Bone Tissue Types and Structure

Types of Bone Tissue

Bone tissue can be classified in several ways, including texture, matrix arrangement, maturity, and developmental origin.

Based on texture of cross sections, bone tissue can be classified as follows:

• Compact bone (dense bone, cortical bone): Compact bone is ivorylike and dense in texture without cavities. It is the shell of many bones and surrounds the trabecular bone in the center. Compact bone consists mainly of haversian systems or secondary osteons.
• Sponge bone (trabecular bone, cancellous bone): Sponge bone is so named because it is spongelike with numerous cavities. It is located within the medullary cavity and consists of extensively connected bony trabeculae (see the images below) that are oriented along the lines of stress.Trabecular bone and the surrounding hematopoietic cells, as well as adipose tissue in medullary cavity.Mature trabecular bone exhibits lamellae and osteocytes between the lamellae. Inactive osteocytes are also present on the bone surface with a flattened shape.
  

  • In contrast to compact bone, complete osteons are usually absent in sponge bone due to the thinness of the trabeculae.Sponge bone is also more metabolically active than compact bone because of its much larger surface area for remodeling.

Based on matrix arrangement, bone tissue can be classified as follows:

• Lamellar bone (secondary bone tissue): Lamellar bone is mature bone with collagen fibers that are arranged in lamellae. In contrast to sponge bone, in which lamellae are arranged parallel to each other, in compact bone, the lamellae are concentrically organized around a vascular canal, termed a haversian canal.
• Woven bone (primary bone tissue): Woven bone (see the first image below) is immature bone, in which collagen fibers are arranged in irregular random arrays and contain smaller amounts of mineral substance and a higher proportion of osteocytes than lamellar bone. Woven bone is temporary and is eventually converted to lamellar bone; this type of bone is also pathologic tissue in adults, except in a few places, such as areas near the sutures of the flat bones of the skull, tooth sockets (see the second image below), and the insertion site of some tendons (see the third image below).Woven bone under a polarized microscope; collagen fibers are arranged in disorganized arrays.Tooth socket in adults.Tendon insertion site in adults.

Based on maturity, bone tissue can be classified as follows:

• Immature bone (primary bone tissue): Immature bone is woven bone.
• Mature bone (secondary bone tissue; see the image below): Mature bone is characteristically lamellar bone. Almost all bones in adults are lamellar bones.Osteon of mature bone viewed under a polarized microscope; lamellae are shown as alternating dark and bright layers due to the perpendicular orientation of the collagen fibers in the neighboring lamellae.

Based on developmental origin, bones can be classified as follows:

• Intramembranous bone (mesenchymal bone): Intramembranous bone develops from direct transformation of condensed mesenchyme. Flat bones are formed in this way.
• Intracartilaginous bone (cartilage bone, endochondral bone): Intracartilaginous bone forms by replacing a reformed cartilage model. Long bones are formed in this way.

Microscopic Structure of Bone

Bone cells

• Osteoblasts
  

  • Osteoblasts (see the image below) are located on the surface of bone or osteoid, and they are responsible for synthesizing the organic components of the bone matrix, including type I collagen, proteoglycans, and glycoproteins. Osteoblasts also synthesize the enzyme alkaline phosphatase, which is needed locally for the mineralization of osteoid.Osteoblasts in cytologic preparation (Diff-Quik stain). Each active osteoblast has eccentrically located nuclei with a conspicuous nucleus and a perinuclear halo, resembling a plasma cell. However, the osteoblast does not exhibit the clock-face or wheel-like chromatin pattern that is seen in a plasma cell.
  • Although an active osteoblast (see the image below) has a cuboidal or columnar shape with an eccentrically located nucleus and a perinuclear cytoplasmic halo, an inactive osteoblast has a flattened shape with low alkaline phosphatase activity. Osteoblasts contact their neighboring osteoblasts cytoplasmically.Active osteoblasts depositing osteoid on the surface of a woven bone trabecula. Osteoblasts are columnar or cuboidal shaped, with eccentric nuclei and perinuclear halo. These cells also have polarity, with the cytoplasm toward the bone but the nuclei at the end away from the bone.
  • Osteoblasts do not divide. They give rise to osteocytes, remain as osteoblasts, or return to the state of osteoprogenitor cells from which they derived.
• Osteocytes
  

  • An osteoblast becomes an osteocyte when the cell is encased by osteoid matrix that it synthesizes itself. Lacunae and canaliculi form around the osteocyte and its cytoplasmic processes, respectively.^[17] Thus, an osteocyte lies in its own lacuna and contacts its neighboring osteocytes cytoplasmically through canaliculi (see the image below).Osteocytes are present in lacunae; their cytoplasmic processes contact each other through the canaliculi.
  • The processes of adjacent cells make contact via gap junctions, maintaining the vitality of osteocytes by passing nutrients and metabolites between blood vessels and distant osteocytes, regulating ion homeostasis, and transmitting electrical signals in bone.
  • Although osteocytes have reduced synthetic activity and are not capable of mitotic division, they are actively involved with the maintenance of the bony matrix. Some of the osteocytes die during remodeling, but most probably return to the state of osteoprogenitor cells or persist as osteocytes for a long time.
• Osteoclasts
  

