Principles of Structural Organization
Like woven bone tissue, lamellar bone consists of osteocytes, collagen fibers, and a mineralized matrix. However, here these elements are organized into bone lamellae.
Within a single individual lamella, all fibers are directed strictly parallel to each other. Meanwhile, in adjacent, neighboring lamellae, the fiber direction alternates to an opposite or oblique angle. This cross-ply arrangement creates a robust framework and provides bones with exceptional mechanical strength.
Depending on the spatial arrangement of these lamellae, two forms of bone substance are distinguished in the body:
- Spongy (trabecular) bone.
- Compact bone.
It is important to clearly differentiate between concepts: bones exist as anatomical organs (flat, spongy, long), while bone substance is a histological structure. Both spongy and compact substance are present in all bone types; only their ratio changes.
Spongy Bone Substance
This structure visually resembles a sponge: it is a three-dimensional anastomosing network. The fundamental lamellae here have a flat or arched shape and combine into struts called trabeculae.
Inside the trabeculae, the lamellae lie parallel to each other, and the struts themselves are generally devoid of their own blood vessels (avascular). The spaces between the struts form bone marrow cavities. It is within these spaces that nutritive vessels run and red bone marrow—the vital hematopoietic organ—resides.
Spongy substance fills the interior volumes of flat and short bones. In long bones, it forms the epiphyses (except for their thin outer margin) and the inner layer of the diaphysis.
Compact Bone Substance and the Osteon System
Unlike the trabecular network, compact bone is characterized by exceptional density—it contains virtually no free spaces. It forms the external parts of all bones and the bulk of the shaft (diaphysis) of long bones.
The lamellae in compact bone are arranged predominantly in a concentric pattern, forming hollow cylinders nested within one another. Depending on their location, four types of lamellae are distinguished:
- Outer circumferential lamellae — encircle the bone externally, lying beneath the periosteum.
- Osteonal lamellae — concentric layers surrounding blood vessels, forming osteons.
- Interstitial lamellae — fragments of degraded old osteons that fill the gaps between newly formed ones.
- Inner circumferential lamellae — line the wall of the medullary cavity.
The osteon serves as the primary unit of compact bone. It is demarcated from adjacent tissues by a resorption (cement) line. A canal containing a blood vessel runs through its center. Lacunae containing osteocyte cell bodies are located between the osteon lamellae. Their long processes radiate outward inside canaliculi, penetrating the lamellae. This canalicular network ensures the delivery of nutrients from the central vessel to the most distant cells.
Microscopic Structure of the Diaphysis
When examining a decalcified long bone (for example, using Schmorl's staining method, which stains collagen in the canalicular walls brown), the wall of the diaphysis displays a distinct layered structure:
- Outer layer (periosteal zone): includes the periosteum with osteoblasts, osteoclasts, and cambial cells, as well as the underlying layer of outer circumferential lamellae.
- Middle layer (osteonal zone): the thickest part of the compact substance. It consists of functioning osteons and interstitial lamellae between them. Within the osteon canals (in the perivascular space), tissue fluid and cells analogous to periosteal elements are present.
- Inner layer (endosteal zone): formed by inner circumferential lamellae covered by the endosteum. Where they contact spongy bone, these lamellae may be resorbed, transitioning directly into trabeculae.
In adults, the central medullary cavity of the diaphysis is filled with yellow bone marrow (adipose tissue).
The structure of flat bones has specific features: beneath a thick layer of compact substance (consisting of circumferential lamellae and primary osteons) lies a deep layer of spongy substance with small cavities.