• CHAPTER  7 The Skeletal System




    1.         List and discuss at least 6 functions of bone tissue.


    2.         Name the four classifications of bones by shape, and give an example of each.


    3.         Define the terms sesamoid bone and Wormian (sutural) bone and give an example of each.


    4.         Illustrate the major features of a long bone including the following: diaphysis, epiphyses, epiphyseal line, periosteum, endosteum, medullary cavity, nutrient foramen and note the locations of spongy bone, compact bone, yellow marrow, red marrow and articular cartilage.


    5.         List the functions of the periosteum.


    6.         Compare and contrast the organic and inorganic components of bone matrix, in terms of structure and function.


    7.         List the terms that are synonymous with inorganic bone matrix. 


    8.         Discuss the different types of bone cells in terms of origin, location and function.


    9.         Distinguish between compact bone and spongy bone, in terms of structure and function.


    10.       Discuss the Haversian (Osteon) System as the structural unit of compact bone using the following terms:  osteocytes, lacunae, lamellae, Haversian canal, blood vessels, bone matrix, and canaliculi.


    11.       Explain how adjacent Haversian Systems communicate with one another (i.e. exchange nutrients, gases and wastes).


    12.       Discuss the significance of the spongy bone within a flat bone.


    13.       Define the term hematopoiesis and name the major skeletal locations where it occurs.


    14.       Name the important function that the trabeculae in spongy or cancellous bones allow for.


    15.       Define the term ossification.


    16.       Distinguish between intramembranous and endochondral ossification, and denote which parts of the skeleton are formed by each.


    17.       Discuss the structure of the epiphyseal plate, explain its significance, and discuss its fate.


    18.       Compare and contrast appositional bone growth and longitudinal bone growth.

    19.       Explain why ossification is a lifelong event.


    20.       List the vitamins and minerals involved in bone remodeling and discuss the action (and any resulting deficiency) of each.


    21.       List the major hormones involved in bone development and remodeling.


    22.       Compare and contrast the functions of osteoblasts and osteoclasts in bone remodeling.


    23.       Fully discuss the negative feedback mechanisms involved in blood calcium (Ca++) homeostasis, and explain how this is related to bone remodeling.


    24.       Distinguish between the axial and appendicular skeleton.


    25.       Define the term suture and designate the major sutures on a diagram of the skull.


    26.       Be able to distinguish right from left of any paired bone.


    27.       Name the eight bones that protect the brain (i.e. cranium).


    28.       Identify the 4 skull bones that contain paranasal sinuses and give two possible functions for sinuses.


    29.       Illustrate the location of the bony structure on all figures in Chapter 7.


    30.       Name the major bones that shape the face.


    31.       Define the parts of the zygomatic arch.


    32.       Name the seven bones that compose the orbit of the eye.


    33.       Explain how the nasal septum is actually composed of two different bones.


    34.       Identify the only skull bone, which is not fused or locked in place, and name the joint at which it moves.


    35.       Describe the structure, location and function of the hyoid bone.


    36        List the 4 major curvatures of the vertebral column and 5 regions of the vertebral column and identify the number of vertebrae in each.


    37.       Explain how the 33 infantile vertebrae become 26 adult bones.


    38.       Name the substance that acts as a "shock absorber" between individual vertebrae.


    39.       Denote the 10 structures all vertebrae have in common.

    40.       Distinguish between the three types of vertebrae.


    41.       List the components of the thoracic cage.


    42.       Distinguish between true, false, and floating ribs.


    43.       Distinguish between the manubrium, body and xiphoid process of the sternum.


    44.       Name the bones in the upper limbs and denote them on a skeleton.


    45.       Distinguish between capitulum and trochlea, name the bone they are part of, and discuss their significance.


    46.       Note the relative positions of the radius and ulna, and name the significance of the olecranon (process).


    47.       Identify the number of bones that make up the wrist, palm region of hand, and fingers, and give the scientific name for each.


    48.       Explain how the bones of the pelvis articulate anteriorly and posteriorly.


    49.       Name the tissue that composes the anterior articulation of the coxal bones.


    50.       Distinguish between a male and female pelvis, in terms of differences in the greater (false) pelvis, the pelvic brim (inlet), the pubic arch (angle), the acetabulum.


    51.       Name the longest, strongest, and largest bone in the body.


    52.       Identify the significance of trochanters.


    53.       Explain why the patella is unique.


    54.       Compare and contrast the structure, location and function of the tibia and fibula, and denote the location of the lateral and medial malleolus.


    55.       Identify how many bones compose the ankle, foot and toes, and give the scientific name for each.


    56.       Distinguish between the talus and calcaneus.

    I.          INTRODUCTION


    The organs of the skeletal system include the bones and the structures that connect bones to other structures, including ligaments, tendons, and cartilages.


    II.        BONE STRUCTURE:  See Figure 7.1, page 183.


    Bones are classified according to their shape:


    A.        Long bones consist of a shaft with two ends.


    1.         Examples include:


    a.         thigh bone = femur,

    b.         upper arm bone = humerus.


