By Betty Kennedy,2014-11-28 13:00
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    By the end of this tutorial students should know:

    1. Basic classification of bone, based on its gross and

    microscopic structure.

    2. Haversian system and its structure

    3. Difference between spongy and compact bone, and

    woven and lamellar bone.

    4. Types of bone cells.



     Bone is a variety of connective tissue characterized by

    deposition of mainly salts of calcium phosphate with

    collagenous fibers and a large amount of water in its matrix.

     The three basic components of connective tissue i.e., cells,

    fibers and ground substance form the organic part of bone.

     Calcium is the inorganic part of ground substance.

     Bone is hard, its hardness is due to the inorganic minerals.

    Bone serves as a storage site for calcium and phosphate.


    There are three kinds of mature bone:

    ; Compact bone: also called dense bone or cortical


    ; Spongy bone: also called cancellous bone,

    trabecular bone or medullary bone.

    ; Woven Bone.


    A long bone e.g.., the femur consists of:

     Centre piece, the shaft (diaphysis) and a thickened head

    (epiphysis) at each end.

     Bone is covered with a tough, strong membrane, the

    periosteum which is richly supplied with blood vessels. Beneath the periosteum is a layer of compact bone which is

    thicker in the shaft than in the two heads.

     The shaft encloses a hollow, the marrow cavity, which is

    lined with a thin soft membrane known as the endosteum.

     The marrow cavity contains a soft tissue richly supplied with

    fat cells and blood corpuscles, the yellow marrow.

     The epiphysis of a long bone consist of spongy (or cancellous)

    bone covered with a thin layer of compact bone. This is made up of bony bars (or trabeculae) arranged in such

    a way that they are able to resist any force which is applied

    upon the bone.

     Between the bars are many tiny cavities filled with a red

    marrow which contains numerous red blood corpuscles in

    different stages of development



     The matrix is the major constituent of bone, surrounding the


     It has inorganic and organic parts.


     The inorganic matter is mainly crystalline mineral salts and

    calcium, present in the form of Hydroxyapatite.

     The matrix is initially laid down as unmineralized osteoid

    (manufactured by osteoblasts).

     Mineralisation involves osteoblasts secreting Vesicles.


     The organic part of matrix is mainly Type I collagen.

     Other factors present include:

glycosaminoglycains, osteocalcin and osteonectin.

    One of the main things that distinguishes the matrix of a bone from

    that of another cell is that the matrix in bone is hard.


     Under the microscope dense, compact bone shows a definite

    and a characteristic pattern of arrangement.

     The ground substance of bone is arranged in concentrated

    layers (lamellae) round the small canals.

     The canals called Haversian canals run parallel to the long

    axis (shaft) of the bone.

     Haversian canals of different osteons are interconnected with

    one another and the marrow cavity via Volkmann's canals

    and contain a blood vessel, a nerve and a lymph vessel.

     Volkmann's canals provide major route of blood vessels

    from marrow cavity to the Haversian canals.

     Each Haversian canal is surrounded by concentric layers of

    bone matrix (called lamallae) and concentric rings of bone

    forming cells (osteoblasts).

     Once bone cells have been completely surrounded by the

    hard bone matrix, they are called osteocytes.

     The osteocytes are embedded in fluid-filled cavities within

    the concentric lamellae.

     These cavities are known as lacunae and occur at regular

    intervals in these concentric layers of bone tissue. The lacunae are connected to one another and to the

    Haversian canals by a system of interconnecting canals

    known as canaliculi.

     Each Haversian canal, its concentric lamellae, lacunae with

    osteocytes and canaliculi forms a long cylinder and is called

    a Haversian system or osteon, the structural & functional

    unit of bones.


     Compact bone and spongy bone are found in specific


     In long bones, most of the thickness of the diaphysis is made

    of compact bone, with a small amount of spongy bone facing

    the marrow cavity.

     The ends (epiphyses) of long bones, however, consist

    mostly of spongy bone covered with a shell of compact bone. The flat bones of the skull have a middle layer of spongy

    bone sandwiched between two relatively thick layers of

    compact bone.


     The marrow surface of compact bone, and the spicules of

    spongy bone, are lined by an (often single) layer of cells

    called the endosteum (endosteal cells).

     Like the periosteal cells, these endosteal cells are also

    osteoprogenitor cells, capable of becoming osteoblasts. (The

    two names periosteal cells and endosteal cells refer to their

    different locations, both function as osteoprogenitor cells).


     Spongy bone is composed of bone spicules, also called

    trabeculae, of varying shapes and sizes.

     The spaces between the spicules are filled with marrow. The composition of spongy bone (cells and matrix) is the

    same as that of compact bone.

     In spongy bone, however, the lamellae of collagen are not

    arranged concentrically around a central canal, but run

    parallel to one another.

     Osteocytes sit in lacunae between lamellae.


     Bone can also be either woven or lamellar (layered).

     Woven bone is weak, with a small number of randomly

    oriented collagen fibers, during periods of repair or


     After a break, woven bone quickly forms and is

    gradually replaced by slow-growing lamellar bone on

    pre-existing calcified Hyaline Cartilage through a

    process known as "bony substitution”.

     Non-lamellar form of bone is normally found in rapidly

    growing areas of embryonic or developing bone, and in

    healing fractures.

     Most woven bone is replaced by lamellar bone during


     The process of bone remodeling occurs continually in

    the embryo, fetus and the adult.


     Osteoblasts secrete the collagen fibres and ground substance (matrix) of bone and are responsible for the calcification of the matrix.

     When an osteoblast is completely surrounded by matrix, it is called an osteocyte.

     Osteocytes are responsible for maintaining the matrix, and can both secrete and resorb matrix.

     Another type of cell called the osteoclast is responsible

    for the resorption of bone.

     These large, multinucleated cells arise from monocytes.

    They help to break down bone matrix.

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