Human bone structure and Histology with Classification

Bone is a specialized and constantly changing connective tissue, having general characteristics of connective tissues like

  • Composed of cells
  • Have dense intercellular substance impregnated with calcium phosphates, carbonates, etc.
  • Have well-developed circulation.

Classification of Bones: Bones are classified on the basis of various characteristics. They are

A. According to position:

  • Axial bones: They form the axial skeletal system of human as well as other animals. These categories include bones like skull bones, vertebrae, ribs, and sternum.
  • Appendicular bones: Bones not covered within the axial skeletal system comes under this category. They include bones of hands and legs. And also pectoral girdle and pelvic girdle.

B. According to Ossification:

  • Cartilaginous bones, those who develop from embryonic cartilages
  • Membranous bones, those who develop from embryonic membranes
  • Membrano-cartilaginous bones, those who develop from both embryonic membranes and cartilages.

C. According to shapes:

  • Bones have different shapes. Like long, short, flat, irregular, pneumatic, sesamoid, and accessory.

Structure of Bone: Bones are generally formed of two parts- one is compact, with fewer pores, even fewer blood vessels penetrating them. The other is spongy, with numerous pores, and blood vessels entering and leaving the bone.Bone structure

Each bone is covered by a covering, called periosteum. This periosteum covers the whole bone except at its articular surfaces. The sesamoid bones are devoid of periosteum.

Long bones are one of the main concern in the human body. The long bones include the bones of hand like humerus, radius, and ulna, and the bones of legs like femur, which is the longest bone of the body, the tibia, and the fibula, the feeblest bone of the body. So let us first discuss the structure of long bones.

The long bones are cartilaginous in origin that means, they are preceded by the cartilages in their embryonic period. The slow depositions of calcium salts in the cartilages make them complete bone, which is called Ossification. The points where the Ossification starts, are particular for each bone and are called center of Ossification. The main center of Ossification from where the major Ossification takes place is called primary Ossification center. And the rest is secondary.

The parts of a young ossifying bone are:

  • Epiphysis
  • Metaphysis
  • Diaphysis

Diaphysis: Diaphysis is the part of bone which is ossified from the primary center of Ossification. It mainly consists of the long shaft of the bone.

Inside the Diaphysis lies the bone marrow cavity, inside which lies the bone marrow. It is of two types- the red bone marrow and the yellow bone marrow. Yellow bone marrow mainly contains fats. But the red bone marrow that is found here has a lot of functions. This is the seat of blood corpuscles production. All the red Blood cells, maximum of the white blood cells and all the Platelets are formed here and streamed into the blood vessel.

Epiphysis: Epiphysis is the part of bone mainly having an articulating surface. It is connected with the main bone via the epiphyseal cartilage, and it protects the epiphyseal cartilage. It is formed from the secondary Ossification center. The types of Epiphysis found in human beings are:

  • Pressure Epiphysis: The type of Epiphysis formed due to pressure on the bone, and the Epiphysis this formed transmits the pressure to the bone shaft itself is called pressure Epiphysis. These Epiphysis are mostly found in the long bones of hand and legs. Examples include head of the femur, head of humerus, the condyles of the tibia, condyles of femur.
  • Traction Epiphysis: The type of Epiphysis formed due to pulling or traction on a part of Bone mainly due to muscle pull is called Traction Epiphysis. If a bone has both traction and pressure Epiphysis, the centers of Ossification for later one appears first. Centers of Ossification for the traction Epiphysis appears first. Examples include the greater and lesser trochanters of the femur, the greater and lesser tubercle of the humerus.
  • Atavistic Epiphysis: The type of Epiphysis which are actually independent of the bone by birth, but remain attached with the bone for nutrition secondarily, is called Atavistic Epiphysis. They are just like parasites. Two of the Epiphysis in humans are Atavistic Epiphysis, they are
    • Os Trigonum, or the posterior tubercle of the talus.
    • Coracoid process of the scapula.