    B.        Short bones are cube-like.


    1.         Examples include:


    a.         wrist bones = carpals,

    b.         ankle bones = tarsals.


    C.        Flat bones are thin and usually curved.


    1.         Examples include:


    a.         most skull bones,

    b.         breast bone = sternum,

    c.         shoulder blades = scapulae,

    d.         ribs.

    e.         see H below


    D.        Irregular bones are not long, short, or flat.


    1.         Examples include:


    a.         vertebrae,

    b.         auditory ossicles.


    E.         Sesamoid bones develop within a tendon.

    1.         The patella is a human sesamoid bone.


    F.         Wormian bones (or sutural bones) are tiny bones within the skull that lie between major skull bones.

    See Fig 7.16, page 197.



    G.        Parts of a Long Bone


                                        See Figure 7.2, page 183 and Figure 7.3a and b, page 184.


                                                    1.         Diaphysis = shaft.  


    a.         consists of a central medullary cavity (filled with yellow marrow)

    b.         surrounded by a thick collar of compact bone.


                                                    2.         Epiphyses (pl) = expanded ends.


    a.         consist mainly of spongy bone  

    b.         surrounded by a thin layer of compact bone.


                                                    3.         Epiphyseal line = remnant of epiphyseal disk/plate.


    a.         cartilage at the junction of the diaphysis and epiphyses (growth plate).


    4.         Periosteum = outer, fibrous, protective covering of diaphysis.


    a.         richly supplied with blood & lymph vessels, nerves (nutrition):


    ·          Nutrient Foramen = perforating canal allowing blood vessels to enter and leave bone.


    b.         Osteogenic layer contains osteoblasts (bone-forming cells) and osteoclasts (bone-destroying cells);


    c.         serves as insertion for tendons and ligaments.


                                                    5.         Endosteum = inner lining of medullary cavity.


    a.         contains layer of osteoblasts & osteoclasts.


                                                    6.         Articular cartilage = pad of hyaline cartilage on the epiphyses where long bones articulate or join.


    a.         "shock absorber



    H.        Flat bones       See Fig 7.3c, page 184.


    1.         covered by periosteum-covered compact bone;

    2.         surrounding endosteum-covered spongy bone. 

    3.         In a flat bone, the arrangement looks like a sandwich:

    m  spongy bone (meat), sandwiched between

    m  two layers of compact bone (bread).   

    *          Hematopoietic tissue (red marrow) is located in the spongy bone within the epiphyses of long bones and flat bones.


    I.          Chemical Composition of Bone (both organic and inorganic)

                Review Ch 5 pages 147-148


                                                    1.         Organic components (35%):

                                        a.         Cells:

    o          osteoprogenitor cells

    1.         derived from mesenchyme,

    2.         can undergo mitosis and become osteoblasts.

    o          osteoblasts

    1.         form bone matrix by secreting collagen,

    2.         cannot undergo mitosis.

    o          osteocytes       See Fig 7.4, page 185, and

                                        Fig 7.7, page 186.

    1.         mature bone cells derived from osteoblasts;

    2.         principle bone cell,

    3.         cannot undergo mitosis,

    4.         maintain daily cellular activities (i.e. exchange of nutrients & wastes with blood).

    o          osteoclasts       See Fig 7.10, page 189.

    1. functions in bone resorption (i.e. destruction of bone matrix),

    2.         important in development, growth, maintenance & repair of bone.

    b.         Osteoid

    o          primarily collagen which

    o          gives bone its high tensile strength.

    o        Also contains glycolipids and glycoproteins


                                                    2.         Inorganic component (65%):


                                                                            a.         Hydroxyapatite (mineral salts), which is primarily


    o          calcium phosphate [Ca3(PO4)2.(OH)2] which

    o          gives bone its hardness or rigidity.



    J.          Microscopic Structure of Compact Bone


                                        1.         Compact Bone is solid, dense, and smooth.


    2.         Structural unit = Haversian system or Osteon.


    See Fig 7.4, page 185 and Fig 7.5, page 185.


    a.         elongated cylinders cemented together to form the long axis of a bone;

                                                                            b.         Components of Haversian system:

    o          osteocytes (spider-shaped bone cells that lie in "lacunae") that have laid down a   

    o          matrix of collagen and calcium salts in

    o          concentric lamellae (layers) around a  

    o          central Haversian canal containing

    o          blood vessels and nerves.


                                                                            c.         Communicating canals within compact bone:

    o          Canaliculi connect the lacunae of osteocytes;

    o          Volkmann's canals connect the blood & nerve supply of adjacent Haversian systems together.

    1.         run at right angles to and connects  adjacent Haversian canals.


    K.        Microscopic Structure of Spongy (Cancellous) Bone       

                See Fig 7.3b and c, page 184.


    1.         consists of poorly organized trabeculae (small needle-like pieces of bone)

    2.         with a lot of open space between them.

