Histology of Bone Tissue

Microscopic examination of bone shows two varieties:

1.  Primary, immature or woven bone

In addition to irregular array of collagen fibres, other characteristics of primary bone tissue are a lower mineral content and a higher proportion of osteocytes than that in secondary bone tissue. The newly formed bone during development or during healing process are woven bone.

2. Secondary bone tissue or lamellar bone

Secondary bone tissue is the variety usually found in adults. It characteristically shows the collagen fibers arranged in lamellae (3-7 micrometer thick) that are parallel to each other or concentrically organized around a vascular canal. The whole complex of concentric lamellae of bone surrounding a canal containing blood vessels, nerves and loose connective tissue is called a Haversian system or osteon. Lacunae consisting of osteocytes are found between and occasionally within the lamellae. In each lamella, collagen fibers are parallel to each other.

In compact bone, the lamellae exhibit a typical organization consisting of haversian system, outer circumferential, inner circumferential and interstitial lamellae.

Each haversian system is a long, often bifurcated cylinder parallel to the long axis of diaphysis. It consists of a central canal surrounded by 4-20 concentric lamellae. Each endosteum lined canal contains blood vessels, nerves and loose connective tissue. The haversian canal communicates with marrow cavity, periosteum and one another through Volkman’s canal.

Structural unit of bone
Bone is a specialized connective tissue. It consists of bone cells or osteocytes that are widely separated by intercellular substance, bone matrix, which consists of a homogenous ground substance or matrix in which collagen fibers and mineral slats (mainly calcium and phosphorus) are deposited.
In addition to mature bone cells (osteocytes), osteoblast or bone producing cells and osteoclasts or bone removing cells are present in the developing bone. In addition, osteoprogenitor cells are also present that give rise to osteoblasts and osteocytes.

Cells of the bone tissue:

1. Osteoprogenitor cells
These are cells of mesenchymal origin that can proliferate and convert themselves into osteoblasts whenever there is need for bone formation. They resemble fibroblasts in appearance.

2. Osteoblasts 
These are bone forming cells derived from osteoprogenitor cells. They are found lining growing surfaces of bone.the cells are of varied shapes (oval, triangular, cuboidal etc.). The nucleus of these cells is ovoid and euchromatic. The cytoplasm is basophilic. Numerous cytoplasmic processes radiate from each cell and come into contact with similar processes of neighboring cells. These cells are responsible for laying down the organic matrix of bone including collagen fibers. They are also responsible for calcification of the matrix.

3. Osteocytes 
These are the cells of mature bone. They lie in the lacunae of bone and represent osteoblasts that have been imprisoned in the bone matrix during bone formation. There is connection between the osteocyte through cytoplasmic processes. Osteocytes have eosinophilic cytoplasm.  The main function of osteocytes are i) to maintain integrity of the lacunae and canaliculi and thus to open the channels for diffusion of nutrition through bone and ii) to remove or deposit matrix and calcium when required.

4. Osteoclasts 
These are bone removing cells. These cells occupied pits called lacunae of Howship or resorption bays. These are very large cells (20 -100 micrometer in diameter). They have numerous nuclei – up to 20 at sites of bone resorption. The surface of an osteoclast shows many folds known as ruffled membrane. Removal of bone by osteoclasts involves demineralization and removal of matrix, stimulated by factors secreted by osteoblasts and parathyroid hormone. They are derived from monocytes of blood.

5. Bone lining cells
These are flattened cells and form a continuous epithelium like layer on bony surfaces where active bone deposition or removal is not taking place. They line periosteal as well as endosteal surfaces and also spaces and canals within the bone.

Ground Substance of Bone
The ground substance of bone consists of an organic matrix in which inorganic salts are deposited.

1. The organic matrix
This consists of ground substance in which collagen fibers (Type I) are embedded. The ground substance consists of glycosamine glycans, proteins and water. Two special glycoproteins – osteonectin and osteocalcin are present in large quantity. They readily bind to the calcium and play a role in mineralization. Collagen fibers are type I and are usually arranged in layers, the fibers within the layer running parallel to one another. They are synthesized by oseoblasts.

2. Inorganic salts
The ions present are mainly calcium and phosphorus. Magnesium, carbonate, hydroxyl, chloride, fluoride, citrate, sodium and potassium are also present in significant amounts. Most of the calcium, phosphate and hydroxyl ions are in the form of needle shaped crystal known as hydroxyapatite. These crystals lie parallel to collagen fibers and contribute to lamellar appearance of bone.

The amount of organic and inorganic component of bone is 35% and 65% respectively.

The main functions of these layers are nutrition of osseous tissue and provision of continuous supply of new osteoblasts for repair and growth.

Coverings of the bone
All bones are lined on both internal and external surfaces by layers of tissue containing osteogenic cells- endosteum on the internal surface and periosteum on the external surface.

The periosteum consists of an outer layer of collagen fibers and fibroblasts. Bundles of periosteal collagen fibers, called Sharpey’s fibres, penetrate the bone mmatrix, binding periosteum to bone. The inner more cellular layer of the periosteum is composed of osteoprogenitor cells.

The endosteum lines all internal cavities within the bone and is composed of single layer of flattened osteoprogenitor cells and a very small amount of connective tissue.

Following resources are used while preparing this post (readers are strongly recommended to go through them for more details):

Wheater's Functional Histology: A Text and Color Atlas
Junqueira's Basic Histology: Text and Atlas