6.1: Introduction to Bones as Organs

I. Functions of the Skeletal System

The skeletal system provides several critical functions beyond simple structural support:

• Protection: Acts as a hard physical barrier that shields vital internal organs, such as the brain (cranium), heart, and lungs (rib cage).

• Mineral Storage and Acid-Base Homeostasis:

  • Bone serves as the body’s primary reservoir for minerals, specifically calcium ($Ca^{2+}$) and phosphorus ($PO_4^{3-}$), which are vital for muscle contraction and nerve signaling.

  • It aids in acid-base balance by releasing or absorbing alkaline salts (like calcium phosphate) to buffer the blood's $\text{pH}$.

• Blood Cell Formation (Hematopoiesis; hemato = blood, poiesis = to make): Red bone marrow, located within certain bones, contains hematopoietic stem cells responsible for producing all types of blood cells.

• Fat Storage: In adults, many bone cavities contain yellow bone marrow, which consists primarily of adipocytes (fat cells) that store triglycerides as a chemical energy reserve.

• Movement: Bones serve as levers. When skeletal muscles—which are attached to bones via tendons—contract, they pull on the bones to produce movement at the joints.

• Support: The skeleton provides the structural framework that supports the weight of the body and maintains its shape.

II. Classification of Bones by Shape

Bones are categorized into five distinct classes based on their general morphology rather than their size:

1. Long Bones

   • Characteristics: Longer than they are wide; possess a central shaft and two distinct ends.

   • Examples: Includes the humerus, femur, radius, ulna, metacarpals, and phalanges.

2. Short Bones

   • Characteristics: Roughly cube-shaped; approximately equal in length and width.

   • Examples: Carpals (wrist) and tarsals (ankle).

3. Flat Bones

   • Characteristics: Thin, broad, and often curved.

   • Examples: Most skull bones, the sternum, ribs, and the pelvis.

4. Irregular Bones

   • Characteristics: Possess complex shapes that do not fit into the other categories.

   • Examples: Vertebrae and certain facial bones like the ethmoid.

5. Sesamoid Bones

   • Characteristics: Specialized small, flat bones that develop within tendons. They serve to reduce friction and modify the pressure exerted on the tendon.

   • Examples: The patella (kneecap) is the most prominent sesamoid bone.

III. Gross Anatomy and Bone Structure

3.1 Structure of a Long Bone

• Periosteum: A dense, irregular connective tissue membrane covering the outer surface. It is rich in blood vessels and nerves. It is anchored to the bone by Perforating (Sharpey’s) fibers, which are collagen bundles that penetrate deep into the bone matrix.

• Diaphysis: The long, tubular shaft of the bone.

• Epiphyses: The expanded ends of the long bone. They are covered with articular cartilage (hyaline cartilage) to reduce friction at joints.

• Marrow (Medullary) Cavity: A hollow space within the diaphysis. In adults, this usually contains yellow marrow, whereas in children, it often contains red marrow.

• Compact Bone: The hard, outer shell that resists mechanical stress, linear compression, and twisting (torsion). Looks like tree rings

• Spongy Bone (Cancellous Bone): Located internally, especially in the epiphyses. It consists of a honeycomb-like network of bony struts called trabeculae, which help the bone resist forces from multiple directions and house bone marrow.

• Endosteum: A thin membrane lining the internal surfaces of the marrow cavity and the trabeculae of spongy bone; has different bone cells that aid in bone homeostasis; similar to the periosteum, but it lacks fibrous outer layer and is thinner

• Epiphyseal Lines and Plates:

  • Epiphyseal Plate: A layer of hyaline cartilage found in growing children, responsible for longitudinal bone growth.

  • Epiphyseal Line: The bony remnant of the plate seen in adults once growth has ceased.

3.2 Structure of Short, Flat, Irregular, and Sesamoid Bones

• These bones lack a diaphysis and epiphyses, thus lack a medullary cavity.

• They consist of a "sandwich" of compact bone on the outside (covered by periosteum) and spongy bone on the inside.

• In flat bones, the internal spongy bone layer is specifically called the diploë.

• Some of these bones may contain sinuses, which are air-filled, mucous-membrane-lined cavities that lighten the weight of the skull.

IV. Blood and Nerve Supply

Bones are highly vascularized tissues:

• Nutrient Arteries: Most blood is supplied by one or two main arteries that enter the diaphysis through a small opening called the nutrient foramen.

• Internal Circulation: These arteries bypass the compact bone to supply the spongy bone and marrow. The epiphyses receive their own dedicated blood supply from smaller vessels.

• Innervation: Nerves accompany the blood vessels, primarily concentrated in the periosteum, which is why bone injuries (like fractures) are extremely painful.

V. Bone Marrow

1. Red Bone Marrow

   • Composition: Loose connective tissue (reticular fibers) housing hematopoietic (blood-forming) cells.

   • Location: In infants, almost all bone cavities contain red marrow. In adults, it is restricted to the axial skeleton (pelvis, vertebrae, ribs, sternum, skull) and the proximal heads of the femur and humerus.

2. Yellow Bone Marrow

   • Composition: Primarily adipose tissue (fats/triglycerides).

   • Function: Replaces red marrow as the body ages; however, in cases of severe anemia, yellow marrow can revert back to red marrow to increase blood cell production.