Secondary (lamellar) bone

Dense areas without cavities are corresponding to compact bone (1); areas with numerous interconnecting cavities – corresponding to spongy (cancellous) bone (2). (Fig.31). Under the microscope, however, both compact bone and the trabeculae separating the cavities of cancellous bone have the same basic histologic structure. The cavities of spongy bone contain bone marrow.

Secondary bone is the variety usually found in adults. Characteristically, it shows collagen fibers arranged in lamellae that are parallel to each other or concentrically organized around a vascular canal.

The whole complex of concentric lamellae of bone surrounding a canal containing blood vessels, nerves, and loose connective tissue is called a haversian system or osteon.

Fig. 28. Lamellae of Compact Bone

The osteon is the basic structural unit of compact bone. In 3-dimensional view, an osteon is a cylinder composed of 4 to 20 concentric lamellae arranged around a central opening, the osteonal canal. Collagen fibers within each lamella are highly ordered: they lie parallel with each other. Within adjacent lamella collagen fibers are arranged in another direction.

Lacunae containing osteocytes are found between the lamellae. In each lamella, collagen fibers are parallel to each other. Surrounding each haversian system is a deposit of amorphous material called the cementing substance that consists of mineralized matrix with few collagen fibers.

Fig. 29. Osteon

In compact bone (eg, the diaphysis of long bones), the lamellae exhibit a typical organization consisting of:

• haversian system,

• outer circumferential lamellae,

• inner circumferential lamellae,

• interstitial lamellae.

Each haversian system (Fig.28) is a long, often bifurcated cylinder parallel to the long axis of the diaphysis. It consists of a central canal surrounded by 4-20 concentric lamellae. Each endosteum-lined canal contains blood vessels, nerves, and loose connective tissue.

The haversian canals communicate with the marrow cavity, the periosteum, and each other through transverse or oblique Volkmann’s canals. Volkmann’s canals do not have concentric lamellae; instead, they perforate the lamellae.

Fig.30.

The outer circumferential lamellae are located immediately beneath the periosteum. Their lamellae have a circular distribution. The inner circumferential lamellae, deposited by the endosteum, encircle the marrow cavity.

Between the 2 circumferential systems are numerous osteons, and among the latter are triangular or irregularly shaped groups of parallel lamellae called interstitial (or intermediate) lamellae. These structures represent angular fragments of older Haversian systems, which have been partially destroyed in the internal reorganization of the bone.

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