Histological Organization of Bone

Histological Organization of Bone

Compact Bone

Compact bone consists almost entirely of extracellular substance, the matrix. Osteoblasts deposit the matrix in the form of thin sheets which are called lamellae. Lamellae are microscopical structures. Collagen fibres within each lamella run parallel to each other. Collagen fibres which belong to adjacent lamellae run at oblique angles to each other. Fibre density seems lower at the border between adjacent lamellae, which gives rise to the lamellar appearance of the tissue. Bone which is composed by lamellae when viewed under the microscope is also called lamellar bone.
In the process of the deposition of the matrix, osteoblasts become encased in small hollows within the matrix, the lacunae. Unlike chondrocytes, osteocytes have several thin processes, which extend from the lacunae into small channels within the bone matrix , the canaliculi. Canaliculi arising from one lacuna may anastomose with those of other lacunae and, eventually, with larger, vessel-containing canals within the bone. Canaliculi provide the means for the osteocytes to communicate with each other and to exchange substances by diffusion.
In mature compact bone most of the individual lamellae form concentric rings around larger longitudinal canals (approx. 50 µm in diameter) within the bone tissue. These canals are called Haversian canals. Haversian canals typically run parallel to the surface and along the long axis of the bone. The canals and the surrounding lamellae (8-15) are called a Haversian system or an osteon. A Haversian canal generally contains one or two capillaries and nerve fibres.
Irregular areas of interstitial lamellae, which apparently do not belong to any Haversian system, are found in between the Haversian systems. Immediately beneath the periosteum and endosteum a few lamella are found which run parallel to the inner and outer surfaces of the bone. They are the circumferential lamellae and endosteal lamellae.
A second system of canals, called Volkmann's canals, penetrates the bone more or less perpendicular to its surface. These canals establish connections of the Haversian canals with the inner and outer surfaces of the bone. Vessels in Volkmann's canals communicate with vessels in the Haversian canals on the one hand and vessels in the endosteum on the other. A few communications also exist with vessels in the periosteum.

Trabecular Bone

The matrix of trabecular bone is also deposited in the form of lamellae. In mature bones, trabecular bone will also be lamellar bone. However, lamellae in trabecular bone do not form Haversian systems. Lamellae of trabecular bone are deposited on preexisting trabeculae depending on the local demands on bone rigidity.
Osteocytes, lacunae and canaliculi in trabecular bone resemble those in compact bone.

Note the distinction between macroscopic (visible to the eye) and microscopic (only visible under the microscope) appearance when the bone is named. Lamellar bone forms both trabecular bone and compact bone, which are the two macroscopically recognizable bone forms.

Suitable Slides

sections of compact bone (usually part of the diaphysis of a long bone) - ground (unstained), Schmorl stainedor H&E

Compact bone, human - Schmorl stain
Lamellae which run parallel to the surface of the bone are visible both on the outer, convex surface of the bone (circumferential lamellae) and on the inner, concave surface of the bone facing the marrow cavity (endosteal lamellae). The surface formed by the endosteal lamellae is often more irregular than the surface formed by the circumferential lamellae. The space between these two sets of lamellae is filled by Haversian systems and interstitial lamellae. Only few of the Haversian systems are "textbook" circular. Osteocyte lacunae are visible between the lamellae. Canaliculi become visible at high magnification (illustrated in the ground section below).
You should be able to see, draw and identify Haversian systems, interstitial and circumferential lamellae and/or endosteal lamellae.
Compact bone, human - ground (unstained)
The osteocyte lacunae are the main feature of the ground section. They are visible as elongated black spots in the bone matrix. Canaliculi, radiate from the lacunae into the surrounding bone matrix. Some lamellae are visible in the ground section. There is actually no distinct border between most lamellae, but our brain can use the elongated osteocyte lacunae and their orientation to "reconstruct" the lamellae. Volkman's canals connect to a few of the Haversian canals.

Suitable Slides

sections of part of a vertebra or an epiphysis of a long bone - H&E, van Gieson
Sections prepared to show articular cartilage will often also contain trabecular bone in the epiphysis below the articular cartilage.

Articular cartilage, bovine - H&E
Thin sheets and bars of bone, trabeculae, are visible at low magnification. Although they may appear as individual pieces in sections, they form an interconnected meshwork in the living bone. The spaces between the trabeculae, the marrow cavity of the epiphysis, is filled by either red bone marrow or yellow bone marrow. At high magnification, elongated osteocyte lacunae, which in well preserved tissue still contain osteocytes, are visible in the matrix. If the H&E stain also turned out well, it should be visible that the matrix of the trabecular bone is formed by lamellae. Canaliculi are present but hard to identify in most H&E stained sections. Haversian systems are not present in the trabeculae.
In trabecular bone obtained from young individuals, in which the bone is still growing, small areas of calcified cartilage are occasionally seen in the bone trabeculae. They are remnants of the cartilage scaffold on which bone matrix was deposited during the formation of the trabeculae. With the reorganisation of bone such areas will eventually be lost.