Chapter 6: Skeletal System - Detailed Study Notes

Chapter 6: Skeletal System

Bones and Bone Structure

Part 1: Introduction to Bones and Their Composition

Bones are dynamic, living organs composed of several tissue types, primarily bone tissue (osseous tissue), but also nerves, blood vessels, cartilage, fibrous connective tissue, and adipose tissue. They perform numerous vital functions within the body.

• Skeletal System:

• Main Structures:

  • Bones (main structural components)

  • Tendons (connect muscle to bone)

  • Ligaments (connect bone to bone)

  • Cartilage (cushions joints, provides flexible support)

• Main Functions:

  • Supports and protects body parts

  • Provides leverage for body movements,

  • Stores minerals,

• 6-1: Functions of the Skeletal System

• The skeletal system includes:

  • Bones (primary organs)

  • Cartilages (e.g., articular, costal, hyaline, elastic, fibrocartilage)

  • Ligaments (connect bone to bone, stabilizing joints)

  • Tendons (connect muscle to bone, transmitting force)

• Primary Functions of the Skeletal System:

  • Support:

    • Provides the structural framework (endoskeleton) that supports the entire body, bears weight, and provides points of attachment for skeletal muscles, helping to maintain body posture.

  • Storage of minerals and lipids:

    • Stores calcium ($Ca^{2+}$) and phosphorus ($PO_4^{3-}$), which are essential for muscle contraction, nerve impulse transmission, and blood clotting. Lipids (fats) are stored in yellow bone marrow as an energy reserve.

  • Blood cell production:

    • Hematopoiesis, the process of producing red blood cells, white blood cells, and platelets, occurs in the red bone marrow, primarily in specific bones like the sternum, pelvis, and ends of long bones in adults.

  • Protection:

    • Encapsulates and protects vital internal organs. For example, the skull protects the brain, the vertebral column protects the spinal cord, and the rib cage protects the heart and lungs from external trauma.

  • Leverage:

    • Functions as a system of levers for skeletal muscles. Muscles contract and pull on bones, using joints as fulcrums, to produce varied body movements, ranging from fine motor skills to powerful gross movements.

6-2: Classification of Bones

Types of Bones According to Shape and Structure

• Sutural Bones:

  • Also known as Wormian bones, reflecting their discoverer.

  • Small, flat, irregularly shaped bones found between the flat bones of the skull within the sutures.

  • Sizes range from a grain of sand to a quarter or larger.

  • Their irregular borders resemble pieces of a jigsaw puzzle and are thought to arise from additional ossification centers.

• Flat Bones:

  • Have thin, roughly parallel, compact bone surfaces (cortices) sandwiching a layer of spongy bone.

  • Form the roof of the skull (parietal bones), sternum, ribs, and scapulae.

  • Provide broad protection for underlying soft tissues and offer extensive surface area for muscle attachments.

• Irregular Bones:

  • Have complex shapes with various short, flat, notched, or ridged surfaces that don't fit into other categories.

  • Examples include vertebrae (which protect the spinal cord and support body weight), bones of the pelvis, and several skull bones (e.g., sphenoid, ethmoid).

  • Their complex shapes allow for specific functions such as muscle attachment and articulation with multiple other bones.

• Long Bones:

  • Relatively long and slender, characterized by a diaphysis (shaft) and two epiphyses (ends).

  • Located in the arm (humerus), forearm (radius, ulna), thigh (femur), leg (tibia, fibula), palms (metacarpals), soles (metatarsals), fingers (phalanges), and toes (phalanges).

  • The femur, the largest and heaviest bone in the body, is a prime example. They act as levers and are crucial for movement and support.

• Short Bones:

  • Boxlike (cuboidal) in appearance, with roughly equal dimensions.

  • Examples include carpal bones (wrists) and tarsal bones (ankles).

  • Provide stability, some movement, and help to transfer forces.

