resting zone

•      Proliferation zone – cartilage cells undergo mitosis, pushing the epiphysis away from the diaphysis

•      Transformation zone – older cells enlarge, the matrix becomes calcified, cartilage cells die, and the matrix begins to deteriorate

•      Osteogenic zone – new bone formation occurs

Long Bone Growth and Remodeling

•      Growth in length – cartilage continually grows and is replaced by bone as shown

•      Remodeling – bone is resorbed and added by appositional growth as shown

Growth in Bone Length

•      Appositional growth

•     New bone on old bone or cartilage surface

•      Epiphyseal plate zones

•     Resting cartilage

•     Proliferation

•     Hypertrophy

•     Calcification

•     Osteogenic

Factors Affecting Bone Growth

•      Nutrition

•     Vitamin D

•   Necessary for absorption of calcium from intestines

•   Insufficient causes rickets and osteomalacia

•     Vitamin C

•   Necessary for collagen synthesis by osteoblasts

•   Deficiency results in scurvy

•      Hormones

•     Growth hormone from anterior pituitary

•     Thyroid hormone required for growth of all tissues

•     Sex hormones as estrogen and testosterone

Hormonal Regulation of Bone Growth During Youth

•      During infancy and childhood, epiphyseal plate activity is stimulated by growth hormone

•      During puberty, by testosterone and estrogens

•    Initially promote adolescent growth spurts

•    Cause masculinization and feminization of specific parts of the skeleton

•    Later induce epiphyseal plate closure, ending longitudinal bone growth 

Bone Remodeling

•      Remodeling units – adjacent osteoblasts and osteoclasts deposit and resorb bone at periosteal and endosteal surfaces

Bone Remodeling

•      Coverts woven bone into lamellar bone

•      Bone constantly removed by osteoclasts and new bone formed by osteoblasts

Bone Deposition

•      Occurs where bone is injured or added strength is needed

•      Requires a diet rich in protein, vitamins C (collagen synthesis), D (Ca++ absorption), and A (balance between deposit and removal), calcium, phosphorus, magnesium, and manganese

•      Alkaline phosphatase is essential for mineralization of bone

Bone Resorption

•      Accomplished by osteoclasts

•      Resorption bays – grooves formed by osteoclasts as they break down bone matrix

•      Resorption involves osteoclast secretion of:

•    Lysosomal enzymes that digest organic matrix

•    Acids that convert calcium salts into soluble forms

•      Dissolved matrix is transcytosed across the osteoclast’s cell where it is secreted into the interstitial fluid and then into the blood

Importance of Ionic Calcium in the Body

•      99% of Calcium is present in bone

•      Calcium is necessary for:

•    Transmission of nerve impulses

•    Muscle contraction

•    Blood coagulation

•    Secretion by glands and nerve cells

•    Cell division

Control of Remodeling

•      Two control loops regulate bone remodeling

•    Hormonal mechanism that maintains calcium homeostasis in the blood (whether and when)

•    Mechanical and gravitational forces acting on the skeleton (where)

Calcium Homeostasis

•      Bone is the major storage site for calcium in the body

•    Calcium moves into bone as osteoblasts build new bone

•    Calcium moves out of bone as osteoclasts break down bone

•    When osteoclast and osteoblast activity is balanced, the movement of calcium in and out is equal

Hormonal Mechanism

•      Rising blood Ca2+ levels trigger the thyroid to release calcitonin

•      Calcitonin stimulates calcium salt deposit in bone and inhibits resorption

Hormonal Mechanism

•      Falling blood Ca2+ levels signal the parathyroid glands to release PTH

•      PTH signals osteoclasts to degrade bone matrix and release Ca2+ into the blood

Bone Fractures (Breaks)

•      Bone fractures are classified by:

•    The position of the bone ends after fracture (displacement)

•    Completeness of the break

•    The orientation of the break to the long axis (linear or transverse)

•    Whether or not the bones ends penetrate the skin (compound - open or simple – closed)

Types of Bone Fractures

•      Nondisplaced – bone ends retain their normal position

•      Displaced – bone ends are out of normal alignment

•      Complete – bone broken all the way through

•      Incomplete – bone is not broken all the way through (hairline)

•      Linear – the fracture is parallel to the long axis of the bone

•      Transverse – the fracture is perpendicular to the long axis of the bone

•      Compound (open) – bone ends penetrate the skin

•      Simple (closed) – bone ends do not penetrate the skin

Common Types of Fractures

•      Comminuted – bone fragments into three or more pieces; common in the elderly

•      Compression – bone is crushed; common in porous bones

Stages in the Healing of a Bone Fracture

•      Hematoma formation (Osteomyelitis)