Epiphyseal Cartilage: Between Epiphysis and Metaphysis, there lies the epiphyseal cartilage. It has a major function, i.e. It allows space and possibility for further growth of the bone. As the long bone grows, the epiphyseal cartilage persists, but only up to age, mainly puberty, when the Epiphyseal cartilage closes and the growth of the bone ceases.

Closer of the epiphyseal cartilage is earlier in females than in males by as much as 1 to 2 years. On the other hand, the Ossification also starts earlier in females than in males.

There are two types of Epiphysis depending on the fusion of epiphyseal cartilages with the Diaphysis. They are

  • Simple Epiphysis: In the bones with multiple Epiphysis, if each Epiphysis joins with the Diaphysis having an individual epiphyseal plate, it is called simple Epiphysis. The example includes head and greater and lesser trochanters of the femur, which joins with a shaft of femur individually.
  • Compound Epiphysis: In the bones with multiple Epiphysis, if each Epiphysis joins with one another and then as a whole joins with the Diaphysis, its called compound Epiphysis. Example include upper end of the humerus where the head of humerus, its greater and lesser tubercle joins with each other and then as a whole joins with the shaft of the humerus.

The law of union of Epiphysis with the Diaphysis states that ‘’the center of Ossification of Epiphysis appearing first, joins the last’’. It is actually the direction opposite to the entry of the nutrient foramen of the bone.

Metaphysis: Metaphysis is the small portion of Diaphysis that faces towards the Epiphysis and connects Diaphysis with epiphyseal cartilage. Metaphysis has several important features:

  • The metaphysis is the most active and growing region of Bone. The bone grows at the expense of the Metaphysis.
  • Metaphysis has the highest blood supply of any bone, being supplied by all the epiphyseal, periosteal, juxta-epiphyseal, and the nutrient artery. Being most vascular, it is the seat of the infection in the bone.
  • Metaphysis gives 90% attachments of bones with muscles.

Histology of Bone: Bone is connective tissue. Being a connective tissue, it has the characteristic cell in a matrix structure, without fibers.

Cells: Four major types of cells are found in the bone matrix:

1. Osteoprogenitor cells: These cells are the stem cells of the bone. These cells give rise to all other types of cells. They are born from mesenchyme of mesoderm. They may be committed i.e. predetermined to form a particular cell, or inducible i.e. decide to form which cell according to chemicals present.

2. Osteoblasts: Being derived from Osteoprogenitor cells, they are determined to form bone. They add substances to the bone matrix, mainly calcium components, by a pinch-off mechanism. They have cell projections which pinch off with the matrix, and that gets deposited forming the hard bone matrix. Thyroxine and growth hormone increase their functioning.

3. Osteocytes: Osteoblasts after forming enough bone matrix, gets trapped within a cage of calcium formed by them. Then they are called Osteocytes and the cages are called lacunae. They become profusely cytoplasmic processes. Secreting alkaline phosphatase they maintain the bone Structure and integration.

4. Osteoclasts: They act opposite to that of Osteoblasts. They are bone absorbing cells. Secreting acid phosphatase, they help in reabsorption of bones, thereby maintaining the perfect structure. They also help in maintaining the calcium amount in the blood. Affected by parathormone, they increase blood calcium.

Haversian System of Compact Bones:

In the compact bones, the bone cells are arranged in a particular pattern. It is called the Haversian System. These Haversian systems have long cylindrical Haversian canals, surrounded by concentric lamella of bony tissue.

Haversian canals contain small blood vessels, i.e. Arteries and veins, and sometimes nerve fibers passing through it.

Haversian canals are connected to each other by Volksmaan Canals. But they lack those concentric circular structures.

Functions of Bones:

    1. Bone forms the main structure of our body.
    2. They can and does act as the levers for muscles to work.
    3. Bones like ribs and skull protect the vital organs like heart and brain respectively.
    4. The bone marrow of bones produces blood corpuscles.
    5. Bones act as reservoirs of minerals like calcium and phosphates.
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