• Sesamoid Bones:

  • Usually small, round, and flat, resembling a sesame seed.

  • Develop within tendons and are often found near joints of the knees, hands, and feet.

  • The patella (kneecap) is the largest and most famous example. They protect tendons from wear and tear and can alter the angle of muscle pull, providing a mechanical advantage.

Bone Markings (Surface Features)

Bone surfaces have various projections, depressions, and openings that serve as sites for muscle attachment, articulation, or passage for neurovascular structures.

1. Projections: - Locations where muscles, tendons, and ligaments attach; or at articulations where bones meet another bone (joints).

   • Examples of Projections for Muscle/Ligament Attachment:

     • Trochanter: Large, rough projection (e.g., greater trochanter of femur).

     • Tuberosity: Smaller, rough projection (e.g., deltoid tuberosity of humerus).

     • Tubercle: Small, rounded projection (e.g., greater tubercle of humerus).

     • Crest: Prominent ridge (e.g., iliac crest).

     • Spine: Pointed process (e.g., spinal process of vertebrae).

     • Line (Linea): Low ridge (e.g., linea aspera).

     • Ramus: Extension of a bone making an angle with the rest of the structure (e.g., ramus of mandible).

   • Examples of Articular Projections (forming joints):

     • Head: Expanded articular end of an epiphysis, often rounded (e.g., head of femur).

     • Condyle: Smooth, rounded articular process (e.g., occipital condyle).

     • Trochlea: Smooth, grooved articular process, like a pulley (e.g., trochlea of humerus).

     • Fascet: Small, flat articular surface (e.g., articular facets of vertebrae).

     • Epicondyle: Projection above a condyle.

2. Openings and Depressions: - Allow passage of blood vessels and nerves, or form depressions for other structures.

   • Depressions:

     • Fossa: Shallow depression (e.g., olecranon fossa).

     • Sulcus (Groove): Narrow groove (e.g., intertubercular sulcus).

   • Openings:

     • Sinus: Chamber within a bone, typically filled with air, lined with mucous membrane (e.g., paranasal sinuses).

     • Foramen: Rounded passageway for blood vessels and/or nerves (e.g., foramen magnum).

     • Fissure: Deep furrow, cleft, or slit (e.g., superior orbital fissure).

     • Meatus: Passage or channel, especially the opening of a canal (e.g., external acoustic meatus).

     • Canal: Duct or channel (e.g., optic canal).

6-2: Structure of Long Bone

• Diaphysis (shaft):

  • The long, central axis of the bone.

  • Composed of a thick layer of compact bone, providing strength.

  • Contains a central space called the medullary cavity (marrow cavity), which houses yellow bone marrow in adults (primarily fat storage) and red bone marrow in infants (hematopoiesis).

• Epiphysis (wide part at each end):

  • Expanded ends of the bone, covered by articular cartilage where joints are formed.

  • Primarily consists of spongy bone (trabecular bone), which is lighter and helps to distribute stress.

• Metaphysis:

  • The narrow zone where the diaphysis and epiphysis meet.

  • In growing bones, this region contains the epiphyseal plate (growth plate), a layer of hyaline cartilage where interstitial growth occurs, increasing bone length.

Structure of Flat Bones

• Example: Parietal bones of the skull.

  • Consist of a layer of spongy bone (trabecular bone) sandwiched between two layers of compact bone (known as the inner and outer cortices).

  • Within the cranium, this layer of spongy bone is specifically known as the diploë, which helps absorb shock and reduce the weight of the skull without sacrificing strength.

6-3: Bone Tissue

• Characteristics:

  • Bone (osseous) tissue is a dense, supportive connective tissue containing specialized cells (osteocytes) and a solid extracellular matrix.

  • The matrix is a unique blend of organic and inorganic components.

  • Bone Matrix:

    • Dense due to calcium salt deposits (inorganic component).

    • Enriched with collagen fibers (organic component) that provide flexibility and tensile strength.

