Skeletal System: Composition, Functions, Divisions, and Anatomy

Overview of the Skeletal System

• Definition and Components:

  • The skeletal system consists of bones, cartilage, ligaments, and other supportive connective tissues.
  • The human skeleton typically contains a standard total of $206$ bones.

• Bone as an Organ:

  • Each individual bone is classified as an organ because it is composed of several tissue types functioning together.
  • Tissues present within a bone include:
  • Osseous connective tissue (bone tissue)
  • Blood vessels
  • Nerves
  • Lymphatic tissue
  • Cartilage
  • Connective tissue coverings (membranes)

Functions of the Skeletal System

• Support:

  • Provides a structural framework for the body.
  • Supports soft tissues and provides attachment sites for skeletal muscles.

• Protection:

  • Encloses and protects internal organs from injury (e.g., the cranial cavity protects the brain, and the thoracic cage protects the heart and lungs).

• Assistance in Movement and Leverage:

  • Serves as levers for attached skeletal muscles.
  • Enables rotation around joints during muscle contraction to facilitate body movement.

• Mineral Homeostasis:

  • Acts as a reservoir for critical minerals, primarily calcium and phosphorus.
  • Stores and releases minerals on demand to maintain system-wide chemical equilibrium.

• Triglyceride Storage:

  • Serves as an energy reserve by storing fat/triglycerides.
  • Stored within yellow bone marrow located in the medullary cavity of long bones.

• Hemopoiesis:

  • The physiological process of red blood cell and white blood cell production.
  • Occurs in red bone marrow, located within spongy bone tissue and at the ends of long bones.

Divisions of the Skeletal System

• Axial Division:

  • Positioned along the central longitudinal axis of the human body.
  • Key structural groups include:
  • Skull: Subdivided into the cranial portion (enclosing the brain) and the facial portion (forming the face structure).
  • Thorax: Comprises the sternum and ribs.
  • Vertebral Column: Formed by stacked vertebrae extending down the trunk.

• Appendicular Division:

  • Comprises the bones of the upper and lower limbs (extremities) and the girdles attaching limbs to the axial skeleton.
  • Key structural groups include:
  • Pectoral (Shoulder) Girdle: Includes the clavicle and scapula.
  • Upper Limb (Extremity): Includes the humerus, radius, ulna, carpals, metacarpals, and phalanges.
  • Pelvic (Hip) Girdle: Formed by the hip/pelvic bones.
  • Lower Limb (Extremity): Includes the femur, patella, tibia, fibula, tarsals, metatarsals, and phalanges.

Classification of Bones by Shape

• Long Bones:

  • Characterized by having a greater length than width, consisting of a central shaft with two expanded ends.
  • Examples: Femur, humerus, tibia, fibula, radius, and ulna.

• Short Bones:

  • Roughly cube-shaped, having near-equal length, width, and thickness.
  • Examples: Carpals (wrist bones) and tarsals (ankle bones).

• Flat Bones:

  • Thin, flattened, and generally curved structures.
  • Examples: Cranial bones and sternum.

• Irregular Bones:

  • Feature complex, irregular shapes that do not fit standard geometric categories.
  • Example: Vertebrae.

• Sesamoid Bones:

  • Small, round, sesame-seed-shaped bones formed within tendons where high friction and physical stress occur.
  • Example: Patella (kneecap).

• Sutural Bones (Wormian Bones):

  • Small bone fragments located within the suture joints between major cranial bones.
  • Variable presence; individuals may or may not possess them in addition to the standard $206$ bones.

• Pneumatized Bones:

  • Bones that contain hollow air cells or sinus cavities (e.g., ethmoid, frontal, and maxillary skull bones).

Anatomical Structure of Bones

• Structural Regions of Long Bones:

  • Diaphysis:
  • The long, cylindrical shaft forming the main body axis of the bone.
  • Composed mainly of dense compact bone organized into osteons.
  • Encloses the hollow Medullary Cavity, which houses red and yellow marrow (fat storage) at the center of the bone.
  • Epiphysis:
  • The expanded proximal and distal ends of the bone that participate in joint formation.
  • Composed internally of light, porous spongy bone (trabecular bone) containing red marrow.
  • Metaphysis:
  • The intermediate region positioned between the diaphysis and epiphysis.
  • Contains the epiphyseal plate (growth plate) in growing bone, replaced by the epiphyseal line upon maturation.
  • Articular Cartilage:
  • A thin layer of hyaline cartilage covering the epiphyseal ends where bones articulate at joints.
  • Reduces friction and cushions kinetic impact.

• Bone Tissue Types:

  • Compact Bone:
  • The most dense, hard outer surface layer of bone.
  • Composed structurally of osteon units.
  • Spongy Bone (Trabecular Bone):
  • Lighter, highly porous internal bone matrix.
  • Composed of a latticework of thin struts called trabeculae.

Bone Membranes and Cellular Components

• General Tissue Organization:

  • Every bone features a layer of compact bone lined by outer periosteum and inner endosteum membranes.
  • Both endosteum and periosteum are crucial for ongoing bone growth, repair, and tissue remodeling.

• Osteogenic Cells:

  • Unspecialized stem cells capable of cell division.
  • Essential for continuous bone growth, repair, and tissue remodeling.

• Periosteum:

  • Connective tissue membrane covering the outer/external surface of bones.
  • Continuous with tendons, ligaments, and joint capsule connective tissues.
  • Firmly attached to the underlying bone via perforating fibers (Sharpey's fibers).
  • Composed of two functional layers:
  • Outer Fibrous Layer: Dense connective tissue layer providing structural protection.
  • Inner Osteogenic Layer: Contains active osteogenic cells involved in bone growth and remodeling.

• Endosteum:

  • Delicate connective tissue lining internal bone surfaces and spaces.
  • Lines the internal medullary cavity, covers spongy bone trabeculae, and lines central osteon canals in compact bone.
  • Contains osteogenic cells essential for internal bone growth and remodeling.

Structural Variations Across Bone Types

• Short Bones:
  • Outer boundary lined by periosteum.
  • Outer shell composed of compact bone enclosing a core of spongy bone.
  • Internal spongy bone spaces are lined by endosteum.
  • Example: Carpals.

• Flat Bones:
  • Consists of two outer parallel plates of dense compact bone enclosing a central layer of spongy bone.
  • Outer surface is lined by periosteum.
  • Internal spongy bone cavities are lined by endosteum.
  • Example: Cranial bones.

• Sutural Bones:
  • Small, irregular accessory bones found within the suture joints of skull plates.
  • Variable features present in addition to the standard $206$ skeletal bones.