BONES

BONES

Bone formation, also called ossification, process by which new bone is produced. Ossification begins about the third month of fetal life in humans and is completed by late adolescence. The process takes two general forms, one for compact bone, which makes up roughly 80 percent of the skeleton, and the other for cancellous bone, including parts of the skull, the shoulder blades, and the ends of the long bones.

Types of ossification:

Endochondral ossification

It begins in the embryonic skeleton with a cartilage model, which is gradually replaced by bone. Specialized connective tissue cells called osteoblasts secrete a matrix material called osteloud, a gelatinous substance made up of collagen, a fibrous protein, and mucopolysaccharide, an organic glue. Soon after the osteoid is laid down, inorganic salts are deposited in it to form the hardened material recognized as mineralized bone. The cartilage cells die out and are replaced by osteoblasts clustered in ossification centres. Bone formation proceeds outward from these centres.

Both endochondral and intermembranous ossification produce immature bone, which undergoes a process of bone resorption and deposition called bone remodeling to produce mature bone.

Intramembranous ossification

Intramembranous ossification is type of bone ossification where the bone tissue is created directly over the mesenchymal tissue [and not on cartilage as in endochondral ossification]. It occurs in healing of bone fractures and the initial formation of the flat bones of our skull. This process is also responsible for forming our jaw and clavicles, or collar bones

Creation of bone tissue

• Mesenchymal stem cells within mesenchyme or the medullary cavity of a bone fracture initiate the process of intramembranous ossification. A small group of mesenchymal stem cells begin to replicate and form a cluster of cells, a nidus.                                                                                          
• Once a nidus has been formed the mesenchymal stem cells within it stop replicating.                         
• Osteoprogenitor cells also undergo morphological process to turn into osteoblasts. Osteoblasts create an extracellular matrix. Mineralization occurs resulting in bone tissue and bone spicules.                                                                                                  
• Further secretion of osteoid increases the size of spicules which fuse with each other and become trabeculae. With continuous growth, trabeculae become interconnected and woven bone is formed. The term primary spongiosa is also used to refer to the initial trabecular network.                                                                                                                                                                 
• The periosteum is formed around the trabeculae. Osteogenic cells that originate from the periosteum increase appositional growth and a bone collar is formed.

Eventually, woven bone is replaced by lamellar bone.

Types of Bone

The bones of the human skeleton are classified by their shape, the basis of the structure, or the basis of development.

BY THEIR SHAPE

Short bones : They're bones that are the same width and length, giving them a cube-like shape. For example, the bones of the wrist (carpals) and ankle (tarsals) are short bones.

Long bones are longer than they are wide and have a shaft and two ends. The diaphysis, or central shaft, contains bone marrow in a marrow cavity. The rounded ends, the epiphyses, are covered with articular cartilage and are filled with red bone marrow, which produces blood cells. Most of the limb bones are long bones—for example, the femur, tibia, ulna, and radius. Exceptions to this include the patella and the bones of the wrist and ankle.

Flat bones are thin and relatively broad bones that are found where extensive protection of organs is required or where broad surfaces of muscle attachment are required. Examples of flat bones are the sternum (breast bone), ribs, scapulae (shoulder blades), and the roof of the skull.

Irregular bones are bones with complex shapes. These bones may have short, flat, notched, or ridged surfaces. Examples of irregular bones are the vertebrae, hip bones, and several skull bones.

Sesamoid bones are small, flat bones and are shaped similarly to a sesame seed. The patellae are sesamoid bones. Sesamoid bones develop inside tendons and may be found near joints at the knees, hands, and feet.

Sutural bones are small, flat, irregularly shaped bones. They may be found between the flat bones of the skull. They vary in number, shape, size, and position.

BY THEIR BASIS OF STRUCTURE:

All bones are composed of same material. The difference only is in the pattern of arrangement. This classification is based on the same thing that is pattern of arrangement of bony tissue. The structural classification has two approaches that are macroscopic approach and microscopic approach.

Macroscopic approach divides the bones into two categories that are:

Compact bone:

The part of a bone where bone substance to bone space ration is a bigger quantity is called compact bone. This means that there is more bone tissue and less empty space.

Spongy bone: 

The part of a bone where bone substance to bone space ratio is a smaller quantity. This means that there is more empty space and less bone tissue.

Microscopic approach divides the bones into following categories;

Lamellar bone:

The type of bone which are composed of thin plates (lamellae) of bony tissue. Most mature human bones are lamellar bones.

Fibrous bone:

These have more fibers in them. In humans they are found only in fetus.

BY THE BASIS OF DEVELOPMENT :

On the basis of the pattern of development a bone follows, there are three categories which are described in detail below;

Membranous bones: 

These are also known as dermal bones and the process by which they ossify is called intra-membranous ossification. These bones ossify from mesenchymal condensations in the intrauterine life. Examples are bones of the skull and facial bones.

Cartilaginous bones: 

These bones ossify from a cartilage model and this type of ossification is known as intra-cartilaginous ossification. These bones do not form from mesenchymal condensations but from preformed cartilage models. Examples of this type of bones include bones of limbs, vertebral column and thoracic cage.

Membrocartilaginous bones:

These bones ossify partly from cartilage and partly from mesenchymal condensations. Examples of this class of bones include clavicle, mandible, occipital, temporal and sphenoid etc.