The Structure of Bones and Bone Tissue

Bones are hard, tough connective tissues that make up the human skeleton. They are composed of an outer shell of compact bone, with spongy bone in the interior. Compact bones are highly organized and made up of tubular units called osteones, also known as Haversian systems. In the cross section of an osteon, bone cells, called osteocytes, are housed in lacunae, which are tiny chambers arranged in concentric circles around a central canal. The matrix of an osteon is laid down in concentric rings called lamellae, which are hard and contain calcium phosphate and collagen fibers. Tiny canals called canaliculi run through the matrix, connecting the lacunae with each other and with the central canal.

Spongy bones, on the other hand, contain numerous minute spikes of bone called trabeculae, which give a lattice-like appearance. They are lighter than compact bone and highly vascular. The spaces of spongy bone are often filled with red bone marrow, and the osteocytes are irregularly placed within the trabeculae.

 

Bone Growth and Repair

Several different types of cells are involved in bone growth and repair. Osteoprogenitor cells (stem cells) are present in the inner portion and central canal of the compact bone. Osteoblasts are bone-forming cells that secrete matrix, while osteocytes are mature bone cells derived from osteoblasts. Once the osteoblasts are surrounded by the matrix, they become osteocytes. Osteoclasts, thought to be derived from monocytes, perform bone resorption, which means they break down bone and deposit calcium and phosphate in the blood. They are important for growth and repair of bone.

 

Cartilage: The Flexible Connective Tissue

Cartilage is a flexible connective tissue that is not as strong as bone. It is present at the end of joints and supports the flexible portion of the nose and external ears. It is more flexible than bone because the matrix is gel-like and contains many collagenous and elastic fibers. The living cells of cartilage are called chondrocytes, which secrete matrix. However, cartilage has no blood vessels, and therefore, injured cartilage is slow to heal.

 

Types of Cartilage

There are three types of cartilage, which differ according to the type and arrangement of fibers in the matrix:

Hyaline Cartilage: This type of cartilage is firm and somewhat flexible. The matrix has an abundance of collagenous fibers. It is found at the ends of long bones and in the nose, larynx, and trachea.

Fibrocartilage: The matrix of this type of cartilage contains wide rows of thick collagenous fibers. It is found where support is of prime importance, such as in the disks located between the vertebrae and the cartilage of the knee.

Elastic Cartilage: This type of cartilage is more flexible than hyaline cartilage because the matrix contains mostly elastic fibers. It is found in the ear flaps and epiglottis.