Principle 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 separate lamella, all fibers are directed strictly parallel to each other. Meanwhile, in adjacent, neighboring lamellae, the direction of the fibers changes to opposite or oblique. Such cross-ply arrangement creates a reliable framework and provides bones with the highest 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 distinguish between concepts: bones exist as anatomical organs (flat, spongy, tubular) and bone substance as a histological structure. Both spongy and compact substance are present in all types of bones; only their ratio changes.
Spongy bone substance
This structure visually resembles a sponge: it is a three-dimensional anastomosing network. The basic lamellae here have a flat or arched shape and combine into trabeculae — trabeculae.
Inside the trabeculae, the lamellae lie parallel to each other, and the trabeculae themselves are generally devoid of their own blood vessels (avascular). The spaces between the trabeculae form bone marrow cavities. It is in these spaces that nutrient vessels run and red bone marrow is located — the most important hematopoietic organ.
Spongy substance fills the internal volumes of flat and spongy bones. In tubular bones, it forms the epiphyses (with the exception of their thin outer edge) 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 — there are virtually no free spaces in it. It forms the outer parts of all bones and the bulk of the body (diaphysis) of tubular bones.
The lamellae in compact bone are arranged predominantly concentrically, forming hollow cylinders nested one inside another. Depending on their location, four types of lamellae are distinguished:
- Outer circumferential lamellae — surround the bone externally, lying beneath the periosteum.
- Osteonal lamellae — concentric layers around blood vessels that form osteons.
- Interstitial lamellae — fragments of degraded old osteons that fill the gaps between new ones.
- Inner circumferential lamellae — line the wall of the medullary cavity.
Osteon acts as the main unit of compact bone. It is separated from neighboring tissues by a resorption (cement) line. A canal with a blood vessel runs through its center. Lacunae containing the bodies of osteocytes are located between the osteon lamellae. Their long processes radiate outward inside canaliculi, penetrating the lamellae. Such a network of canaliculi ensures the delivery of nutrients from the central vessel to the most distant cells.
Microscopic structure of the diaphysis
When studying a decalcified tubular bone (for example, using Schmorl's stain, which stains the collagen in the canaliculi walls brown), the wall of the diaphysis demonstrates a clear layered structure:
- Outer layer (periosteal zone): includes the periosteum containing 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. Inside 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 with endosteum. At sites of contact with 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 its own specifics: beneath a thick layer of compact bone (consisting of circumferential lamellae and primary osteons) lies a deep layer of spongy bone with small marrow spaces.
Briefly: answers to topical questions
What runs through the central canal of an osteon?
Blood and lymphatic vessels, as well as nerves, run through the central canal of an osteon. Inside this canal, directly between its wall and the blood vessel, lies the perivascular space. Its main contents include the following components:
- Tissue fluid — the basic medium of the space.
- Cellular elements — represented by osteogenic cells, osteoblasts, and bone macrophages (osteoclasts), which are analogous to periosteal cells.
Through which channels do blood vessels penetrate from the periosteum into the bone to the osteons?
Blood vessels penetrate from the periosteum into the bone through Volkmann's (perforating) canals. Branches depart from the periosteal vessels, entering the bone and ensuring its vascularization. The subsequent pathway of substance transport: vessels (capillaries) → perivascular spaces → bone canaliculi with osteocyte processes → osteocytes.
To avoid confusing levels of organization: bones (as organs) are classified into flat, spongy, and tubular. Meanwhile, the bone substance inside them is divided into compact and spongy — and both types are constructed exclusively from lamellar tissue.
Class and exam questions
What is the difference between a bone lamella and a trabecula?
A bone lamella is the elementary "brick" of tissue with parallel-laid collagen fibers. A trabecula is a larger structure of spongy bone that consists of several combined bone lamellae.
Where are blood vessels located in spongy bone?
The trabeculae of spongy bone are usually avascular. Blood vessels run in the intertrabecular spaces (marrow spaces) together with red bone marrow tissue.
Where do interstitial lamellae in compact bone come from?
Interstitial lamellae are remnants of previous generations of osteons. During continuous bone remodeling, old osteons are partially broken down (resorbed), and their undegraded fragments remain between newly formed osteons.
What else to study on this topic
- Cellular composition of the periosteum and endosteum (functions of cambial cells, osteoblasts, and macrophages)
- Histological features of the perivascular space within the osteon
- Mechanisms of formation of the resorption (cement) line
- Schmorl's method and visualization of the bone canaliculus system
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