  • Osteocytes:

    • Mature bone cells located within small pockets called lacunae, which are organized around blood vessels.

    • They maintain the protein and mineral content of the matrix and help repair damaged bone.

  • Canaliculi:

    • Narrow, interconnected passageways that radiate through the matrix, allowing for nutrient, waste, and gas exchange between blood vessels and osteocytes.

  • Periosteum:

    • A tough, fibrous membrane that covers the outer surfaces of bones (except at articular cartilage( around joints)).

    • Consists of an outer fibrous layer (for protection and attachment of tendons/ligaments) and an inner cellular layer (containing osteogenic cells).

    • Functions as a site for attachment of muscles, tendons, and ligaments, aids in bone growth, repair, and provides nourishment.

Bone Matrix Components

• Calcium Phosphate:

  • Chemical Formula: $Ca<em>3(PO</em>4)_2$

  • Comprises almost two-thirds of bone mass and is the primary inorganic component, providing hardness and compressional strength.

  • It interacts with calcium hydroxide ($Ca(OH)<em>2$) to form resilient hydroxyapatite crystals ($Ca</em>{10}(PO<em>4)</em>6(OH)_2$), which are the main mineral component of bone.

  • The matrix also incorporates other calcium salts, such as calcium carbonate ($CaCO_3$), and ions including magnesium ($Mg^{2+}$), fluoride ($F^{-}$), potassium ($K^{+}$), and sodium ($Na^{+}$).

• Matrix Proteins:

  • Approximately one-third of bone mass includes collagen fibers, which are flexible and contribute tensile strength to the bone, preventing it from being overly brittle.

  • Other organic substances like proteoglycans and glycoproteins are also present, forming the ground substance.

Bone Cells

• Types:

  • Osteogenic Cells (Osteoprogenitor Cells):

    • Undifferentiated mesenchymal stem cells found in the inner layer of the periosteum, the endosteum, and the canals that contain blood vessels.

    • Responsible for producing osteoblasts through cell division.

    • Important for bone growth and fracture repair.

  • Osteoblasts:

    • Immature bone cells responsible for osteogenesis (bone formation).

    • They actively synthesize and secrete the organic components of the bone matrix (osteoid), primarily collagen fibers and ground substance, before mineralization occurs.

    • When trapped within the calcified matrix, they differentiate into osteocytes.

  • Osteocytes:

    • Mature bone cells, typically the most numerous bone cells.

    • Located within lacunae and maintain the protein and mineral content of the surrounding bone matrix.

    • They play a crucial role in maintaining bone density and health, detecting mechanical stress, and signaling for bone remodeling.

  • Osteoclasts:

    • Large, multinucleate cells that originate from hematopoietic stem cells (related to macrophages, not osteogenic cells).

    • Secrete acids (e.g., lactic acid) and proteolytic enzymes (e.g., collagenase) to dissolve the bone matrix, a process called osteolysis or bone resorption.

    • This process is critical for bone remodeling, releasing stored minerals (calcium) into the blood and repairing damaged bone.

6-4: Compact Bone and Spongy Bone

Structure of Compact Bone

• Osteon (Haversian System):

  • The fundamental functional unit of compact bone.

  • Consists of concentric lamellae (layers of bone matrix) that surround a central (Haversian) canal, forming a strong, weight-bearing cylinder.

  • Osteons are aligned parallel to the long axis of the bone, allowing the bone to resist stress from a single direction.

• Central Canal (Haversian Canal):

  • A longitudinal canal at the center of each osteon.

  • Contains blood vessel(s) (capillaries and venules), nerves, and lymphatic vessels, providing nutrients to the osteocytes within the lacunae.

• Perforating Canals (Volkmann's Canals):

  • Horizontal canals that extend perpendicularly from the surface of the bone to the central canals.

  • Help carry blood vessels and nerves into deep bone tissue and the medullary cavity, connecting adjacent osteons and nutrient supply from the periosteum and endosteum.

