Structure And Functions Of Bone tissue

Introduction

Bone is a skeletal tissue assist in providing support, locomotion, and the protection to many vital organs. The human body is distinctively special as we are blessed with a wonderful network of hard tissues-bones, without which we shouldn't have been upright, tall and well-built.  The clinical management of bones itself is an independent specialization; which speaks the degree of its importance. The study of structure and functions of bones is known as osteology while the orthopaedics is a specialization that deals with the medical and surgical conditions of bones.

 

 

Relationship between bones and the body

Bones, a type of connective tissues help in connecting many structures in our body. Similarly, blood is a  liquid connective tissue, has the ability to connect the entire body. A ligament is the fibrous connective tissue help to join bones with other bones while tendon connects bone s to muscles. When bones join together, they form the skeletal system, which is made up of different types of bones; long, short, small, and flat bones. These bones provide the framework that helps in the structural and functional integrity of the surrounding muscles, organs, and tissues. 2 or more bones join together to form joints. Joints ensure the flexibility required for the body. It is essential to keep up the health of the bones always; a disease or injury to a bone requires a very long time to heal. Furthermore, bones are linked with many other body systems so they impact the health and wellbeing of the entire body. For instance, a healthy bone is essential for healthy blood cells. This is because blood cells are produced with the help of bone marrow found in the long bones. Therefore, when bones are affected by a disease condition; cardiovascular and haematological systems are most likely to be damaged. Bones work with muscles and joints to hold our body parts together during locomotion, in this way, the musculoskeletal system is formed, which is essential to maintain the shape of the body. The musculoskeletal system protects delicate internal organs such as the brain, heart and lungs. Unhealthy bones influence the overall health of the body as they can cause substantial alterations in the biochemical values necessary for good health. Some of the biochemical changes caused by bone pathology are fluctuations in the levels of thyroid, plasma calcium, vitamin D, liver enzymes including SGPT and SGOT. To get into the depth of the subject orthopaedics is beyond the scope of this discussion so, let's focus on the basic concepts of bones: formation, structure, functions, classification, and some common conditions that affect human bones.

 

 

Formation Of Bone tissue

Formation of bone is scientifically termed as osteogenesis or ossification.  The process requires assistance from vitamin D, calcium, proteins and hormones such as pituitary and growth hormones. The formation and growth of the baby`s bones start as early as when the mother is in her first trimester of pregnancy. After the birth of the baby, bones continue to grow up to 25 years, when the growth plates are permanently closed. Growth plates are found at the epiphyseal ends. Ossification is the process by which the bone matrix (collagen fibres and ground substance) is formed. During the process, the minerals such as calcium, and phosphorus along with the salts get deposited on the collagen fibres. The collagen fibres give tensile strength necessary for the bone while the calcium provides compressional strength that adds quality. 

 

 

 

Gross Anatomy of a typical bone

Bones contain 2 types of tissues, cancellous (trabecular) and cortical (compact) tissues. Cancellous tissues form the mid-portion of most of the long bones while the rest of the bone is made up of compact tissues. Bone is anatomically divided into epiphysis and diaphysis along its length. Epiphysis refers to the 2 terminal ends of long bones while the diaphysis makes up the shaft, which is the middle part. The diaphysis primarily comprised of the compact tissues, which is otherwise called the cortical bone tissues. Epiphyseal ends are made up of cancellous bone which gives a cushion to join with the terminal end of other bone to form a joint. Furthermore, the junction between the diaphysis and epiphyses is made up of growth plates, which are at least 2 in number, makes up the epiphyseal ends. The epiphyseal plates are responsible for the verticle growth of children by enabling the longitudinal expansion of the long bones. Growth plates cease after  25 years of age, making it impossible for the bones to grow further. The innermost hollow space in a long bone is the medullary cavity, which is responsible for the production of blood.

 

 

 

 

 

Microanatomy of bones

Outer covering

The outermost thin covering layer of the bone is the periosteum, a membranous layer comprises of connective tissue that supports bone health and development. The periosteum is further divided into 2 layers, the outer fibrous periosteum, and the inner Endosteum. The outer layer allows for attachment of muscle tissue with the bone in addition to its ability to provide pathways for blood and lymphatic tissues. Endosteum is a thin, vascular membrane covering the marrow cavity of long bones.

