Bones are a part of the skeletal system along with the cartilage and serve as a framework for the body’s tissues. Though bones are hard, they do perform the functions of an organ.
Introduction
A human mind generally visualises bone as a long white cylindrical structure with two bulges at the end. It is indeed true but with slight modifications.
Bones are constantly changing structurally and biologically, and bone remodelling occurs throughout life as a result of the pressures imposed on them.
What is a Bone?
Bone is a rigid living tissue that constitutes the skeletal system of the human body. There are about 300 bones in a baby’s body at the time of birth. Adults have 206 bones, which are formed through the fusion of smaller bones.
Bones are usually immobile structures that are ideally present to provide structural support to the body.
Bones, like other organs, are valuable and perform various functions. They protect important organs, promote breathing, allow mobility, maintain homeostasis and produce cells in the marrow that are vital for survival.
Types of Bones
The size and shape of the bones in the human body vary greatly. Five main types of bones include short, long, irregular, flat and sesamoid bones.
Long Bones
Long bones are those whose length exceeds their width. They are made up of a long shaft with two hefty ends, often known as extremities.
They are mostly compact bones with a lot of spongy bone at the ends and extremities. Thigh, leg, arm, and forearm bones are among the long bones. Long bones support the body's weight and make mobility easier.
The appendicular system consists the majority of long bones. Long bones are present in
- The femur - the body's largest bone
- Lower limbs (tibia, fibula, metatarsals and phalanges) and
- Upper limbs (radius, ulna, humerus, metacarpals and phalanges)
Short Bones
Short bones are usually cube-shaped, with roughly equal vertical and horizontal diameters. Short bones are mostly spongy bones and have a layer of compact bone on top.
Short bones give support and help in a few movements. Short bones are mostly present in the wrist and the ankle.
Short bone examples
- The tarsals in the ankles (talus, calcaneus, navicular, cuboid, lateral cuneiform, intermediate cuneiform and medial cuneiform)
- The carpals in the wrist (scaphoid, lunate, pisiform, triquetral, hamate, capitate, trapezoid and trapezium)
Flat Bones
Flat bones are generally flattened, curved and thin. The cranium has many flat bones.
Flat bones are present in
- The head (frontal, occipital, parietal, nasal, vomer and lacrimal)
- Thoracic cage (sternum and ribs)
- Pelvis (ilium, ischium and pubis).
Internal organs such as the brain, heart and pelvic organs are protected by flat bones.
Flat bones are slightly flattened which makes it easy for them to give protection, similar to a shield. They can also provide broad regions for muscle attachment.
Irregular Bones
These bones don't fall into any of the other categories due to their unique shapes, sizes and structures, thus the name Irregular bones.
Their complicated shape aids in the protection of interior organs. A perfect example would be the spinal cord which is protected by the vertebrae, which are irregular bones in the vertebral column.
Irregular bones in the body include
- The uneven bones of the pelvis (the pubis, ilium and ischium) - Protect organs in the pelvic cavity
- Vertebrae (bones of the vertebral column) - Protects the spinal cord
- Few bones of the skull
Irregular bones are mostly spongy with a thin covering of compact bone on top. The surface patterns and properties of each of the irregular bones are exclusive that help distinguish them from the others.
These bones have holes, smooth facets, depressions, projections, lines and certain marks. Irregular bones generally are passages for nerves and blood vessels. They are also the point of articulation with other bones and attachment points for tendons and ligaments.
Sesamoid Bones
Sesamoid bones are tiny, spherical bones that are lodged in the tendons. The purpose of the sesamoid bones is to protect tendons from wear and tear.
Example of Sesamoid bone
- Patella - Commonly referred to as ‘kneecap’
Functions of Bones
Bone provides the body with shape and stability, while also protecting a few organs. Bones are also a mineral storage site, and the bone marrow serves as a medium for the growth and storage of blood cells.
