Skeletal System

Overview of the Skeletal System

  • Definition and Importance: The skeletal system consists of bones that are alive and multifunctional, playing crucial roles in various bodily functions.

7.1 Overview of the Skeletal System

  • Components of the Skeletal System:

    • Organs: Bones serve as organs composed of various tissues including:

    • Bone tissue

    • Cartilage

    • Dense connective tissue

    • Blood

    • Nervous tissue

    • Functions:

    • Support for softer tissues

    • Protection of internal organs

    • Movement facilitation

    • Blood cell formation (hematopoiesis)

    • Storage of minerals

  • Divisions of the Skeletal System:

    • Axial Division: Comprising the skull, vertebral column, ribs, and sternum.

    • Appendicular Division: Comprising the shoulders, arms, hips, and legs.

Bone Shape and Structure

  • Variation in Bone Structure: Bones differ in their size and shape but share foundational structural similarities.

7.2 Bone Shape and Structure

  • Differences in Bones:

    • Size: Vary greatly between different bones.

    • Shape: Differ in shape defined by specific categories.

  • Commonalities:

    • Similar structure, development, and function.

Bone Shapes

Classification by Shape:

  1. Long Bones:

    • Characteristics:

      • Long and narrow structure

      • Expanded ends (epiphyses)

  2. Short Bones:

    • Characteristics:

      • Cube-like; length = width

      • Includes sesamoid bones (embedded in tendons)

  3. Flat Bones:

    • Characteristics:

      • Plate-like with broad surfaces

  4. Irregular Bones:

    • Characteristics:

      • Variety of shapes

      • Connect to several other bones

Structure of Long Bones

Parts of a Long Bone:

  • Epiphysis: Expanded end of the bone.

  • Diaphysis: The shaft of the bone.

  • Metaphysis: The widening part between the diaphysis and epiphysis.

  • Articular Cartilage: Covers the epiphysis to provide a smooth surface for joint movement.

  • Periosteum: Encloses the bone with dense connective tissue.

  • Compact (Cortical) Bone: Forms the wall of the diaphysis, providing strength.

  • Spongy (Cancellous) Bone: Composes the epiphyses, containing trabeculae (branching bony plates).

  • Medullary Cavity: Hollow chamber within the diaphysis; filled with marrow.

  • Endosteum: Lines the medullary cavity and spaces within spongy bone.

  • Bone Marrow: Can be red (produces blood cells) or yellow (store fat).

Microscopic Structure of Bone

Bone Cells and Matrix:

  • Osteocytes: Mature bone cells found in lacunae.

  • Lacunae: Chambers occupied by osteocytes.

  • Canaliculi: Tiny passageways allowing cell processes of osteocytes to exchange nutrients and waste.

  • Extracellular Matrix:

    • Composed primarily of collagen fibers and inorganic salts,

    • Collagen: Provides bone resilience.

    • Inorganic Salts: Contribute to bone hardness.

Types of Bone Tissue

Compact Bone:

  • Features:

    • Composed of cylindrical units known as osteons.

    • Rigidity provided by layers of matrix called lamellae surrounding a central canal.

    • Strong and solid; designed to bear weight and resist compression.

Spongy Bone:

  • Features:

    • Composed of trabeculae (branching plates).

    • Provides flexibility and reduces overall weight due to spaces between trabeculae.

Functions of Bones

Major Functions of Bones:

  • Provide shape to body

  • Support body structures

  • Protect internal organs

  • Aid movement through muscle connections

  • Produce blood cells (hematopoiesis)

  • Store inorganic salts.

Support, Protection, and Movement

  • Bones are pivotal in shaping and supporting the head, face, thorax, and limbs while also providing structural support against body weight (particularly in lower limbs and vertebral column).

  • Protection provided by:

    • Skull bones safeguarding the brain, ears, and eyes.

    • Ribs and shoulder girdle protect the heart and lungs.

    • Pelvic girdle guards internal reproductive organs and lower abdominal organs.

  • Movement facilitated through the interplay of bones and muscles.

Blood Cell Formation

  • Hematopoiesis: The process of blood cell formation, occurring in red bone marrow which produces:

    • Red blood cells

    • White blood cells

    • Platelets

  • Age Effect on Bone Marrow:

    • Some red bone marrow is replaced by yellow bone marrow (fat storage), which does not produce blood cells.

    • Adult locations for red marrow include:

    • Skull

    • Ribs

    • Sternum

    • Clavicles

    • Vertebrae

    • Hip bones

  • Bone Marrow Transplants: Used to treat various hematopoietic conditions.

