Compact (Cortical) Bone

Compact Bone Definition

It is a bone is one of two kinds of bone tissue that can be found in the body of a human being. It is also known as Cortical Bone.

Compact Bone Function

Picture 1 – Compact (Cortical) Bone
Source – wikimedia

Some of the main functions of Compact Bone are :

Protection of Cancellous Bone

The compact type of bone wraps around and protects the only other type of bone tissue known as the Cancellous Bone.

Supports weight

Compact Bone supports the weight of the human skeleton. The Cancellous Bone is soft and spongy in nature and would have been unable to support the human skeletal structure without the back up of the Compact Bone.

Provides rigidity and strength

This bone tissue is the main component of the long bones of the leg, the arm and other bones. It provides more strength and rigidness to the bones when needed.

Contains blood supply

Mature compact bone is structurally layered or lamellar. It is penetrated by a detailed system of Haversian systems and interlinking vascular canals that contain blood to be supplied to the Osteocytes. The bone is set up around these canals in concentrical layers, thus constituting structural divisions that are known as osteons.

Helps in new bone formation

Immature compact bones have a woven structure and do not contain Osteons. It forms a collagen fibre framework all around and is finally replaced by mature bone in forming new bones that make the osteons. It is also involved in bone resorption remodelling.

Disposal of waste

These bone tissues, along with the aid of Haversian and Osteons, dispose of waste and deposit minerals that help in building strength and providing solid growth. Cancellous and cortical bone tissue constitutes the overall bony component of the human skeletal system.

Supports the skull

Cortical bones constitute the skull which is one of the most important parts of the human body. Presence of this tissue provides humans with the ability to bear trauma to the skull and balance the weight.

Microscopic Structure of Compact Bone

It is a dense bone where the bony matrix is soundly filled with inorganic salts and organic ground substance. Only tiny spaces (lacunae) are left which that contain the bone cells or Osteocytes. Compact bone constitutes as high as 80 percent of the skeletal structure of the human body. The remaining part is filled with Cancellous Bone that has an appearance similar to a sponge with many large spaces. It is found in the Medullary cavity (marrow space) of a bone. Both Compact and Cancellous types can be found in most human bones. Bone tissues are distinguished as spongy or compact depending on the proportions of soft and mineralized tissues.

Compact bone is mostly detected in long bones that can be found in the, legs, arms, toes and fingers. These bones are greater in length than width and need added support and strength that can the compact bone tissue can provide. Compared to long bones, short ones are approximately cube-shaped and consist of only a lean, compact bony tissue layer. Short bones can be found in the ankle and wrist.

Compact Bone Disorders

Some common disorders affecting the cortical bone tissue include

• Osteoporosis
• Osteomyelitis infection
• Congenital musculoskeletal defects which affect muscle and bone function

This bone tissue requires regular calcium intake along with other hormones and minerals through diet. Vitamin K and Vitamin D are also very important for the development. Lack of any of such substances can lead to changes in the structure of the bone and consequently affect the function of bones.

Certain amount of demineralization naturally takes place during the aging process. However, severe loss of calcium loss results in weakening and thinning of bones. Due to this, bones can become susceptible to fractures.

Compact Bone Structural Composition

This type of bone tissue is made up of osteons, or cells that are composed of frail layers of the membrane called lammelae. This membrane protects the blood and nerves of the internal core of the compact bone. When osteons die, they change into lammelae and forms obstruction within osteon cells that can be found inside the matrix of the compact bone. The density and strength of compact bones develops as a result of this. The osteons appear as the rings of tree and vary in appearance in different animals. This is reason why an examination of the physical composition of osteon remnants can help identify their age, gender or taxonomic category (type of species). Haversian canal maintains the source of blood and nerves that are located in the inner centre of the cortical bone tissue and is shielded by a very compact outer shell.

The cortical bone cells appear to be closely clustered together into a compact mass. This kind of bone tissue is not entirely solid even though they are close together. There are tiny canals running through the bone that allow blood vessels to get through it. Spongy bone, which resembles as sponge, contains large open spaces.

Compact Bone Vs Spongy Bone

There are very few spaces and gaps within the Compact bone tissue. Its porosity is very little. Cortical Bone tissue constitutes approximately 80% of this bone. It is due to its low porosity that it is also known as dense bone.

Cancellous or spongy tissues can be found in the interior position of the bone. This tissue comprises of an almost porous structural network that has shapes of plates and rods. Only 20% of bone mass is made up of Spongy tissues though their surface area is ten times greater than that of compact bone tissues.

Spongy bone tissues are surrounded by blood while compact bone tissues are bordered by marrow. Bone marrow appears as a yellowish fluid and can be found in the cavities within the compact bone tissues. It is in the spongy bone tissues where the red bone marrow can be found.

Cortical Bone tissues are strong and dense and may be found on the outer part of the bone. Spongy tissues are porous and may be found on the inner part of a bone. Blood is made on the spongy part of the bone.

Haversian system can be found in cortical bone tissues but is missing in spongy bone tissues.

References:
http://www.britannica.com/EBchecked/topic/129490/compact-bone

http://www.wisegeek.com/what-is-compact-bone.htm

http://www.innerbody.com/image_skel09/skel61.html

http://www.daviddarling.info/encyclopedia/B/bone.html#compact