Bone Anatomy – Functions, and Types

Bones are essential organs of the skeletal system in vertebrates, providing structural support and protection to internal organs. They also facilitate movement, blood cell production, and mineral storage. Bones in the human body are made of bone tissue, cartilage, marrow, blood vessels, and nerves.

Each bone has a specific function and shape. Bone growth and remodeling occur throughout life. These processes are regulated by ossification and bone resorption. Let us discuss bones, their structure, function, and more in detail.

What is Bone?

The bone is a rigid structure formed of an organic part i.e. connective tissue made up of collagen fibrils and an inorganic part i.e. hydroxyapatite and a very small amount of water. The bone has different structural parts such as compact bone tissue, spongy bone tissue, bone marrow, epiphyseal line, and network of blood vessels.

Bone Definition: A bone is a rigid, mineralized organ that forms the major part of the vertebrate skeletal system.

Types of Bone Cell

Morphology and Structure of Bones in Human Body

Typically, a bone has the following structure:

• Compact Bone Tissue: It is the outermost part of bones and is very hard and dense.
• Spongy Bone Tissue: It is a sponge-like structure present inside the bone and is also called cancellous tissue or trabeculae. It forms a honeycomb-like structure. Thus it is soft and less dense as compared to compact bone tissue.
• Epiphyseal Line: It is present in long bones of the skeleton. It is a flat bony structure within the spongy bone tissue that supports the growing bone. It also provides strength to the weak part of growing bone.
• Blood Vessels: There is a network of blood vessels surrounding each bone of the body. It provides mainly oxygen and nutrients to different parts of bones.
• Bone Marrow: It forms the central part of bones. It is made up of soft and jelly-like living cells. These cells are also called stem cells and are carrying out a very important function of forming blood cells. Bone marrow is present in almost all bones where there are spongy bones present.

Morphology of bones vary depending on their location and function. They show diverse shapes, including long, short, flat, irregular, and sesamoid. For example:

• Long bones, like the femur, have a shaft and expanded ends, while short bones, such as those in the wrist, are cuboidal.
• Irregular bones, such as vertebrae, have complex shapes.
• Flat bones apprear dorsoventrally flat as in skull.

Cells of the Bone

Bone cells are made up of four types of cells:

Osteoblasts

They form 4-6 % of are present on a bone surface. They have cuboidal shape. They are considered to have originated from mesenchymal cells i.e. pluripotent bone marrow cells. Osteoblasts form a new bone structure and also facilitate calcium and collagen deposition at the new bone surface. The cell to cell communication by association of osteoblasts to messenger system and arrangement at the bone surface area has a role in bone remodelling.

Osteoclasts

They are large multinucleated phagocytic cells. Osteoclasts are formed in hematopoietic stem cells and migrate to specific skeletal sites. They have a role in bone resorption in other words dissolving the bone which results in bone remodelling. Abnormal increase in osteoclasts lead to some bone diseases such as osteoporosis decreasing bone density and increasing chances of bone fracture.

Osteocytes

Osteocytes are entrapped osteoblasts. There are about 25,000 osteocytes per mm3 of bone forming 95% of total bone cell volume. They have an important function in the bone remodelling cycle. By sensing the pressure and cracks in the bone structure, they direct osteoclasts to dissolve the bone thus filling the gaps within the cracks.

Osteogenic Cell

Osteogenic cells, also known as osteoprogenitor cells, are a type of mesenchymal stem cell found in bone tissue. Osteogenic cells are located in the inner layer of the periosteum and the endosteum. These cells are responsible for the formation of new bone tissue through a process called osteogenesis.

Types of Bones

Types of Bones

There are five major types of bones as below:

• Long Bones: Their length is more than their width. The long bones in the human body are present in legs and arms. These bones are femur, tibia, fibula of legs and humerus, radius and ulna of arms.
• Short Bones: They are as long as they are wide. These bones are cuboid shaped. Examples: Bones of wrist and ankle.
• Flat Bones: The name suggests the shape and surface of bones is flat. They are typical bones with the compact layer, spongy layer and bone marrow. Examples: Ribs, skull bones, sternum, and shoulder bones.
• Irregular Bones: Irregular bones do not possess any specific shape and thus are called irregular. They play a role in giving protection to internal delicate structures such as Vertebrae giving protection to the spine.
• Sesamoid Bones: This type of bone is small and embedded in muscles or tendons. This type of bone is present at the joints. Example: Patella bone of knee.

