Leukocytes (white blood corpuscles).

Differences between erythrocytes and leucocytes. Leucocytes are different from erythrocytes in several ways.

a. They are true cells, each leucocyte having a nucleus, mitochondria, Golgi complex, and other organelles.

b. They do not contain hemoglobin and, therefore, appear colourless in unstained preparations.

c. Unlike erythrocytes which do not have any mobility of their own, leucocytes can move actively.

d. Erythrocytes do not normally leave the vascular system, but leucocytes can move out of it to enter surrounding tissues.

Features of different types of leukocytes. Leucocytes are of various types. Some of them have granules in their cytoplasm and are, therefore, called granulocytes. Depending on the staining characters of their granules granulocytes are further divided into neutrophil leucocytes (or neutrophils), eosinophil leucocytes (or eosinophils), and basophil leucocytes (or basophils).

Apart from these granulocytes there are two types of agranular leucocytes. These are lymphocytes and monocytes.

Relative number. There are about seven thousand leucocytes in every cubic millimetre of blood. Of these about two thirds (sixty to seventy per cent) are neutrophils, and about one fourth (twenty to thirty per cent) are lymphocytes. The remaining types are present in very small numbers. The eosinophils are about three per cent, the basophils about one per cent, and monocytes about five per cent.

Nuclei. In lymphocytes the nucleus is spherical, but may show an indentation on one side. In monocytes the nucleus is ovoid and may be indented, it is placed eccentrically. In basophils the nucleus is S-shaped. The nucleus of the eosinophil leucocyte is made up of two or three lobes that are joined by delicate strands. In neutrophil leucocytes the nucleus is very variable in shape and consists of several lobes: that is why these cells are also called polymorphonuclear leucocytes, or simply polymorphs. The number of lobes increases with the life of the cell.

Life span. The life of a neutrophil leucocyte is only about fifteen hours. Eosinophils live for a few days, while basophils can live for nine to eighteen months. The life span of lymphocytes is variable. Some live only a few days (short-lived lymphocytes) while others may live several years (long-lived lymphocytes).

Blood platelets.Blood platelets are round, oval, or irregular discs about three micrometers in diameter. They are also known as thrombocytes. The discs are biconvex. Each disc is bounded by a plasma membrane within which there are mitochondria and membrane bound vesicles. There is no nucleus. The platelets appear to have a clear outer zone (hyalomere) and a granular central part (granulomere).

Platelets are concerned with the clotting of blood. As soon as blood is shed from a vessel, platelets stick to each other and to any available surfaces (specially to collagen fibres). Platelets break down into small granules and threads of fibrin appear around them.

The life of a platelet is about ten days.

Plasma.Plasma is a homogenous, slightly alkaline fluid that transports all nutritive materials.

Questions:

1. What does the blood consist of?

2. Three main types of formed elements of blood.

3. Structural an functional features of the erythrocytes.

4. Differences between erythrocyte and leucocytes.

5. Features of different types of leucocytes.

6. Structural and functional features of blood platelets.

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NERVOUS TISSUE

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