*Characteristic features of bone *Bone types in the body *Microstructure of compact & spongy bone *Histophysiology of bone

Functions:  It supports and protects the more fragile tissues and organs.  Harbors hematopoietic tissue (bone marrow).  Forms a system of levers and pulleys that multiply and focus the contractile forces of muscle.  The bone matrix function as an important storage site for calcium and other essential minerals.

Types of Bone Tissue: - Regular (long) or compact bone. - Irregular (flat) or spongy bone. Parts of Long Bones: Most bones of the arms and legs (eg. the femur) are termed long bones. The different parts in bone are:- - The diaphysis is the shaft of a long bone, - The epiphysis is its bulbous end.

Types of Bone Tissue: - Regular (long) or compac bone. - Irregular (flat) or spongy bone. Parts of Long Bones: Most bones of the arms and legs (eg. the femur) are termed long bones. The different parts in bone are:- - The diaphysis is the shaft of a long bone, - The epiphysis is its bulbous end.

Surfaces:  The outer surfaces of bones are covered by two layers of connective tissue, the periosteum.  The outer or fibrous layer is formed of dense connective tissue;  the inner or osteogenic layer is formed of loose tissue containing bone cell precursors.  Sharpey's fibers are periosteal collagen fibers (nail shape) that penetrate bone matrix and anchor the periosteum to the bone.  The internal surfaces of bones are covered by a thin, condensed reticular connective tissue (endosteum) that contain bone and blood cell precursors.  The endosteum lines the marrow cavity and sends extensions into the haversian canals.

Composition: Bone tissue composed of cells (osteocytes, osteoblasts. and osteoclasts), fibers (type I collagen), and ground substance containing abundant mineral salts. It differs from other connective tissues primarily in having large quantities of inorganic salts in its matrix, accounting for its hardness. Structure of bone: As other connective tissues, bone is formed of:-  Cells: Osteogeneic cells, Osteoblasts, Osteocytes, Osteoclasts.  Matrix: organic and non-organic parts.  Fibers: collagen type I fibers.

Bone cells 1- Osteogenic (Osteoprogenitor) cells  They are mesenchymal stem cells found in the periosteum and endosteum.  They are small spindle-shaped cells with pale cytoplasm and ovoid nuclei.  Two types are distinguishable by the electron microscope: one can differentiate into osteoblasts, and the other into osteoclasts.  Osteoblast precursors derive from embryonic mesenchyme and have sparse RER and Golgi complexes.  Osteoclast precursors derive from blood monocytes and have abundant free ribosomes and mitochondria.

2- Osteoblasts:  They are bone forming cells, found in the growing surface of the bone (in the periosteum and endosteum) they form one-cell-thick sheets.  They are dividing cells that synthesize the organic components of bone matrix.  They are large rounded or cuboidal cells, with deep basophilic cytoplasm, well-developed RER and Golgi; and eccentric nucleus.  They synthesize and secrete all the organic components of bone matrix and may be involved in bone mineralization.  Once surrounded by matrix, osteoblasts are considered mature and called osteocytes.

3- Osteocytes  The cells are branched, smaller than osteoblasts, and not divide.  They are terminally differentiated bone (mature) cells found in cavities in the bone matrix called lacunae, and their processes (branches) extend into canaliculi in the classified matrix.  Osteocytes are isolated from one another by an impermeable bone matrix and contact one another at the tips of their filopodia, often through gap junctions.  Osteocytes recently derived from osteoblasts are located near bone surfaces in rounded lacunae; older cells are found farther from the surface in flattened lacunae.  They maintain bone matrix.  The death of osteocytes results in bone breakdown, or resorption.  EM of relatively quiescent osteocyte (left) with cytoplasmic processes extend within canaliculi. And formative osteocyte (right) showing Golgi body (G), rER, and collagen fibers (arrows).

EM of relatively quiescent osteocyte (left) with cytoplasmic processes extend within canaliculi. And formative osteocyte (right) showing Golgi body (G), rER, and collagen fibers (arrows).

4- Osteoclasts  Osteoclasts are bone-resorbing cells that lie on bony surfaces in shallow depressions called Hawship'slacunae.  Osteoclasts are large multinucleated cells (~ 50 nuclei), with acidophilic cytoplasm containing abundant lysosomes and mitochondria and well developed Golgi complex; and brush border (Ruffled border) of plasma-membrane facing the bone marrow.  They are derive from the fusion of blood monocyte derivatives and are considered components of the mononuclear phagocyte system.  The cells release acid, collagenase, and other lytic enzymes into the compartments; these break down bone matrix and release minerals, a process called bone resorption

Bone matrix. Bone matrix contains organic components (osteoid), and inorganic components (bone mineral). The organic components constitutes about 50% of bone volume and 25% of bone weight. It is composed of type I collagen fibers and unmineralized ground substance, which is composed of proteins, carbohydrates, and small amounts of proteoglycans and lipids. The inorganic components (bone mineral) makes up about 50% of bone volume and 75% of bone weight. It is composed of calcium and phosphate, with some bicarbonate, citrate, magnesium and potassium and trace amounts of other metals.

Compact Bone 1- Periosteum: covering the long bone, and formed of two layers:- - Outer fibrous layer of collagen fibers. - Inner cellular layer of osteogenic cells and osteoblasts. 2- Endosteum: a cellular layer lining the bone cavities, and formed of osteogenic cells and osteoblasts.

3- Haversian system (osteon):- Bone lamellae are arranged concentrically around the blood vessels. The bone lamellae are formed of osteocytes inside lacunae and canaliculi embedded in calcified matrix. 4- Volkmann`s Canals:- They are transverse canals connecting blood vessels in the Haversian canals to each other and to those in the periosteum and in marrow cavities. 5- External circumferential lamellae. 6- Internal circumferential lamellae. 7- Interstitial lamellae.

Diagrammatic illustration of a section of compact bone showing the arrangement of osteocytes around Haversian canal to form the Osteon.

Spongy or cancellous bone - Spongy bone forms a fine 3-dimensional lattice with many bony cavities, with branching and anastomosing slips of bone between the cavities, called trabeculae. - Spongy bone is found at the core of the epiphyses of mature long bones, at the core of short bones, and between the thick plates, or tables, of the flat bones of the skull, where it is called the diploii.

Macroscopic and microscopic characters of Spongy or Cancellous bone, where multiple bone cavities filled with bone marrow.