Bones and typers

What is a bone?

Bone is a dense type of connective tissue impregnated with inorganic salts mainly the salts of calcium such as calcium phosphate, calcium carbonate etc. The organic portion of the bone constitutes one third  and the inorganic salt component constitutes two third .The inorganic salts are mainly responsible for rigidity and hardness, which make bone resist compression caused by the forces of weight and impact. The organic connective tissue portion of the bone makes it resilient and thus the bone can afford resistance to tensile forces. In strength bone is comparable to iron and steel.

Bone forms an important component of the skeleton of human body. Because of its rigidity and hardness, it provides the strength and form necessary to keep human body in its shape. It also protects the delicate organs vital for life, for example the skull protects the brain and the ribcage protects heart and lungs.

Types of bone

Human bones are classified on different bases of classification. Generally four types of classifications are followed each dividing bones into different types.

Blood Supply of Bones

Although bone is not an actively growing tissue in adults, it does need a constant blood supply to stay alive. In fact, the supply of blood to long bones is profuse and derived from a number of sources that are explained with detail in the lines below.

Nerve Supply of Bones

As with all other living tissues, the bone is innervated by peripheral nerves so that it can coordinate with the central nervous system. The important part of this coordination is formed by the sensory signals coming from the bones. The brain reads these signals and make necessary changes to avoid any damage to bones and other body tissues.

Parts of a Young Bone

A typical long bone, which is still young remains under the process of ossification for many years. It ossifies in three parts; the two ends and one shaft. The ends ossify from secondary centers of ossification while the shaft ossifies from primary center of ossification. Primary center of ossification is the center where the process of ossification starts first of all. Secondary centers of ossification are the centers where the process of ossification starts after the primary center.
In a young bone in which the process of ossification is not complete and the following important parts can be distinguished from the others;

• Epiphysis
• Diaphysis
• Metaphyses

Epiphysis :

The ends and the tips of a long bone that ossify from secondary centers are known as epiphyses.

Epiphyses are of 4 types and they can be better understood by explaining their types;

• Pressure epiphyses : These epiphyses are articular that is they take part in joint formation. These epiphyses are the weight transmitting epiphyses. Examples are head of the humerus, lower end of the radius etc.

Diaphysis :

Diaphysis is the term used for elongated shaft of the long bone. It ossifies from a primary center.

Metaphyses :

The ends of diaphysis near the epiphyses are known as metaphyses. Since a long bone has two ends so there are two metaphyses. Each metaphysis is the zone of active growth of a long bone. Before the fusion of diaphysis and epiphyses, the metaphyses are richly supplied with blood through end arteries forming hair pin bends. This is the common site of osteomyelitis in children.

Types of Cells in Bone

Types of Cells in BoneBone is formed by three primary cell types: Osteoblasts, Osteocytes and Osteoclasts. 

Osteoblasts:

Osteoblasts are  bone-forming cells that descend from osteoprogenitor cells. They form a protein mixture known as osteoid, which mineralizes to become bone. Osteoid is primarily composed of Type I collagen. Osteoblasts also manufacture hormones, such as prostaglandins, to act on the bone itself. They robustly produce alkaline phosphatase, an enzyme that has a role in the mineralization of bone, as well as many matrix proteins. Osteoblasts are the immature bone cells, and eventually become entrapped in the bone matrix to become osteocytes, which are the mature bone cells. All bone lining cells are osteoblasts.

Osteocytes:

Osteocytes are mature bone cells that originate from osteoblasts, which have migrated into and become trapped and surrounded by bone matrix, produced by themselves.

The spaces they occupy are known as lacunae. Osteocytes have many processes that reach out to meet osteoblasts and other osteocytes probably for the purposes of communication. Their functions include formation of bone, maintenance of matrix and homeostasis of Calcium.

Osteoclasts:

Osteoclasts are the cells responsible for bone resorption and remodelling. They are large, multinucleated cells located on bone surfaces in what are called Howship’s lacunae or resorption pits. These lacunae, or resorption pits, are left behind after the breakdown of the bone surface. Because the osteoclasts are derived from a monocyte stem-cell lineage, they are equipped with phagocytic-like mechanisms similar to circulating macrophages.

Ossification of Bones

Ossification is the process by which bone is formed. The formation of bone is in fact conversion of other types of connective tissues into bone. Based on the type of tissue converted into bone, the process of ossification is of three types. These types with appropriate detail are explained in the lines below.

Intra-membranous ossification:

This type of ossification is also known as mesenchymal ossification. In this type the bone is ossified from mesenchymal condensations. The bones formed by this process of ossification are known as membranous bones or dermal bones.