Support and movement
The human’s structural stability is provided by bones. The ability of humans to maintain an erect posture, walk on two feet and perform tasks impracticable to other animals is due to the development of complex bone structures like the spine.
Protection
The internal organs and other internal systems are protected by the rigid structures of the bone.
Bones like sesamoid bones, facilitate the protection of the tissues by minimising stress and friction. The other bones join together to form advanced structures that surround and protect vital organs like the rib cage, skull and pelvis.
Mineral homeostasis
99 percent of calcium, 85 percent of phosphate and 50 percent of magnesium in the human body are retained by the bones and serve as the major mineral storage region.
Bones are very important for maintaining mineral homeostasis in the blood, with minerals held in the bone being released in response to the body's demands. Hormones such as parathyroid hormone maintain and regulate the body’s mineral levels.
Haemopoiesis
Haemopoietic stem cells in the red bone marrow produce the blood cells.
There is a decrease in erythropoietin, the hormone required for boosting the red blood cell (erythrocyte) formation. As a result, babies are born with only red bone marrow, which is gradually replaced by yellow marrow over time.
The amount of red bone marrow in the body is nearly half by adulthood and it lowers to about 30 % by old age.
Triglyceride storage
Yellow bone marrow is primarily composed of adipose cells that store triglycerides making it the body’s potential energy reserve.
Bone tissue
Bone tissue is the hardened, inflexible connective tissue caused by mineralisation. The skeletal system of vertebrates is made up of bone tissue. The bone matrix and the bone cells make up the bone tissue.
Types of bone tissue
Bone tissues are tissues that render structure and strength to the bones. Bone tissues can be classified into three types.
Compact tissue
Compact tissues are the denser, outer layer of a bone's structure.
Cancellous tissue
Cancellous tissues are the sponge-like layer that lines the insides of bones.
Subchondral tissue
The smooth tissues at the extremities of bones that are covered by cartilage are the subchondral tissues. Adults have cartilage, which is a specialised, tough connective tissue. Most children's bones are formed from this tissue.
Structure of bone tissue
The osseous tissue or bone tissue is of two types, namely, compact bone tissue and spongy bone tissue.
Cortical bone tissue
Cortical bone, generally known as Compact bone, is made up of tightly packed osteons, which are commonly referred to as Haversian systems.
The concentric rings (Lamellae) of the matrix that surround the osteonic canal make up the osteons. The bone cells, osteocytes, are present in the spaces between the matrix and the rings called the lacunae.
Passages through the tough matrix happen via small channels named canaliculi that arise from the lacunae to the osteonic canal.
Compact bone tissues have the Haversian systems which are firmly packed together and produce a solid mass.
In the osteonic canals, the blood arteries are parallel to the bone. These arteries connect to the vessels on the bone’s surface through the penetrating canals.
Cancellous bone tissue
Cancellous bone tissue is also known as spongy bone tissue. Unlike compact bone tissues, cancellous bone tissues are less dense and lighter.
Cancellous bone tissues have plates and bars of bones that are close to the cavities that hold the red bone marrow. The canaliculi get their blood supply from neighbouring cavities rather than from the central Haversian canal.
The plates may appear to be haphazardly arranged, yet they are ordered to give maximum strength. Spongy bone tissue plates follow stress lines and can realign if the stress direction changes.
Different types of Bone cells
Bone tissues have four different types of bone cells.
Osteoblast
Osteoblasts are cuboidal cells that are found along the bone surface and make up 4–6% of all resident bone cells. They are well known for their role in bone formation.
Osteoblasts are bone cells that are important for producing bone matrix proteins and minerals during early embryonic bone formation, as well as controlling bone growth and mineralization throughout life. These cells are located in places with high metabolism where new bone formation is occurring.
Osteoclast
Osteoclasts are multinucleated cells with specialised functions in bone growth and regeneration.
These cells absorb and destroy bone, allowing new bone to grow, while maintaining bone strength.
