Concentric lamellae are microscopic, circular layers of bone matrix found within the hard, dense outer shell of bones. These layers resemble the growth rings of a tree trunk, but on a much smaller scale. Each lamella forms a distinct, tubular layer, contributing to bone tissue’s organized structure and strength.
The Osteon Structural Unit
Concentric lamellae are integral components of a larger unit known as an osteon, also referred to as a Haversian system. An osteon represents the fundamental structural and functional unit of compact bone. These cylindrical structures typically measure between 0.25 mm and 0.35 mm in diameter and can extend several millimeters to about 1 centimeter in length.
Each osteon features multiple concentric lamellae, ranging from 5 to 20 layers, arranged around a central canal. This central space, known as the Haversian canal, houses the blood vessels, lymphatic vessels, and nerve fibers that supply the bone tissue. Small spaces, called lacunae, are located between these lamellae, housing mature bone cells (osteocytes). These osteocytes communicate with each other and the central canal through tiny channels called canaliculi, facilitating nutrient and waste exchange.
Material Composition and Strength
Each concentric lamella is composed of organic and inorganic materials, providing bone with its mechanical properties. The organic component primarily consists of collagen fibers, specifically Type I collagen, which provides tensile strength. Interspersed within these collagen fibers are inorganic mineral salts, predominantly hydroxyapatite crystals, which contribute to the bone’s compressive strength and overall rigidity.
A distinguishing feature of these lamellae is the alternating orientation of collagen fibers in adjacent layers. Within a single lamella, collagen fibers are generally arranged parallel to each other. However, the fibers in neighboring lamellae are oriented in different directions, often at angles to one another, creating a “twisted plywood” or helical pattern. This arrangement helps bone resist torsional, or twisting, forces. The varying fiber angles distribute stress, preventing crack propagation and enhancing the bone’s overall durability.
Placement within Compact Bone
Osteons are densely packed in the outer layer of most bones, forming compact bone tissue. This arrangement provides support and protection. The osteons are typically aligned parallel to the long axis of the bone, helping them withstand compressive forces along that axis.
Beyond the osteons, compact bone also contains other types of lamellae, providing additional structural integrity. Interstitial lamellae are irregular fragments of older osteons, filling the spaces between intact osteons. These remnants result from the continuous process of bone remodeling. Circumferential lamellae are layers that run around the outer and inner circumference of the bone shaft, lying just beneath the periosteum (outer membrane) and endosteum (inner lining), respectively. These distinct types of lamellae work in concert to form the strong framework of compact bone.