Bone Structure and Growth Lecture

Bone Structure and Growth Lecture Notes

Section 1: Introduction to the Structure and Growth of Bones

• Osteology is the study of bones.

• The skeletal system consists of bones, cartilages, and ligaments.

  • Cartilage serves as a precursor for most bones; many bones in the human skeleton develop from a cartilage model.

  • Cartilage also covers many joint surfaces of mature bone.

• Ligaments hold bones together at joints.

• Tendons, which are part of the muscular system, attach muscles to bones.

Section 2: Bone Tissue

• Bone, or osseous tissue, is a connective tissue characterized by a matrix hardened by calcium phosphate and other minerals.

• The hardening process of bone is referred to as mineralization or calcification.

• A bone (an organ) not only consists of osseous tissue but also includes other tissues such as:

  • Blood

  • Bone marrow

  • Cartilage

  • Adipose tissue

  • Nervous tissue

  • Fibrous connective tissue

  • Epithelial tissue.

Module 6.1: The Skeletal System

• The skeletal system is divided into the axial and appendicular divisions, consisting of about 206 separate bones in adults.

  • Axial Skeleton (80 bones):

  • Includes bones of the skull, thorax, and vertebral column.

  • Forms the long axis of the body.

  • Appendicular Skeleton (126 bones):

  • Comprises bones of limbs and the girdles that attach limbs to the axial skeleton:

    • Shoulder girdles

    • Pelvic girdle

Functions of the Skeletal System

• The skeletal system has several primary functions:

  • Support: Bones support the body and muscles; the mandible and maxilla provide support for teeth.

  • Store minerals and lipids:

  • The skeleton stores and releases mineral reserves, with calcium being the most abundant mineral.

  • Contains adipose tissue that stores lipids as energy reserves.

  • Produce blood cells:

  • Red bone marrow is the main site for producing red blood cells, white blood cells, and platelets.

  • Protection: Bones protect delicate organs such as the brain, spinal cord, heart, and lungs.

  • Leverage: Limb movements and breathing are facilitated by the action of muscles on bones.

  • Acid-base balance: Bone tissue buffers blood against excessive pH changes.

Module 6.2: Classification of Bones

• Bones are classified based on shape and structure into six categories:

  • Flat Bones

  • Sutural Bones

  • Long Bones

  • Irregular Bones

  • Sesamoid Bones

  • Short Bones

Surface Markings of Bones

• Bone surfaces exhibit characteristic markings or surface features, including:

  • Projections/Elevations: Bumps where muscles, tendons, and ligaments attach, as well as articulation points between bones.

  • Depressions/Grooves/Openings: Sites where blood vessels or nerves lie adjacent or penetrate bone.

Types of Bone Markings

• Canal or Meatus: A large passageway through a bone.

• Sinus: A chamber within a bone, typically filled with air.

• Foramen: A small, rounded opening for blood vessels or nerves to pass through.

• Fissure: An elongated cleft or gap.

• Process: Any projection or bump.

• Trochanter: A large, rough projection.

• Head: An expanded proximal end of a bone that forms part of a joint.

• Neck: A narrow connection between the head and shaft of a bone.

• Diaphysis (Shaft): The elongated tubular body of a long bone.

• Facet: A small, flat articular surface.

• Tubercle: A small rounded projection.

• Sulcus: A deep, narrow groove.

• Tuberosity: A small, rough projection that may occupy a broad area.

• Trochlea: A smooth, grooved articular process that is shaped like a pulley.

• Condyle: A smooth, rounded articular process.

• Crest: A prominent ridge.

• Fossa: A shallow depression in a bone.

• Line: A low ridge, more delicate than a crest.

• Spine: A pointed or narrow process.

• Ramus: A thick projection that forms an angle with the main body of the bone.

Module 6.3: Long Bone Structure

• Long bones are adapted to transmit forces along their shaft and possess a rich blood supply.

Long Bone Features

• Epiphysis: An expanded area at each end of the bone, mostly consisting of spongy (trabecular) bone with a thin outer layer of compact bone.

  • It is covered with articular cartilage at joints.

• Metaphysis: The zone connecting the epiphysis to the shaft (diaphysis).

• Diaphysis (Shaft): Contains the medullary (marrow) cavity that holds two types of marrow:

  • Red Bone Marrow: Highly vascular and involved in producing blood cells.

  • Yellow Bone Marrow: Adipose tissue, functioning as an energy reserve.

Blood Supply in Long Bones

• Growth and maintenance of bone require an extensive blood supply.

• Vascular Features:

  • Nutrient Artery and Nutrient Vein: Typically one of each per bone, with a nutrient foramen providing access to the marrow cavity.

  • Metaphyseal Artery and Vein: Carry blood to/from the metaphysis and connect with epiphyseal arteries/veins.

Module 6.4: Bone Cells

• Bone consists of cells, fibers, and ground substance, with four principal types of bone cells:

  • Osteogenic Cells (Osteoprogenitor Cells): Stem cells residing in the endosteum, periosteum, and central canals, producing new osteoblasts.

