Skeletal System: Structure and Function

Functions of the Skeletal System

• Support: Bones cradle soft tissues (e.g., cranial bones protect the brain) and allow the body to maintain an upright posture (e.g., long leg bones).

• Protection: Bones surround and safeguard soft tissues and vital organs (e.g., vertebrae protect the spinal cord). They also provide passageways for nerves and blood vessels through openings like foramina, fissures, or meatuses.

• Anchorage (Movement): Muscles attach to bones at specific points called the origin and insertion. When muscles contract, they pull on the bones, creating a system of levers that allows for movement. The muscle itself does the moving, but the bone provides the strong anchor.

• Mineral Storage: The skeletal system stores crucial minerals, primarily calcium and phosphate, which contribute to bone density. These ions are vital for both bone and blood and are constantly exchanged. High blood calcium can lead to deposition in bone, while low blood calcium results in bone resorption.

• Blood Cell Formation (Hematopoiesis): All blood cells are produced in the red bone marrow, located within the bones.

• Triglyceride Storage: Fats (triglycerides) are stored as adipose tissue in yellow bone marrow.

Bone-Associated Tissues

• Cartilage: A type of connective tissue that acts as a cushion between bones.

• Ligaments: Connect bone to bone.

• Tendons: Connect bone to muscle.

Classification of Bones

By Location

• Axial Skeleton: Includes the skull, ribs, and sternum. This forms the central axis of the body.

• Appendicular Skeleton: "Append" means to hang. This includes the limbs (arms and legs) and the girdles that attach them to the axial skeleton:

  • Pectoral Girdle: Clavicle (collarbone) and scapula (shoulder blade), attaching the upper limbs.

  • Pelvic Girdle: Os coxa (hip bones), attaching the lower limbs.

By Makeup (Tissue Type)

Both types are hard and dense but differ in internal structure. Both are perforated with vessels and nerves.

• Spongy Bone (Trabecular Bone):

  • Not squishy; still hard and dense.

  • Characterized by many open spaces formed by trabeculae (beams).

  • Ideal for holding bone marrow and decreasing bone density, making bones lightweight.

  • Typically found sandwiched between layers of compact bone.

• Compact Bone (Lamellar Bone):

  • Looks very smooth, like a thin plate (lamellar).

  • Forms the outer layer of all bones and often the ends of long bones.

  • More dense and provides superior protection compared to spongy bone.

  • Gross anatomy (visible to the naked eye) shows it as smooth, but microscopically it has perforations for vessels and nerves.

By Shape

• Long Bones: Longer than they are wide. Examples include limb bones (e.g., femur, humerus), bones of the hands (metacarpals), feet (metatarsals), and fingers/toes (phalanges).

  • Features: An epiphysis (proximal and distal ends or "heads") and a diaphysis (the shaft).

• Flat Bones: Relatively thin, flattened, and often curved. Examples include most cranial bones (frontal, parietal, temporal, occipital) and the ribs.

• Irregular Bones: Have complex, varied, and often bizarre shapes that do not fit into other categories. Examples include vertebrae, scapula, and the os coxa.

  • Often feature processes (outward projections) for muscle attachment and numerous holes for nerve and blood vessel passageways.

• Short Bones: Cube-shaped or somewhat round. Examples include the patella (kneecap) and many carpals (wrist bones) and tarsals (ankle bones).

Microscopic Structure of Bone

Cells of Bone Tissue

• Chondrocytes: Cartilage cells (chondro means cartilage). They give rise to cartilage and are responsible for its cushioned properties. Often appear like "eyeballs" under a microscope and are the most prevalent cell type in hyaline cartilage.

• Osteoprogenitor Cells (Osteogenic Cells):

  • Stem cells that undergo extensive mitosis.

  • Can differentiate into other bone cells.

  • Found in the periosteum (outer layer of bone) and endosteum (inner lining of bone). They are the most superficial bone cells in these areas.

• Osteoblasts: "To fill" or build bone.

  • Responsible for secreting osteoid, a bone matrix rich in collagen.

  • Actively produce new bone tissue, constantly remodeling and rebuilding bone in response to diet, exercise, age, and health.

  • Also found in the periosteum and endosteum, typically deeper than osteoprogenitor cells.

  • Structurally stout, allowing for efficient packing of collagen and osteoid.

• Osteocytes: Mature bone cells.

  • Differentiate from osteoblasts when osteoblasts become completely surrounded by osteoid.

  • Reside in small pockets called lacunae.

  • Have long cellular extensions (tentacles) that monitor mechanical stress/movement on bones and calcium levels within the bone matrix.

  • Communicate with other osteocytes and bone cells through these extensions and canaliculi.

• Osteoclasts: "To break" bone.

  • Unique; derived from a white blood cell lineage, not the osteoprogenitor -> blast -> cyte pathway.

  • Primary function is to break down bone tissue (bone resorption).

  • Possess a high number of lysosomes (vesicles with digestive enzymes) and a highly folded membrane (ruffled border) to maximize surface area for enzyme action.

  • Multi-nucleated.

  • Crucial for bone remodeling, releasing minerals like calcium into the blood, and responding to calcium imbalances.

Gross and Microscopic Features of Bone

• Bone Marrow:

  • Red Bone Marrow: Site of hematopoiesis (blood cell formation). Found in the pockets of spongy/trabecular bone.

  • Yellow Bone Marrow: Stores adipose (fat) for energy metabolism. In long bones, red marrow often converts to yellow marrow with age.

• Nutrient Foramen: A hole in the bone that allows arteries and nerves to pass through, providing nutrients and innervation to the bone tissue.

• Periosteum: A dense, irregular connective tissue membrane covering the outside of most bones (except articular surfaces).

  • Contains osteoprogenitor cells and osteoblasts.

  • Site of attachment for ligaments and tendons, secured by perforating fibers.

  • Plays a role in bone strengthening: mechanical stress (e.g., from exercise) on ligaments/tendons pulls on the periosteum, stimulating osteoprogenitor cells to deposit more bone tissue, increasing bone density.

• Endosteum: A thin membrane lining the internal surfaces of the bone (e.g., the medullary cavity and trabeculae of spongy bone).

  • Composed of dense regular connective tissue.

  • Contains osteoprogenitor cells, osteoblasts, and osteoclasts.

• Articular Cartilage: Hyaline cartilage that covers the ends of bones at joints, providing a smooth, cushioned surface for articulation.

• Osteon (Haversian System):

  • The fundamental structural unit of compact bone, visible microscopically as tree-trunk-like structures stacked side-by-side.

  • Composed of concentric layers called lamellae.

  • Lamellae: Concentric circles of collagen fibers within an osteon. The collagen fibers in adjacent lamellae run perpendicular to each other ($90^{\circ}$ rotation). This alternating arrangement makes them "twister resistors" that effectively resist torsion (twisting forces) on the bone.

  • Central Canal (Haversian Canal): Located at the center of each osteon, containing blood vessels and nerves that supply the bone cells with nutrients and remove waste.

  • Canaliculi: Tiny canals that connect the lacunae (where osteocytes reside) to each other and to the central canal, facilitating communication between osteocytes and nutrient/waste exchange.

  • Lacunae: Small, fluid-filled pockets or pits within the lamellae where osteocytes are housed.