Functions of osteoclasts are not just restricted to bone resorption. They release cytokines, function as immune cells in inflammatory bone disorders and regulate haematopoietic stem origination from bone marrow
The activity of osteoclasts is inhibited by many of the medications that are used to treat common bone diseases.
Osteocyte
Osteocytes are 'entrapped' osteoblasts that reside within the bone itself. Osteocytes are the longest-living bone cells, accounting for 90–95 percent of bone tissue cells, compared to only 5% for osteoclasts and osteoblasts.
When osteoblasts become buried in the mineral matrix of bone and develop separate characteristics, osteocytes arise.
It also helps with bone remodelling by sending signals to other osteocytes in response to minor bone deformations generated by muscle action.
Haematopoietic
HPCs (haematopoietic progenitor cells) and HSCs (haematopoietic stem cells) are cells found in the blood and bone marrow. Blood cells including the white blood cells, red blood cells and platelets originate from an immature cell.
HPCs have the ability to produce mature blood cells such as red blood cells (which carry oxygen), platelets (which help stop bleeding) and white blood cells (the cells that fight infections).
Within the bone marrow stem cell, haematopoietic stem cells (HSCs) are protected in a metabolically inactive condition.
Blood supply
The well vascularised bone marrow accounts for 10 - 20 % of cardiac output. All major functions including the nutrient and oxygen transport, repair and homeostasis require blood vessels in the bones.
The long bones get blood supply from the nutrient artery as well as the periosteal, metaphyseal and epiphyseal arteries. Alongside each artery, nerve fibres branch into the marrow cavities.
Arteries are the principal source of blood and nutrients for long bones. They enter through the nutrition foramen and divide into ascending and descending branches
The metaphyseal and epiphyseal arteries, which emerge from the arteries of the associated joint, supply the ends of long bones.
When there is disruption in the blood flow to the bone, the bone tissues die. This process is known as osteonecrosis. A common example occurs after a femoral neck fracture, which affects the blood flow to the femoral head and causes necrosis of the bone tissue. The structure of the femoral head then collapses, resulting in discomfort and dysfunction.
Bone development
The terms osteogenesis and ossification are frequently used interchangeably to describe the formation of bones. During the first few weeks after conception, the skeleton begins to form.
The skeletal pattern is created in cartilage and connective tissue membranes by the end of the eighth week following conception, and ossification occurs.
The bones of the adults continue to develop throughout their lives. Even after reaching adult stature, bone formation continues for fracture repair and remodelling to accommodate changing lifestyles.
Ossification can be of two forms - Intramembranous and Endochondral.
Growth
A process similar to endochondral ossification causes bones to grow in length at the epiphyseal plate.
Mitosis occurs to build the cartilage in the epiphyseal plate. The chondrocytes age and eventually deteriorate next to the diaphysis. Bones are formed when the osteoblasts enter the matrix and ossify it.
The process is continuous throughout youth and adolescence and ceases when the cartilage formation slows and stops. In the early twenties, the cartilage growth stops and the epiphyseal plate ossifies completely leaving only a thin epiphyseal line. The bones will not increase in length after this.
The anterior pituitary gland secretes growth hormone, while the ovaries and testes secrete sex hormones, which regulate bone growth.
Even when bone growth ceases during adulthood, the thickening process continues to occur in response to stress from increasing muscular activity or weight gain. Appositional growth is the term for increasing bone diameter.
In the periosteum, osteoblasts produce compact bone around the exterior bone surface. Simultaneously, osteoclasts in the endosteum break down bone around the medullary cavity on the internal surface of the bone.
These two processes work together to increase the diameter of the bone while also preventing it from becoming too heavy and thick.
Conclusion
Bone is a major part of the skeletal system. They facilitate body movements while also maintaining the structure of the body.
There are four different types of bones - long bones, short bones, irregular bones and sesamoid bones. The bones along with the bone tissue make up the framework of the body.
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