Inorganic Salt Storage

  • Bone Matrix Composition: Approximately 70% consists of inorganic mineral salts, primarily hydroxyapatite (calcium phosphate).

  • Other Salts Include:

    • Magnesium ions

    • Sodium ions

    • Potassium ions

    • Carbonate ions

  • Health Implications: Osteoporosis is a condition resulting from loss of bone mineralization, leading to weakened bones. Parathyroid hormone and calcitonin are crucial in regulating blood calcium levels due to calcium's role in nerve impulses and muscle contractions.

Bone Development, Growth, and Repair

Bone Development Process

  • Begins in the embryonic stage within weeks of conception.

  • Bone structures continue developing through adulthood.

  • Two primary processes by which bones form:

    • Intramembranous Ossification

    • Endochondral Ossification

Intramembranous Bone Development

  • Definition: Formation of bones through direct replacement of connective tissue.

  • Examples: Flat bones in the skull, clavicles, and sternum.

  • Process:

    • Mesenchymal cells (primitive tissue) transform into osteoblasts,

    • Osteoblasts deposit the bone matrix, and when enclosed, they become osteocytes in lacunae.

    • External mesenchyme develops into periosteum.

Endochondral Bone Development

  • Definition: Formation from pre-existing hyaline cartilage models.

  • Examples: Most long bones like femur and humerus.

  • Process:

    • Chondrocytes (cartilage cells) enlarge,

    • Ensuing breakdown of the cartilage matrix,

    • Osteoblasts invade the area to replace cartilage with bone tissue, creating spongy and then compact bone.

Growth at the Epiphyseal Plate

Growth Mechanism:

  • Epiphyseal Plate: Region allowing lengthening of bones, separating the diaphysis from the epiphysis. Consists of four zones:

    1. Zone of Resting Cartilage: Anchors the plate to epiphyseal ends.

    2. Zone of Proliferating Cartilage: Cell mitosis occurs, creating rows of new cells.

    3. Zone of Hypertrophic Cartilage: Older cells thicken the plate and undergo matrix calcification; older cells die.

    4. Zone of Calcified Cartilage: Comprised of dead cartilage cells and calcified matrix.

  • Effect of Osteoclasts: Break down calcified matrix and allow osteoblasts to invade and replace cartilage with bone tissue, leading to bone length growth.

Factors Influencing Bone Development, Growth, and Repair

  • Nutritional Factors: e.g., Vitamin D (calcium absorption), Vitamin A (osteoblast/osteoclast activity), Vitamin C (collagen synthesis).

  • Hormonal Influences: Hormones affecting growth include:

    • Growth hormone: Influences cartilage cell division.

    • Thyroid hormone: Promotes osteoblast activity.

    • Parathyroid hormone: Stimulates osteoclast activity.

    • Estrogen/Testosterone: Facilitate ossification and influence bone formation.

  • Physical Stress: Promotes growth through exercise engagement.

Clinical Applications

Fractures

  • Classification by Cause:

    • Traumatic: Resulting from injury.

    • Spontaneous/Pathologic: Caused by disease.

  • Classification by Nature of Break:

    • Simple (Closed): Bone is not exposed through skin.

    • Compound (Open): Bone is exposed through skin.

Steps in Fracture Repair:

  1. Hematoma Formation: A large blood clot forms immediately after the fracture.

  2. Cartilaginous (Soft) Callus: Formed by osteoblasts producing spongy bone, debris removal by phagocytes, and fibrocartilage formation.

  3. Bony (Hard) Callus: Osteoblastic invasion leads to the hard callus filling the fracture space after cartilaginous callus breakdown.

  4. Remodeling: Restoration of bone to its original shape via osteoclasts removing excess bone material.

Preventing Fragility Fractures:

  • Definition: Fracture from a fall lower than standing height, indicating low bone density.

  • Causes:

    • Aging leads to higher bone resorption than formation, causing osteopenia or osteoporosis.

  • Statistics: About 50% of women and 25% of men over 50 experience bone loss conditions.

  • Prevention Strategies:

    • Regular exercise (30 minutes a day; emphasis on weight-bearing activities).

    • Adequate intake of calcium and vitamin D.

    • Avoid smoking.

Skeletal Organization

  • The adult human skeleton is composed of approximately 206 bones.

  • Variability: Some individuals may possess additional bones (e.g., sutural or sesamoid bones) or might lack certain bones.

  • Examples of Additional Bones:

    • Sutural (wormian) bones found at the sutures of the skull.

    • Small sesamoid bones within tendons to reduce friction.