•    Torn blood vessels hemorrhage

•    A mass of clotted blood (hematoma) forms at the
fracture site

•    Site becomes swollen, painful, and inflamed

Stages in the Healing of a Bone Fracture

•      Fibrocartilaginous callus forms

•      Granulation tissue (soft callus) forms a few days after the fracture

•      Capillaries grow into the tissue and phagocytic cells begin cleaning debris

Stages in the Healing of a Bone Fracture

•      The fibrocartilaginous callus forms when:

•    Osteoblasts and fibroblasts migrate to the fracture and begin reconstructing the bone

•    Fibroblasts secret collagen fibers that connect broken bone ends

•    Osteoblasts begin forming spongy bone

•    Osteoblasts furthest from capillaries secrete an externally bulging cartilaginous matrix that later calcifies

Stages in the Healing of a Bone Fracture

•      Bony callus (ossification) formation

•    New bone trabeculae appear in the fibrocartilaginous callus

•    Fibrocartilaginous callus converts into a bony (hard) callus

•    Bone callus begins 3-4 weeks after injury, and continues until firm union is formed 2-3 months later

Stages in the Healing of a Bone Fracture

•      Bone remodeling

•    Excess material on the bone shaft exterior and in the medullary canal is removed

•    Compact bone is laid down to reconstruct shaft walls

Effects of Aging on Skeletal System

•      Bone Matrix decreases

•      Bone Mass decreases

•      Increased bone fractures

•      Bone loss causes deformity, loss of height, pain, stiffness

•    Stooped posture

•    Loss of teeth

Homeostatic Imbalances

•      Osteoporosis

•    Group of diseases in which bone reabsorption outpaces bone deposit

•    Spongy bone of the spine is most vulnerable

•    Occurs most often in postmenopausal women

•      Treatment

•    Calcium and vitamin D supplements

•    Increased weight bearing exercise

•    Hormone (estrogen) replacement therapy (HRT)

•      Prevented or delayed by sufficient calcium intake and weight-bearing exercise


Bones and Skeletal Tissue

         I.    Skeletal Cartilages

A. Basic Structure, Types, Location

B.          Growth of Cartilage


        II.    Classification of Bones

A. Shape

       III.    Functions of the Bones

A. Support

B.          Protection

C.         Movement

D. Storage

E.          Blood Cell Formation

      IV.    Bone Structure

A.          Gross Anatomy

1.   Types of Tissue

a.     Compact

b.     Spongy

2.   Typical Long Bone

a.     Diaphysis

b.     Epiphysis

c.     Epiphyseal Line

d.    Periosteum

e.     Endosteum

f.      Articular Cartilage

2.   Short, Irregular, and Flat Bone

a.     Compact and Spongy Bone

b.     Diploe

c.     Periosteum

d.    Endosteum

3.   Location of Hematopoietic Tissue in Bones

a.     Red Marrow

b.     Red Marrow Cavities

c.     Yellow Marrow

B.          Microscopic Structure of Bone

1.   Compact Bone

a.     Haversian System (Osteon)

b.     Lamellar Bone

c.     Haversian Canal

d.    Volkmann’s Canals

e.     Osteocytes

f.      Lacunae

g.     Canaliculi

h.    Interstitial Lamellae

i.      Circumferential Lamellae

2.   Spongy Bone

a.     Trabeculae

b.     Lamellae

c.     Osteocytes

C.         Chemical Composition of Bone

1.   Organic Components

a.     Cells

b.     Osteoid

2.   Inorganic Components

             3.   Bone Markings

        V.    Bone Development (Osteogenesis)

A. Formation of the Bony Skeleton

1.   Intramembranous Ossification

2.   Endochondral Ossification

B.          Postnatal Growth

1.   Growth in Length of Long Bones

2.   Growth in Width (Thickness)

3.   Hormonal Regulation of Bone Growth During Youth

      VI.    Bone Homeostasis: Remodeling and Repair

A. Bone Remodeling

1.   Basic Processes

a.     Bone Remodeling Units

b.     Bone Deposition

c.     Bone Reabsorption

2.   Control of Remodeling

a.     Negative Hormonal Feedback

b.     Mechanical Stress

B.          Repair of Fractures

1.   Types of Fractures

2.   Treatment

3.   Phases of Repair

a.     Hematoma Formation

b.     Callus Formation

c.     Bony Callus Formation

d.    Remodeling

     VII.    Homeostatic Imbalances of Bone

A. Osteoporosis  


PART I: The Axial Skeleton

         I.    Basic Regions

Part 2: The Appendicular Skeleton

         I.    Basic Regions