• Lamellae:

  • Concentric layers of bone matrix arranged within the osteon.

  • Concentric Lamellae: Surround the central canal within an osteon, forming rings.

  • Interstitial Lamellae: Irregular fragments of old osteons that fill the spaces between intact osteons, remnants of remodeled bone.

  • Circumferential Lamellae: Layers of bone matrix located at the outer (just deep to the periosteum) and inner (lining the medullary cavity) surfaces of compact bone, contributing to the bone's overall circumference and strength.

Structure of Spongy Bone (Trabecular Bone)

• Lacks organized osteons; instead, its matrix forms an open network of needle-like or flat pieces called trabeculae.

• Trabeculae are irregularly arranged lamellae and osteocytes connected by canaliculi.

• The irregular organization of trabeculae allows spongy bone to withstand stresses applied from multiple directions, while also making the bone lighter.

• Red Bone Marrow:

  • Fills spaces between trabeculae in certain bones (e.g., epiphyses of long bones, flat bones).

  • This highly vascular tissue is the primary site of hematopoiesis (blood cell formation).

  • It contains blood vessels that supply nutrients to the osteocytes within the trabeculae via diffusion.

• Yellow Bone Marrow:

  • Found in other sites of spongy bone (especially in the medullary cavity of adult long bones).

  • Primarily for fat storage (adipose tissue) and can be converted back to red marrow if severe anemia demands increased hematopoiesis.

  • Compact Bone and Spongy Bone

    ▪ – • ▪ – • Weight bearing bones

    Trabeculae in

    epiphysis of femur

    transfer forces from

    pelvis to compact

    bone of femoral

    shaft

    Medial side of

    shaft compresses

    • Causing tension

    on the lateral side

  • ▪ Periosteum—membrane that

    covers outside of bones

    ▪ Except within joint cavities

    ▪ Outer, fibrous layer and inner, cellular layer

    ▪ Fibers are interwoven with those of tendons

    ▪ Perforating fibers—fibers that become

    incorporated into bone tissue

    ▪ Increase strength of attachments

    ▪ Functions of periosteum:

    ▪ Isolates bone from surrounding tissues

    ▪ Provides a route for blood vessels and

    nerves

    ▪ Participates in bone growth and repair

• Endosteum—incomplete

  cellular layer that lines

  medullary cavity

  ▪ Active during bone

  growth, repair, and

  remodeling

  ▪ Covers trabeculae of

  spongy bone

  ▪ Lines central canals of

  compact bone

  ▪ Consists of flattened

  layer of osteogenic cell

6-5: Bone Formation and Growth

• Ossification (Osteogenesis):

  • The process of bone tissue formation.

  • Involves the replacement of pre-existing connective tissue (either cartilage or fibrous membranes) with bone tissue.

• Calcification:

  • The deposition of calcium salts into any tissue.

  • This process occurs during ossification, but can also occur in other tissues, potentially pathologically (e.g., calcified arteries).

• Ossification Types:

  • Endochondral Ossification:

    • The more common mechanism by which most bones of the body (e.g., long bones, vertebrae) form.

    • Bone develops by replacing a hyaline cartilage model.

    • Primary ossification center in hyaline cartilage

  • Intramembranous Ossification:

    • Produces flat bones of the skull, mandible (lower jaw), and clavicles (collarbones).

    • Bone develops directly from mesenchymal (fibrous connective tissue) membranes, without a cartilage model.

• Growth Period:

  • Most human bones grow in length and width through childhood and adolescence, typically ceasing growth in length around age 25, when the epiphyseal plates close.

Endochondral Ossification Steps:

1

As the cartilage

enlarges, chondro-

cytes near the center

of the shaft increase

greatly in size. The

matrix is reduced to

a series of small

struts that soon

begin to calcify. The

enlarged chondro-

cytes then die and

disintegrate, leaving

cavities within the

cartilage.

