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Bone Tissue

Bones are organs containing various types of tissue, such as blood, connective tissue, nerves, and bone tissue.
There are two types of bone tissue: compact and spongy.

Compact Bone Tissue
Compact bone (or cortical bone), forms the hard external layer of all bones
Surrounds the medullary cavity (bone marrow).
Provides protection and strength to bones .

Compact bone tissue consists of units called osteons or Haversianian systems. (Cyindrical structures)
The cylinders aligned along the lines of stress for strength
Each osteon consists of lamellae, layers of compact matrix that surround a central canal (the Haversian canal), which contains the bone's blood vessels and nerve fibers.
Living osteocytes connected by canaliculi which transport blood.

Cancellous bone tissue consists of trabeculae that are arranged as rods or plates with red bone marrow in between.
Spongy bone is  at the ends of long bones - more compressible

Central (Haversian) canals run longitudinally. contain Blood vessels and nerves.
    - around canals are concentric lamella (layers of bone cells & matrix)
    - osteocytes occupy lacunae which are between the lamellae
    - radiating from the lacunea are channels called canaliculi

Types of bone cells
Bone is made up of 4 types  of cells :
  1. Osteoprogenitor cells - Osteogenic cells - the only bone cells that divide - differentiate & develop into osteoblasts which form new bone.
  2. Osteoblast - synthesize and secrete a collagen matrix and calcium salts
  3. Osteocyte -When the area surrounding an osteoblast calcifies, osteoblast becomes trapped and transforms into an osteocyte, the mature type of bone cell.
  4. Osteoclast - breaks down and reabsorbs bone, arise from monocytes and macrophages rather than osteogenic cells ..
There is a continual balance between osteoblasts generating new bone and osteoclasts breaking down bone.

Cells are surrounded by matrix consisting of
25% of water
25% of protein
50% of mineral salts

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