Examination of Bone Tissue

Below is a basic description of the organization both macroscopically and microscopically of a typical bone. Use the illustrations to assist your visualizing the location of these structures. This is especially inportant with the microscopic structure of bone.

Bone Tissue: Macroscopic

Let us examine the basic anatomy of a bone by looking at a long bone. Typically these bones consist of the shaft or diaphysis (length of the bone) which has an internal cavity, and the knob-like ends, called the epiphysis.

The diaphysis is made of a dense type of bone tissue called compact bone. Compact bone is also found covering the ends of a long bone. Although it seems to be extremely solid, compact bone has many microscopic canals running through it (see below, Haversian canals). The shaft of living bone contains a cylindrical cavity filled with yellow bone marrow which is stored lipids (fat), which can be used as an energy source.

Internally the bulk of each epiphysis is composed of spongy bone, which is less dense than compact bone. Spongy bone consists of tiny bars and plates that form a grid-like pattern that is similar to that of a household sponge. In living bone, the spaces between the bars and plates contain red bone marrow, which produces most of the red and white blood cells in the body.

Where the ends of long bones form movable joints, their surfaces are covered with articular cartilage, a tough, smooth connective tissue (hyaline) that reduces friction between the moving surfaces. The outer surface of the shaft is covered by the periosteum, a thin, tough covering of connective tissue whose inner layer contains many specialized bone-forming cells. The periosteum is well supplied with nerves and blood vessels, some of which pass into the compact bone. The medullary or marrow cavity is lined with a fibrous tissue as well, the endosteum. Both the endosteum and periosteum have cellular and fibrous componets.

As you read the description that follows, examine the illustration of bone structure. First note how the three images (L-R: Whole bone- wedge of the whole- wedge of a Haversian canal) relate to each other. The three thumbnail images are clickable, opening new windows with enlarged electron micrographs of bone tissue. Bone is made up of two base componets: a cellular componet and a non-living componet made mostly of Ca (calcium) and PO
4 (phosphate).
Bone Tissue: Microscopic
The main type of bone cell is the Osteocyte ("bone cell", shown as purple in the diagram). In compact bone, these cells are embedded within the solid calcium phosphate matrix of solid bone. Each cell appears to be isolated from other cells, but in reality are connected to neighboring cells by thin cellular extensions that pass through tiny channels in the solid matrix (see picture on left of the diagram), allowing diffusion between cells to occur.
To sustain life and perform their specialized functions, osteocytes (like all other cells) need access to blood vessels to obtain nutrients and excrete waste.Access to these nutrients is provided by microscopic channels called Haversian canals, which the osteocytes are organized into circular patterns around. Each canal contains blood vessels and a nerve (see the middle figure in the diagram).
The osteocyte closest to the blood vessel aquire nutrients from the blood by diffusion and pass them to moredistant cells by means of their cellular extensions, also by diffusion. Likewise waste products from distant osteocytes diffuse through a series of cells until they reach the blood vessels of the Haversian canal. Each of these circular 'clusters' are refer to as an Osteon or Haversian canal System; They form the functional unit of bone.

Spongy bone also contain osteocytes, but they are not organized around Haversian canals. The bars and plates of spongy bone are narrow, only a few cells thick, thus each osteocyte has intimate access to red marrow, allowing for the exchange of nutrients and waste with this material with a rich blood supply.

While osteocytes maintain bone, two other types of cells form new bone and remove bone. Bone-forming cells are called Osetoblasts, and they are located within the periosteum and endosteum. Osteoblasts remove soluble Ca+2 and PO4-3 ions from the blood and deposit them as calcium phosphate (called hydroxyapotite) on the thin collegen fibers of the bone matrix. During bone development, osteoblasts become entrapped in the matrix as it forms. Once this occurs they become transformed into osteocytes. Osteoblasts are formed by precursor cells (similar to stem cells) called osteoprogenitor cells. These cells are also found in both linings of the bone.

Bone-removing cells are called Osteoclasts. Osteoclasts are located on the surface of bone, within the endosteum and remove bone by secreting enzymes and acids that dissolve the solid calcium phosphate. As a result Ca+2 and PO4-3 are released into the blood when insufficient dietary Ca+2is consumed and become available when these ions are needed elsewhere in the body.

The activities of -blast and -clast cells is equal in 'normal', young individuals. Bone is constantly being remodeled as Ca+2 levels of the blood vary. Hormonal activity influences this and as a consequence, as one ages (particularly in women), -blast activity diminishes and -clast activity remains constant. This can ultimately lead to a weakening of the bone.