 

Bone matrix

A typical bone has 2 components, non-cellular and the cellular components. The non-cellular portion comprises of the mixture of protein, matrix and mineral deposits,  coated along the length and breadth of the bone. Conversely, the cellular component is made up of osteoblasts, osteocytes, and osteoclasts. Osteoblasts are bone-forming, pluripotent cells found in their primordial stage while the osteoclasts are slightly matured cells found in lacunae of the bone matrix. Osteocytes are the most advanced and highly matured form of bone cells.  For a healthy bone, there must be equilibrium between osteoblasts and osteoclasts. 

 

 

                                     

 

   Structure of a long bone -Image from Brunner

 

 

 

Haversian system or osteon

Haversian system, also called Osteon, is the major functional unit of bone, named after the British physician Clopton Havers. An osteon makes up lamellae, canaliculi and the lacunae spread evenly between many osteocytes.  Based on the organizational structure, the 3 types of lamellae comprise circumferential, interstitial and the concentric. Circumferential lamella is a single layer found underneath endosteum while the interstitial type occupies the space between the osteocytes inside the osteon whereas, the concentric lamellae accommodate many Haversian canals by evenly spreading in between them. The spaces that spread between many osteocytes are called lacunae, they form a network of tiny canals called canaliculi.  Canaliculi supply nutrients through blood vessels and remove the wastes. They also provide a means of communication between osteocytes. Workman's canal develops communication between the periosteum into the centre of the bone so that the entire bone tissues receive blood supply.

 

 

  

 

 

Classification of bones

When we are born, we have around 270 soft bones but as we grow older, many of these fuses together that ultimately result in 206 bones. Depending upon the location and the function, bones are classified into Long bones, short bones, flat bones, and irregular bones. Long bones are found in the upper and the lower limbs help in locomotion and production of blood cells in their central canals. Most common long bones include, femur, tibia, fibula, humerus, radial and ulnar bones.  Short bones are the intermediaries that connect long bones; for example, metacarpals of the wrist joint.  Flat bones look thin and flat; sternum is the best example of this. Irregular bones are 33 in number with the major portion being found in the vertebral column. irregular bones found here.

 

 

Functions of bones

  1. Long bones assist in weight-bearing and movement, for example, femur (thigh) bones.

  2. Short bones, with the help of surrounding muscles and long bones help in the contraction and relaxation of muscles;  for example, finger bones.

  3. There are many flat bones but the flat bones found in the skull helps to protect the brain.

  4. Irregular bones perform different functions based on their location. 

  5. Bones are the reservoir of many minerals such as calcium, phosphorus, and magnesium. 98% of the total body calcium is found in our bones alone.

  6. Hematopoiesis, a process of formation of new blood cells is assisted by the bone marrow found in the long bones.

  7.  The muscles attached to the bony skeleton contract and relax to produce movement and heat. By this way, bones assist in maintaining the body temperature

 

 

 

Conditions affecting the bones
Bones are vulnerable to many developmental defects, fractures and some infections. The table below provides brief definitions of each of these.

 

Name of the condition

Description

Fractures

A fracture is a breakage in the continuity of bone tissue. It is the most common accidental condition in adults.

Osteoporosis 

It is the gradual loss of bone density and strength due to ageing

Osteomyelitis 

It is the infection of the bone and its surrounding tissues

Osteitis 

It is a condition associated with the bone inflammation, for example, Paget’s disease of the bone

Acromegaly 

It refers to the overgrowth of bones in the face, hands and feet

Fibrous dysplasia 

An abnormal growth or swelling of bone

Rickets

A  child’s growing bones fail to develop due to the deficiency of vitamin D

Multiple myeloma

Cancer of the plasma cells in bone marrow

Bone cancer

It is a painless mass along the length and breadth of bone. Primary bone cancers include osteosarcomas and chondrosarcomas.

Scurvy

Bowed legs due to calcium deficiency

 

 

 

Further reading

1.    

Epithelium                                    

2.

Connective tissues

3.

Tissues in our body

4.  

 Cardiac muscles   

 

 

 

Questions

  1. What type of tissue is the bone?

  2. Why bone tissues are very hard? Explain.

  3. What is ossification? Explain the minerals responsible for ossification.

  4. Classify bones. Explain the structure of a long bone.

  5. Describe the functions of bones.

 

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