Intra-cartilaginous ossification:

In this type of ossification, the mesenchyme has been converted to cartilaginous models and the process of ossification starts in these cartilaginous models. The process of conversion of mesenchymal condensations in cartilage is known as chondrification and this process takes place during the second month of intrauterine life.

Process of ossification:

With the explanation of different types of ossification in bones it is also important to explain what ossification is. Ossification is the process by which bone is formed. It is started at certain sites known as centers of ossification each of which is a point where lying down of lamellae (bone formation) is started by the activity of osteoblasts. Osteoblasts are bone forming cells and secrete collagen and other substances that form the ground substance of bone. 

Growth of a Long Bone

All long bones of the body (except clavicle) are formed from a cartilaginous model by the process of ossification. Once a young bone is formed, the growth takes place in three steps.

Growth in length:

A long bone grows in length my multiplication of cells in the epiphyseal plate of cartilage. The cartilage cells divide and increase in number. The zone of active division in the epiphyseal plate of cartilage lies towards the epiphysis (end of the bone). This means that newly formed cartilage cells will push the older, larger cells towards the diaphysis (shaft of the bone).  Eventually these cartilage cells are replaced by osteocytes (bone cells), thus increasing the length of the bone.

Growth in thickness:

A long bone grows in thickness by multiplication of cells in the deeper layer of periosteum. The cells lying in the deeper layer of periosteum are known as osteoblasts (bone forming cells). These cells divide continuously and form the osteocytes, thus increase the thickness of bone.

Remodeling of bone:

The growth take place by deposition of new bony tissue on the surface and at the ends. The is process of bone deposition by osteoblasts is called appositional growth or surface accretion. The appositional growth can result in shape alterations, however,  in order to maintain the shape, the unwanted bone must be removed. 

Cartilage and Its Types

Cartilage is a type of connective tissue composed of special cells known as chondrocytes along with collagen or yellow elastic fibers. The fibers and the cells are embedded in a firm gel like matrix rich in mucopolysaccharides. Cartilage is not as hard and rigid as bone. It is much more flexible and elastic.

Characteristic features of cartilage:

• Cartilage has no blood vessels or lymphatics and the nutrition of the cells diffuses through the matrix. This explains the slowness of healing process in cartilages.
• Cartilage has no nerves and therefore it is insensitive.
• Cartilage is surrounded by a fibrous membrane known as the perichondrium. This perichondrium is similar to the periosteum in structure and function. The articular cartilage has no perichondrium and thus its regeneration after injury is inadequate because the perichondrium plays a major role in regeneration of the cartilage.

Types of cartilage:

There are three types of cartilage;

• Hyaline cartilage
• Fibrous cartilage
• Elastic cartilage

Hyaline cartilage:

This type of cartilage has very thin fibers having same refractive index as the matrix of the cartilage and thus these fibers are not seen. Hyaline cartilage is the articular cartilage of long bones, sternum, ribs etc. Its color is bluish white and it is flexible.

Fibrous cartilage:

This type of cartilage has numerous white fibers. It is present in the symphysis pubis, and sternoclavicular joint etc. Its color is glistening white and the appearance is opaque.

Elastic cartilage:

This type of cartilage has numerous yellow elastic fibers. It is present in the ear pinna, external auditory meatus, Eustachian tubes, and epiglottis etc. Its color is yellowish and the appearance is opaque. 

Joint

Joint is a junction between two or more bones or cartilages. It is a device to permit movements in a hard and rigid skeleton. Joints are formed in such a way that they keep a balance between the movement, stability and strength of human skeleton. However there are some types of joints where movements are sacrificed for stability and in other cases the stability is sacrificed for movements. For example the shoulder joint and hip joint are both ball and socket joints but shoulder joint is more mobile and less stable while hip joint is more stable and less mobile. Also there are certain types of joints where motion does not occur. These joints are called immovable joints and are primarily meant for growth and they permit molding during child birth.

Synovial Joints

Synovial joints are most evolved and therefore most mobile type of joints. They possess the following characteristic features;

• There articular surfaces are covered with hyaline cartilage. This articular cartilage is avascular, non nervous and elastic. Lubricated with synovial fluid, the cartilage forms slippery surfaces for free movements.

• Between the articular surfaces there is a joint cavity filled with synovial fluid. The cavity may be partially or completely subdivided by an articular disc known as meniscus.

• The joint is surrounded by an articular capsule which is fibrous in nature and is lined by synovial membrane. Because of its rich nerve supply the fibrous capsule is sensitive to stretches imposed by movements.

• The synovial membrane lines the entire joint except the articular surfaces covered by hyaline cartilage. It is this membrane that secretes the slimy fluid called synovial fluid which lubricates the joint and nourishes the articular cartilage.