Bone – Definition
The bone is a rigid body tissue that makes up our body skeleton. The bone is a connective tissue that is made up of different types of cells. Internally, it has a honeycomb-like matrix that gives rigidity to bones. The primary function of the bones is to provide structural support to the body and enable mobility. They also produce RBCs and WBCs and store minerals inside them.
Bone Structure and Morphology
The bone is made up of varied composition, that is about 30% flexible matrix and 70% bound minerals that are embedded with specialised bone cells. This unique constitution of the bone allows it to be strong and hard while being lightweight. The bone matrix is composed of collagen (90-95%) and ground substances. The collagen fibres provide elasticity and resistance to the structure. The matrix also contains calcium phosphate that hardens the bone structure.
The bones have an external layer called the cortex. It is a hard exterior that gives the white and smooth appearance to the bones. It forms around 80% of the total mass of the human skeleton. The cortical surface of the bone is covered by the periosteum on the outside and by endosteum on the inside. The interior of the bone is filled with a spongy tissue that is referred to as cancellous bone or trabeculae. It is an open porous network of intersecting plates and spicules that gives it a higher surface area to volume ratio. It is a vascular tissue that often contains red bone marrow and is responsible for hematopoiesis.
Bone marrow or myeloid tissue is found in the cancellous tissue in newborns that produces red blood cells. As the child grows, the red bone marrow is converted to adipose tissue, that is a fatty substance.
Read more: Bones in Your Body – An Overview
Cells of the Bone
The bone consists of four types of cells which have different functions. Let us look at them briefly.
Osteoblasts
Osteoblasts are large cuboidal cells that form about 4-6% of the total bone cells. These cells synthesise and mineralise bone during bone synthesis and remodelling. They form a closely packed sheet-like surface on the bone. They are formed by the differentiation of osteogenic cells. It produces many cellular products such as alkaline phosphatase, growth factors, collagenase and collagen fibres.
The osteoblasts get surrounded by the bone matrix and cellular products and are then referred to as osteocytes.
Osteocyte
Osteocytes are oblate shaped cells that are derived from osteogenic cells or osteoblasts. They constitute almost 95% of the total bone cells. They are located in small spaces in the bone matrix called lacunae. The cytoplasmic processes of the osteocytes extend towards other osteocytes in small channels called canaliculi. Through these channels, the nutrients and waste products are exchanged that helps in maintaining the viability of osteocytes.
They are the most abundant and long living cells of the bone. They also take part in bone deposition, resorption and remodelling.
Osteoclast
Osteoclasts are large multinucleated cells that break down bone tissues. Bones are dynamic tissues that undergo constant remodelling to put up with the stress and requirements of our body. Osteoclasts are phagocytic cells that are derived from the macrophage-monocyte cell lineage.
The major function of these cells is bone resorption. Bone resorption is the process of breaking down of bone tissues by the osteoclasts to release minerals such as calcium into blood circulation.
Osteoclasts achieve the destruction of bone tissues by secreting certain enzymes such as acid phosphatase. This enzyme is capable of digesting collagen as well as calcium and phosphorus. The bone tissues are first broken down into fragments that are later engulfed by the osteoclasts and digested within the cytoplasmic vacuoles. The minerals released from the digestion are released into the bloodstream.
Osteogenic Cells
Osteogenic cells or osteoprogenitor cells are stem cells present in the bone. These are precursor cells that give rise to specialised cells, viz., osteocytes and osteoblasts. These cells reside in the bone marrow.
Osteogenic cells are proliferative cells that are derived from the mesenchymal stem cells. Morphologically, they appear like a flattened, spindle-shaped cell.
Types of Bone
There are five types of bones found in our body. Let us discuss them one by one.
Long Bones
Long bones are bones that are longer than they are wide. The mid section of the long bone is called diaphysis that is predominantly made up of cortical bone and also contains bone marrow and adipose tissue. The diaphysis flares and forms an internal cancellous structure called metaphysis. The epiphysis is the rounded end of the long bones (on both sides) that has an internal cancellous structure.
Examples of the long bones in humans include the femur, tibia, and fibula of the legs; humerus, radius and ulna of the arms; clavicle (collar bone), metatarsals and metacarpals of the feet and hand respectively.
Short Bones
Short bones are the ones that are as long as they are wide. They are cube shaped bones that have a thin cortical layer and a thick spongy interior. They provide stability and support to our body. Examples of short bones include tarsals and carpals in the foot and hand, respectively.
Flat Bones
Flat bones are thin and curved bones that are composed of spongy cancellous tissue sandwiched between two thin layers of cortical bone. The main function of flat bones is to provide a broader surface area for the attachment of muscles. They usually form broad, flat plates as in the sternum, cranium (skull), rib cage and ilium (pelvis).
Sesamoid Bones
Sesamoid bones are the bones that are embedded in the tendons or muscles. The patella of the knee and pisiform of the wrist are two examples of sesamoid bones.
Irregular Bones
Irregular bones, as the name suggests, do not fit into any of the bone types and form peculiar bony shapes in the human endoskeleton. They are composed of cancellous tissue that is enclosed between a thin layer of cortical bone. Examples of the irregular bones include hyoid, sacrum, coccyx, maxilla, mandible, etc.
Read more: Functions of Irregular Bones
Functions of the Bone
Mechanical Functions
- The bones provide a framework for the human body. They keep the body supported and form attachment points for ligaments, tendons, joints and skeletal muscles.
- They protect the internal organs such as the skull that protects the brain, and ribs that protect the heart and lungs.
- The three ear bones namely malleus, incus, stapes help in the process of hearing by sound transduction.
Synthetic Functions
- The bone consists of bone marrow that carries out the process of hematopoiesis and produces red blood cells, white blood cells and platelets.
- The bone marrow is also the site where defective red blood cells are destroyed.
Metabolic Functions
- The bone is a mineral reservoir for the storage of calcium and phosphate.
- The adipose tissue in the bone marrow serves as a site for storage of fatty acids.
- Bone releases alkaline salts into the bloodstream that maintain an acid-base balance.
- The bones absorb heavy metals and other foregin elements from the tissue to reduce their negative effect and later excretes them.
- It releases the enzyme osteocalcin that regulates blood sugar levels and fat deposition.
- Bone resorption – the process of release of calcium in the bloodstream is a significant bone function that maintains the calcium balance in our body.
Bone Remodelling and Resorption
The process of bone formation is called ossification. The process of continuous creation and replacement of bone cells is known as bone remodelling. Osteoblasts and osteoclasts accomplish the process of remodelling by a cascade of signals. The remodelling process is vigorous till the age of 35 after which it starts to decrease. Bone remodelling is important for repair of damaged bones, maintenance of calcium homeostasis and shaping the skeleton during growth years.
The process of remodelling is achieved through bone resorption. Bone resorption is the breakdown of osteoclasts to release the minerals such as calcium from the bone tissue to the blood. Bone formation is higher than resorption during childhood, but resorption exceeds the process of formation as ageing begins.
That is all about bones. Keep visiting BYJU’S Biology for more interesting topics.
Also Read:
- Difference between Compact and Spongy Bones
- Difference between Male and Female Bones
- What Are the 22 Bones of the Skull?
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