  • Osteoclasts (see the image below) are probably derived from a monocytic-macrophage system and are responsible for bone resorption. They are multinucleated cells with fine, fingerlike cytoplasmic processes and are rich in lysosomes that contain tartrate-resistant acid phosphatase (TRAP).Osteoclast in a cytologic preparation (Papanicolaou stain). This image shows multiple nuclei and cytoplasmic processes.
  • Osteoclasts lie in resorption craters known as Howship lacunae (see the image below) on bone surfaces or in deep resorption cavities called cutting cones. These bone cells can only resorb mineralized bone matrix.Remodeling of bone. Multiple osteoclasts are sitting in the Howship lacunae, resorbing one side of a bony trabecula, while osteoblasts are depositing new bone on the other side.
  • Cells that express the full morphologic and functional properties of mature osteoclasts are known to be restricted to the surfaces of bones.
    • Osteoclast transmigration on the bone surface has been assumed to be for the purpose of bone resorption. A study by Saltel et al appears to have demonstrated a new property of mature osteoclasts: transmigration through bone tissues of various cell types.The authors' results may have implications for therapeutic strategies for bone diseases with an imbalance in bone remodeling that is caused by excessive osteoclast resorption.
    • Research is also under way to investigate whether "components of the bone matrix and specific cell surface receptors on osteoclasts and their precursors play an essential role in determining the genetic profile and functional properties of fully differentiated resorbing osteoclasts."
  • Osteoclasts or their committed precursors do not have receptors for parathyroid hormone. The hormonal signal is mediated by osteoblasts.However, osteoclasts do have receptors for calcitonin.
  • When in an active state, osteoclasts create an effect that always predominates over that of osteoblasts because osteoclasts are three times more efficient at bone resorption than osteoblasts are at bone deposition. In balance, osteoclasts have a much shorter life span than osteoblasts.
  • Osteoclasts are rarely seen in routine histologic sections of normal bone. An increased number of osteoclasts is characteristic of diseases with increased bone turnover.

Bone matrix

Bone matrix consists of organic and inorganic components.The association of organic and inorganic substances gives bone its hardness and resistance. The organic component is composed of collagen fibers with predominately type I collagen (95%) and amorphous material, including glycosaminoglycans that are associated with proteins. Osteoid is uncalcified organic matrix. Inorganic matter represents about 50% of the dry weight of bone matrix, composed of abundant calcium and phosphorus, as well as smaller amounts of bicarbonate, citrate, magnesium, potassium, and sodium. Calcium forms hydroxyapatite crystals with phosphorus but is also present in an amorphous form.

During bone remodeling, osteoblasts deposit a layer of osteoid seam (approximately 10 µm thick) on the surface of preexisting bone, which then begins to mineralize in approximately 20 days. This interval is known as the mineralization lag time.

In the histology of normal bone, as a result of the normal remodeling process, up to 20% of the bone surface may be covered by osteoid (usually 10 µm thick). An increased amount of osteoid is seen in pathologic conditions in which the remodeling rate is accelerated or in which the mineralization lag time is increased.

Microscopic architecture of bone

• Haversian system (secondary osteon)
  

  • The primary structural unit of compact bone is the haversian system. Each haversian system is a long, often bifurcated, cylinder that is parallel to the long axis of bone, formed by successive deposition of 4-20 (average 6) concentric layers of lamellae.
  • Collagen fibers are parallel to each other within each lamella, but they are oriented perpendicularly to the fibers in the neighboring lamellae. Such an arrangement can be highlighted as alternating bright and dark layers in polarized microscopy (see the image below).Osteon of mature bone viewed under a polarized microscope; lamellae are shown as alternating dark and bright layers due to the perpendicular orientation of the collagen fibers in the neighboring lamellae.
  • Lamellar deposition starts from the periphery, so that younger lamellae are closer to the center of the system, and the younger systems have larger canals. Between the lamellae are lacunae that contain the cell bodies and canaliculi that hold the cytoplasmic processes of osteocytes.
  • In the center of each haversian system is a haversian canal, which is lined by endosteum and contains a neurovascular bundle and loose connective tissue.
    • The haversian canals connect with each other by transverse or oblique Volkmann canals that communicate with the marrow cavity and the periosteum to provide channels for the neurovascular system. Volkmann canals are not surrounded by concentric lamellae; rather, they perforate the lamellae.
• Interstitial lamellae
  

  • Interstitial lamellae are incomplete or fragmented osteons that are located between the secondary osteons. They represent the remnant osteons left from partial resorption of old osteons during bone remodeling.
  • The mixture of interstitial lamellae and complete osteons produces a mosaic pattern. Thus, the age of the bone can be deduced from the proportion of interstitial lamellae and intact osteons. Younger bone has more complete osteons and less interstitial lamellae in between the osteons.
    • The age of bone also affects osteoclast activity in bone resorption. In a study by Henriksen et al, the authors demonstrated that osteoclasts preferentially differentiate and resorb bone on aged bone than they do on young bone.
• Circumferential lamellae
  

  • Circumferential lamellae are circular lamellae that line the external surface of the cortex adjacent to the periosteum and line the inner surface of the cortex next to the endosteum.
  • There are more outer than inner circumferential lamellae.