Functions of Bones

Bones perform various metabolic, synthetic, and mechanical functions essential for overall health and body function. Functions of bones in human body are explained below:

Metabolic Functions of Bones

• Mineral Homeostasis: Bones store essential minerals such as calcium and phosphorus, regulating their levels in the bloodstream. When blood levels are low, bones release minerals to maintain balance.
• Hematopoiesis: Bone marrow, located within certain bones, is the site of hematopoiesis, the process of blood cell formation. Red bone marrow produces red blood cells, white blood cells, and platelets, essential for oxygen transport, immune function, and blood clotting.

Synthetic Functions of Bones

• Bone Formation (Osteogenesis): Osteoblasts, specialized bone-forming cells, synthesize and deposit new bone tissue during growth, repair, and remodeling processes. They secrete collagen and other proteins that form the organic matrix of bone, which is later mineralized with calcium and phosphorus.
• Matrix Remodeling: Osteoblasts and osteoclasts work together to remodel bone tissue. Osteoblasts deposit new bone matrix, while osteoclasts remove old or damaged bone tissue. This dynamic process maintains bone strength and integrity, repairs microdamage, and adapts bone structure to mechanical demands.

Mechanical Functions of Bones

• Support and Structure: Bones provide structural support for the body, serving as a framework for muscles, tissues, and organs. They give the body its shape and rigidity, supporting posture and movement.
• Protection: Bones protect vital organs such as the brain, heart, and lungs from injury or trauma. For example, the skull protects the brain, and the rib cage shields the heart and lungs.
• Facilitating Movement: Bones in coordination with joints and muscles, enable movement and locomotion. Skeletal muscles attach to bones via tendons, allowing for coordinated movement and mobility.

What is Bone Resorption and Bone Remodelling

Bone architecture constantly changes based on age, injury, mechanical damages caused due to strain and stress on the body. This continuous process of bone formation and replacing damaged parts by adding new cells is called bone remodelling. Rate and speed of bone remodelling is faster in children of young age and in adults. It slows down with increasing age in older individuals.

Bone Resorption

Bone resorption is a physiological process in which osteoclasts are formed to break down and remove old or damaged bone tissue. Osteoclasts create an acidic environment and secrete enzymes to dissolve the mineral and organic components of bone, releasing minerals like calcium and phosphorus into the bloodstream. This process is tightly regulated by hormones, growth factors, and mechanical stimuli to maintain skeletal health and mineral homeostasis. This process results in formation of small cavities called resorption pits or lacunae.

Conclusion – Bone Anatomy – Functions and Types 

The bone is made up of various cell types viz. Osteoblasts, osteoclasts, osteocytes and bone lining cells. Each type of bone cell has a specific role in bone formation, damage repair, allowing cell to cell communication, and Calcium mineral deposition. Bones are constantly changing. This process is called Bone remodelling. The factors influencing these changes are age, sex, pressure, stress and strain on bones. The physiology of the body also contributes to changes in bone architecture. The key role in bone remodelling is played by bone cells, Vitamin D, and parathyroid hormones.

FAQs on Bone Anatomy – Functions and Types 

How many Bones are Present in the Human Body?

There are a total 206 bones present in the human body.

Which Bones Predominantly Contain Red Bone Marrow?

Bone marrow is present mainly in flat bones such as hip, pelvis bone, shoulder bones etc. and long bones which contain spongy part such as, femur, humerus, tibia etc.

Which Bone Cells are Involved in Bone Resorption?

Osteoclasts are responsible for bone resorption. It is basically dissolving bone as a part of the bone remodelling process.

How is Bone Formed?

Bone is formed by the process of ossification. The process is a part of bone remodelling. Osteoblast cells take part in this process and subsequently Calcium mineral is deposited on growing bones.

What is Osteoporosis?

Osteoporosis is a common disease of bone. It is caused due to decrease in bone density leading to brittleness of bones. This results in a decrease in bone strength and frequent fracture of bones.

What are Bones Made of?

Bones are primarily composed of bone tissue which consists of collagen fibers and mineral deposits such as calcium and phosphorus. They also contain cartilage, marrow, blood vessels, and nerves.

What is Bone Marrow?

Bone marrow is a soft, jelly-like tissue found inside bones which is responsible for producing blood cells, including red blood cells, white blood cells, and platelets.