2.Blood vessels grow

around the edges of

the cartilage, and the

cells of the perichon-

drium convert to

osteoblasts. The

shaft of the cartilage

then becomes

ensheathed in a

superficial layer of

bone

3.Blood vessels penetrate the

cartilage and invade the

central region. Fibroblasts

migrating with the blood

vessels differentiate into

osteoblasts and begin

producing spongy bone at a

primary ossification center.

Bone formation then

spreads along the shaft

toward both ends of the

former cartilage model.

4.Remodeling occurs as

growth continues,

creating a medullary

cavity. The osseous

tissue of the shaft

becomes thicker, and the

cartilage near each

epiphysis is replaced by

shafts of bone. Further

growth involves increases

in length and diameter.

5.Capillaries and osteoblasts

migrate into the epiphyses,

creating secondary

ossification centers

6.The epiphyses eventually become filled

with spongy bone. The metaphysis, a

relatively narrow cartilaginous region

called the epiphyseal cartilage, or

epiphyseal plate, now separates the

epiphysis from the diaphysis. On the

shaft side of the metaphysis,

osteoblasts continuously invade the

cartilage and replace it with bone. New

cartilage is produced at the same rate on

the epiphyseal side

7.At puberty, the rate of epiphyseal cartilage

production slows and the rate of osteoblast

activity accelerates. As a result, the

epiphyseal cartilage gets narrower and

narrower, until it ultimately disappears.

This event is called epiphyseal closure.

The former location of the epiphyseal

cartilage becomes a distinct epiphyseal

line that remains after epiphyseal growth

has ended

Growth Types

• Interstitial Growth- growth in length

• Secondary ossification centers develop

  – Epiphyseal closure—completion of epiphyseal growth

  – Width of epiphyseal cartilages reveals timing of

  endochondral ossification

  – •– •

  Former location of epiphyseal cartilage is visible on

  x-rays as an epiphyseal line

• Remains after epiphyseal closure

• Appositional Growth- growth in width

• Thickens and strengthens long bones

• Layers of circumferential lamellae are added at outer

  surface

  – • Deepest layers become replaced by osteons

  During this process, osteoclasts slowly remove bone

  matrix at inner surface of bone

• Enlarging medullary cavity

Intramembranous Ossification

• Also known as dermal ossification because it occurs within membranes (dermis).

• Occurs in:

  • Primarily responsible for forming the flat bones of the skull (e.g., frontal, parietal bones), the mandible, and the clavicles (collarbones).

• Steps of Intramembranous Ossification:

• 1.Mesenchymal cells cluster together,

  differentiate

  into osteoblasts, and

  start to secrete the

  organic components of

  the matrix. The resulting

  osteoid then becomes

  mineralized with

  calcium

  salts forming bone

  matrix

2.As ossification proceeds,

some osteoblasts are

trapped inside bony pockets

where they differentiate into

osteocytes. The developing

bone grows outward from the

ossification center in small

struts called spicules

3.Blood vessels begin to

branch within the region

and grow between the

spicules. The rate of bone

growth accelerates with

oxygen and a reliable

supply of nutrients. As

spicules interconnect,

they trap blood vessels

within the bone.

4.Continued deposition of bone

by osteoblasts located close

to blood vessels results in a

plate of spongy bone with

blood vessels weaving

throughout.

5.SSSubsequent remodeling

around blood vessels

produces osteons typical

of compact bone.

Osteoblasts on the bone

surface along with

connective tissue around

the bone become the

periosteum

Conclusion

Understanding the skeletal system, including its diverse functions, structures, cell types, and growth mechanisms, is critical for ensuring the integrity of musculoskeletal function, mineral homeostasis (especially calcium and phosphate regulation), and overall structural support for the human body. Its dynamic nature, constantly undergoing remodeling and adaptation, underscores its importance in physiology and health.