Long bones, characterized by a shaft and two expanded ends, are commonly found in limbs like the femur or humerus. To understand their composition, scientists examine a “cross-section,” cutting the bone perpendicular to its length to reveal internal components. This approach provides a detailed view of the layers and structures that contribute to the bone’s function. This article will explore these components, progressing from the outermost covering to the innermost core.
The Outer Covering
The periosteum is a robust, fibrous membrane enveloping the outer surface of a long bone, excluding joint areas. This membrane has two distinct layers. The outer fibrous layer provides protection and attachment points for muscles and tendons.
Beneath this lies the inner cellular layer, containing specialized cells for bone growth and repair. The periosteum also supplies blood vessels and nerves that nourish the underlying bone.
The Dense Outer Layer
Compact bone, also called cortical bone, forms the dense outer shell of the bone shaft. This layer provides strength, rigidity, and protection. Its organized structure efficiently manages mechanical stresses.
Microscopically, compact bone is built from structural units called osteons, or Haversian systems. Each osteon consists of concentric rings of calcified matrix, known as lamellae, surrounding a central canal that houses blood vessels, nerves, and lymphatic vessels. These central canals are interconnected by perforating canals, also known as Volkmann’s canals, which allow blood vessels and nerves to extend to the periosteum and endosteum. This arrangement allows for efficient nutrient delivery and waste removal, while enabling the bone to withstand weight-bearing forces and resist bending.
The Spongy Inner Network
Spongy bone, also known as cancellous or trabecular bone, is primarily located at the ends of long bones and lines the medullary cavity. Unlike compact bone, it features an interconnected network of bony plates and rods called trabeculae. These trabeculae create numerous open spaces.
This porous arrangement makes the bone lighter without compromising strength. It also acts as a shock absorber, distributing forces and reducing stress. The spaces within this network are filled with bone marrow.
The Central Cavity and Marrow
The medullary cavity is the hollow space extending down the center of the bone shaft, also known as the diaphysis. This cavity contains two primary types of bone marrow, each with distinct functions.
Red bone marrow is responsible for hematopoiesis, producing all types of blood cells, including red blood cells, white blood cells, and platelets. Yellow bone marrow is primarily composed of fat cells and serves as an energy reserve. In adults, red bone marrow is predominantly found in the spongy bone at the ends of long bones, while yellow bone marrow occupies the medullary cavity of the shaft.