Dense Support Connective Tissues - Osseous Tissue

Osseous tissue - During the evolutionary history of the early skeleton, bone was found to be superior to cartilage as a support tissue:

  1. Cartilage does not have the rigidity of bony tissue. When cartilage is mineralized to make it more rigid, the ability of nutrients and metabolites to diffuse through the matrix was retarded. In bone, mineral salts are deposited around the processes of osteoblasts forming canaliculi which communicate between bone cells and the vascular tissue. This assures an adequate supply of nutrients for the bone cells.
  2. Cartilagenous tissue is not richly endowed with blood vessels. Bone, on the other hand, has a fairly extensive blood supply through the Haversian and Volkman’s canals.
  3. The rigid nature of bony tissue does not permit internal or interstitial growth as seen in cartilage. Bone grows by apposition, i.e., from the outside as in the trunk of a tree. The elongation of long bones is permitted by the proliferation of the cartilage cells internally within specific growth plates. This is followed by the death of the new chondrocytes and their replacement with bone cells.
  4. Bone is not a static tissue but is continually undergoing local destruction and reformation.

Composition - Like all connective tissues, bone is composed of two major components:

I.  Cells

A. Osteoblasts

    1. Found where ever new bone is being deposited, e.g., in the periosteum, in endosteum, near growth plates or within the mesenchymal sheets forming flat bones.
    2. Tend to be cuboidal cells
    3. Nucleus is large and multilobed.
    4. Cytoplasm is basophilic due to the synthesis of the proteins of ground substance and alkaline phosphatase. The latter is an enzyme responsible for the calcification of matrix.

 

B. Osteocytes

1. Found imprisoned within lacunae in the mineralized matrix of bone.

2. Cytoplasm is slightly basophilic.

3. Fine cytoplasmic processes extend through thread-like canaliculi. In young cells, these processes make contact with those of neighboring cells. In older cells, the processes are withdrawn. The canaliculi remain intact and serve as avenues for nutrients, oxygen and wastes to move between the blood and the osteocytes.

C.  Osteoclasts

1.  Giant, multinucleated cells.

2.  Located on the surfaces of bone in shallow excavations calloed Howship’s lacunae.

3.  Cytoplasm is faintly basophilic and granular containing lysosomes.

4.  Under the electron microscope, the surface of the cells show numerous cytoplkasmic projections referred to as the "ruffled border"

5.  Their origin is disputed. They may arise from osteoprogenitor cells which produce all bone cells or may be descended from monocytes which invade the young bone from the blood.

II.  Bone Matrix

  1. Consists of collagenous fiber bound together by a cementing substance, primarily glycosaminoglycans.
  2. The inorganic components make up 65% of the weight of bone and are chiefly calcium compounds, e.g., calcium phosphate ( 85%) and calcium carbonate (10%).
  3. The minerals are deposited on the collagenous framework. The interiors of lacunae and canaliculi are lined with a special organic cement which lacks fiber.
  4. Within a Haversian system, the matrix is arranged in layers or lamellae (3 to 7um thick). The fibers in a particular lamella are arranged parallel to each other in a helical manner. The fibers in adjacent lamellae are arranged at right angles to each other. This alternating arrangement explains the unique appearance of the layers.

Architecture of Bone

I.  Periosteum -

A. A fibrous sheath which covers bone except on its articulating surfaces.

B. Composed of two layers of collagenous fiber:

1.  A dense, vascular outer layer.

2.  A looser inner layer containing more elastic fiber and cells.

a.  Coarse collagenous fibers called Sharpey’s fibers, arise in this layer and embed themselves in the bone matrix. This helps the periosteum adhere to the bone tissue.
b.  The inner layer also contains active osteoblasts during growth and repair.

3.  It anchors tendons ad ligaments.

4.  Conducts blood vessels and nerves to the underlying bony tissue. For this reason, the periosteum is very important in bone development and repair.

5.  The articulating surfaces of long bones are covered with hyaline cartilage not periosteum.

II. Compact Bone - Consists of bone matrix laid down in highly organized layer or lamellae.

  1. Periosteal and endosteal lamellae are layers of bone matrix, osteocytes and minute canals (canaliculi) that border the internal and external surfaces of bone.
  2. Haversian system - the basic structural sub unit of mature bone. It consists of:
    1. A system of concentrically arranged lamellae, each about 3 to 7um thick, found around a central lumen, the Haversian canal.
    2. The Haversian canal contains blood vessels, nerves and lymphatics.
    3. The lamellae are transversed by minute tubes called canaliculi. The canaliculi all connect with the Haversian canal. At their other end, they enter the lacunae around living bone cells, the osteocytes. They serve to provide these cells with nutrients and carry away wastes.
    4. Osteocytes are arranged concentrically in the lamellae. They fit into a space called a lacuna. The canaliculi branch and communicate with each other, the lacunae and the Haversian canals.
    5. The Haversian system is also transversed by Volkman’s canals which pass through horizontally or at an angle. These canals also carry blood vessels, nerves and lymphatics.

III. Spongy or Cancellous Bone - Consists of a complex pattern of "lattice-work" struts enclosing spaces. The bony struts are called trabeculae and the spaces contain marrow tissue. Most spongy bone is found in the ends (epiphyses) of long bones. In the fetus, the arrow is blood-forming.

IV. Endosteum - lines the marrow cavities and irregular surfaces of spongy bone.

  1. In the adult it consist of condensed reticular tissue which has both osteogenic and hemopoietic potential.
  2. During growth and repair of a fracture, it becomes active and contains osteoblasts.