Biology Today - February 2018

Types of bones

On the basis of texture, a bone is of two types : compact and spongy (cancellate) bone.

Table : Types of bones

Characteristics Compact bone Spongy bone

Location In the shaft (diaphysis) of long bones In the epiphyses of long bones

Lamellae Arranged to form Haversian system Arranged irregularly, lamellae form trabeculae

Bone marrow Yellow; stores fat Red; produces RBCs

Bone marrow cavity Narrow Broad

Nature Hard and compact Spongy

Blood

Blood is a mobile, fluid and softest connective tissue which is mesodermal in origin. It is of reddish colour that flows inside
blood vessels by means of pumping activity of heart. An adult human contains 5.0 – 5.5 litres of blood. Blood is composed of a
watery fluid called plasma and formed elements (blood corpuscles).
Plasma is a complex liquid component of the blood, about 90% of which is formed by the water alone. Several organic and
inorganic substances including proteins, glucose, cholesterol, urea, hormones, vitamins, inorganic salts and fibrinogen are
dissolved in the plasma. It also contains an anticoagulant, heparin.
Formed elements or blood corpuscles are of the three types : erythrocytes, leucocytes and thrombocytes (platelets).

Table : Types of blood cells

Blood cell Description

1. Erythrocyte (Red blood
   corpuscle)
   
   
   • It is circular, biconcave, denucleated, has homogenous cytoplasm with haemoglobin.
   
   
   • Cell organelles such as endoplasmic reticulum, mitochondria, ribosomes, centrioles
     are absent.
   
   
   • It transports oxygen and some amount of carbon dioxide.
   
   
   • Formation of erythrocytes is called erythropoiesis.
   
   
   • Excess RBCs are stored in the spleen.
   
   
   • Life span of RBC is about 120 days.
   
   
   
   


2. Leucocytes (White blood corpuscles)

Granulocytes

Eosinophil – It has bilobed nucleus and has coarse granules in cytoplasm. It takes acidic stain.

• It is non-phagocytic in nature, has antiallergic properties and plays role in
  immunity.


• It also helps in dissolving blood clot.


• It can attach itself to parasitic forms, causes their distruction by liberating lysosomal
  enzymes on their surface.
  Basophil – The nucleus is usually three lobed, there are less number of coarse granules in
  cytoplasm. It takes basic stain.


• It releases heparin, histamine and serotonin.


• It is similar to mast cells of connective tissues.

Neutrophil – It has many lobed nucleus, fine granules in cytoplasm and it takes acidic as

well as basic stains.

• It is phagocytic in nature. It engulfs germs and dead cells.


• Barr bodies are found.