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.
REFERENCES:
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