    3.         nourished by diffusion from nearby Haversian canals.


    III.       BONE DEVELOPMENT AND GROWTH  (Osteogenesis/ossification)


    A.        Introduction


    1.         The "skeleton" of an embryo is composed of fibrous CT membranes (formed from mesenchyme and hyaline cartilage) that are loosely shaped like bones.


    2.         This "skeleton" provides supporting structures for ossification to begin. 


    3.         At about 6-7 weeks gestation, ossification begins and continues throughout adulthood.


    III.       BONE DEVELOPMENT AND GROWTH  (Osteogenesis/ossification)


    B.        Ossification follows one of two patterns:


    See Fig 7.6, page 186 & See Table 7.1, page 187 for a summary.


    Both mechanisms involve the replacement of preexisting CT with bone.


    1.         Intramembranous Ossification is when a bone forms on or within a fibrous CT membrane. 


                                                                            a.         Flat bones are formed in this manner (i.e. skull bones, clavicles);


                                                    2.         Endochondral Ossification occurs when a bone is formed from a hyaline cartilage model. 


    a.         Most bones of the skeleton are formed in this manner.

    b.         Primary Ossification center hardens as fetus and infant.

    c.         Secondary Ossification centers develop in child and harden during adolescence and early adulthood.

    d.         See Table 7.2  page 189.


    During infancy and childhood, long bones lengthen entirely by growth at the epiphyseal plates (called longitudinal growth) and all bones grow in thickness by a process called appositional growth.


                            C.        Growth at the Epiphyseal Plate       See Fig 7.9, page 188.


    1.         Structure of the Epiphyseal Plate or Disc (4 zones):


    a.         Zone of resting cartilage

    o          near epiphysis,

    o          small, scattered chondrocytes,

    o          anchor plate to epiphysis.


    b.         Zone of proliferating cartilage

    o          larger chondrocytes that resemble a stack of coins,

    o          Chondrocytes divide to replace those that die at the diaphyseal surface of the epiphysis.


    c.         Zone of Hypertrophic cartilage

    o          extremely large chondrocytes that are arranged in columns,

    o          maturing cells.

    C.        Growth at the Epiphyseal Plate


                            1.         Structure of the Epiphyseal Plate or Disc (4 zones):


    d.         Zone of calcified cartilage

    o          only a few cells thick,

    o          consists of dead cells because the matrix around them became calcified,

    o          This calcified matrix is destroyed by osteoclasts and is then invaded by osteoblasts and capillaries from the diaphysis.

    o          The osteoblasts lay down bone on the calcified cartilage that persists.

    o          As a result, the diaphyseal border of the plate is firmly cemented to the bone of the diaphysis.


    2.         The epiphyseal plate allows for bone lengthening until adulthood.  As a child grows


    a.         Cartilage cells are produced by mitosis on the epiphyseal side of the plate,

    b.         They are then destroyed and replaced by bone on the diaphyseal side of the plate.


    *          Therefore, the thickness of the plate remains almost constant, while the bone on the diaphyseal side increases in length.


    3.         The rate of bone growth is controlled by:


    a.         human Growth Hormone (hGH) from the pituitary

    b.         sex hormones from the gonads (see below)


    4.         Ossification of most bones is completed by age 25.

    See Ossification Timetable 7.2, page 189.



    A.        Support


    1.         The bones in legs and pelvis support the trunk,

    2.         The atlas (1st vertebra) supports the skull, etc.


    B.        Protection of underlying organs


    1.         The skull protects the brain,

    2.         The rib cage protects the heart and lungs, etc.


    C.        Body Movement


    1.         Skeletal muscles attached to bones by tendons.

    a.         serve as levers to move bones.

    b.         See figure 7.13 page 191 for examples of levers

    c.         See figure 7.14 page 194 for examples of how muscles pull on bones


    D.        Hematopoiesis


    1.         Definition = Blood Cell Formation

    a.         All blood cells are formed in the red marrow of certain bones.


    E.         Inorganic Salt Storage


    1.         Bone stores many minerals

    a.         calcium,

    b.         phosphorus

    c.         others.

                            2.         Also a means of calcium homeostasis

                            3.         See section V: Bone Remodeling and Repair


    F.         Energy Storage


    1.         Yellow marrow in the shaft of long bones

    2.         serve as an important chemical energy reserve.



    Once a bone has been formed, it is continuously being remodeled throughout life.  This process involves the action of osteoblasts and osteoclasts, two hormones (calcitonin & parathyroid hormone) and in turn affects blood calcium homeostasis.


    A.        Rate of Remodeling Varies:


    1.         Distal femur is replaced every four months.

    2.         Diaphysis may not be fully replaced during one's lifetime.


    B.        Osteoclasts are large multinucleated cells responsible for bone resorption;


                                                    1.         secretes lysosomal enzymes that digest the organic matrix;

                                                    2.         secrete acids that decompose calcium salts into Ca++ and PO4- ions, which can then enter blood.