  • Osteoblasts: Immature cells that produce new bone in the process known as osteogenesis or ossification.

  • Osteocytes: Mature bone cells that remain in the lacunae of the bone matrix, helping maintain and repair the bone.

  • Osteoclasts: Cells that remove bone matrix, involved in osteolysis. They develop in bone marrow through the fusion of stem cells.

Bone Matrix Composition

• The bone matrix is composed of:

  • 1/3 Organic Component: Mostly collagen, providing flexibility.

  • 2/3 Inorganic Component: Primarily calcium phosphate, forming hydroxyapatite crystals that provide strength.

• Bone is a composite material, balancing strength and resilience.

• Conditions such as osteomalacia (soft bones) and osteogenesis imperfecta (brittle bone disease) result from imbalances in these components.

Module 6.5: Types of Bone

• Compact Bone: Features organized osteons (Haversian systems), which include:

  • Central Canal: Contains blood vessels.

  • Perforating Canals: Run perpendicularly to Central Canals.

  • Concentric Lamellae: Surrounding the central canal.

  • Canaliculi: Interconnect lacunae.

• Spongy Bone:

  • Contains no blood vessels within the matrix; nutrients reach osteons through open canaliculi.

  • Structure enhances strength while minimizing weight.

Module 6.6: Appositional Growth of Bone

• Appositional Growth increases the bone diameter:

  • Osteogenic Cells differentiate into Osteoblasts which add new bone matrix under the periosteum.

  • Osteoclasts remove matrix at the inner surface to enlarge the medullary cavity.

Periosteum Functionality

• The periosteum covers the outer layer of compact bone, consisting of:

  • Fibrous Outer Layer: Provides isolation from surrounding tissues.

  • Cellular Inner Layer: Actively participates in growth and repair.

Endosteum Functionality

• The endosteum lines the medullary cavity and is active during bone growth, repair, and remodeling. It consists of a layer of flattened osteogenic cells.

Module 6.7-6.8: Bone Development (Ossification)

• Ossification (or osteogenesis) refers to bone formation, occurring by two principal methods:

  • Intramembranous Ossification: Bone forms within a fibrous membrane (e.g., flat bones of the skull, mandible).

  • Endochondral Ossification: Bone forms within a cartilage model, taking place where the initial skeleton is formed of hyaline cartilage.

Bone Growth and Remodeling

• Bone growth occurs in two main directions: length and width.

  • Width Growth: via appositional growth, adding matrix to surfaces.

  • Length Growth: occurs at epiphyseal plates, where cartilage transforms into bone during maturation, culminating in epiphyseal closure at puberty, leaving an epiphyseal line.

Clinical Module: Abnormalities of Bone Growth

• Disorders affecting bone growth can lead to noticeable physical signs:

  • Pituitary Growth Failure: Deficiency in growth hormone, resulting in shorter bones.

  • Achondroplasia: Slow growth of epiphyseal cartilage, leading to short limbs while the trunk remains normal-sized.

  • Marfan Syndrome: Excessive cartilage formation leads to tall stature and potential cardiovascular issues.

  • Gigantism: Results from excess growth hormone before puberty, leading to extreme heights.

  • Acromegaly: Overproduction of growth hormone post-puberty, culminating in thickened bones and altered features.

Module 6.10: Bones as Mineral Reservoirs

• Calcium is the most abundant mineral in the body, stored primarily in bones,

  • Vital for physiological functions, including muscle contraction and nerve impulse transmission.

Module 6.11: Hormonal Regulation of Calcium Ion Metabolism

• Primary hormones include:

  • Parathyroid Hormone (PTH): Increases blood calcium levels via promoting absorption, conservation, and resorption of calcium.

  • Calcitonin: Decreases blood calcium by inhibiting resorption and enhancing deposition in bone.

Module 6.12: Bone Fractures

• Fractures are categorized by:

  • The direction of the fracture line.

  • Skin involvement (Open/Closed).

  • Complexity (Simple/Comminuted).

Steps in Fracture Repair

1. Fracture Hematoma Formation: Initial bleeding and clotting.

2. Callus Formation: Creation of internal and external callus to stabilize the fracture.

3. Spongy Bone Formation: Replacement of cartilage with spongy bone.

4. Compact Bone Formation: Remodeling occurs over time, initially producing a swelling that eventually minimizes.

Specific Types of Fractures

• Transverse Fractures: Break across the long axis.

• Spiral Fractures: Caused by twisting stress.

• Greenstick Fractures: Incomplete break; common in children.

• Comminuted Fractures: Bone fractures into multiple fragments.

• Pott’s and Colles Fractures: Specific breaks at the ankle and wrist, respectively.

Osteoporosis

• Osteoporosis: A disease characterized by porous bones, leading to increased fracture risk.

  • Affects particularly spongy bone; factors include gender, age, race, and lifestyle choices.