Morphology of Blood Cells | Blood | Body Fluids | Biology

In this article we will discuss about the morphology of Erythrocytes, Leukocytes and Platelet.

Morphology of Erythrocytes (RBC):

The erythrocytes are the most numerous blood cells i.e. about 4-6 millions/mm³. They are also called red cells. In man and in all mammals, erythrocytes are devoid of a nucleus and have the shape of a biconcave lens. In the other vertebrates (e.g. fishes, amphibians, reptilians and birds), they have a nucleus.

The red cells are rich in hemoglobin, a protein able to bind in a faint manner to oxygen. Hence, these cells are responsible for providing oxygen to tissues and partly for recovering carbon dioxide produced as waste. However, most CO₂ is carried by plasma, in the form of soluble carbonates.

In the red cells of the mammalians, the lack of nucleus allows more room for hemoglobin and the biconcave shape of these cells raises the surface and cytoplasmic volume ratio. These characteristics make more efficient the diffusion of oxygen by these cells. In so-called “sickle-cell anaemia”, erythrocytes become typically sickle-shaped.

With the electron microscope, biologists saw that red cells can have different shapes-normal (discocyte), berry (crenated), burr (echinocyte), target (codocyte), oat, sickled, helmet, pinched, pointed, indented, poikilocyte, etc. The mean life of erythrocytes is about 120 days. When they come to the end of their life, they are retained by the spleen where they are phagocyted by macrophages.

Morphology of Leukocytes (WBC):

Leukocytes, or white cells, are responsible for the defense of the organism. In the blood, they are much less numerous than red cells. The density of the leukocytes in the blood is 5000-7000/mm³. Leukocytes divide in two categories: granulocytes and lymphoid cells or agranulocytes.

The term granulocyte is due to the presence of granules in the cytoplasm of these cells. In the different types of granulocytes, the granules are different and help us to distinguish them. In fact, these granules have a different affinity towards neutral, acid or basic stains and give the cytoplasm different colors.

So, granulocytes distinguish themselves in neutrophil, eosinophil (or acidophil) and basophil. The lymphoid cells, instead, distinguish themselves in lymphocytes and monocytes. The shape of the nucleus helps us in the recognition of the leukocytes.

Each type of leukocyte is present in the blood in different proportions:

Neutrophil 50 – 70%

Eosinophil 2 – 4%

Basophil 0, 5 – 1%

Lymphocyte 20 – 40%

Monocyte 3 – 8%

i. Neutrophil:

The neutrophil are the more common leukocytes. They have a diameter of 12-15 µm. You can recognize them as their nucleus is divided into 2 – 5 lobes connected by a fine nuclear strand or filament (Fig. 14.2).

The cytoplasm is transparent because its granules are small and faintly pink colored. Immature neutrophils have a band-shaped or horseshoe-shaped nucleus and are known as band cells. In the nucleus of the neutrophil of cells from females, you may see an appendage like a little drumstick (Barr body). It is the second X chromosome, inactivated.

Neutrophils:

Are very active in phagocyting bacteria and are present in large amount in the pus of wounds. Unfortunately, these cells are not able to renew the lysosomes used in digesting microbes and dead after having phagocyted a few of them.

ii. Eosinophil:

The eosinophils are quite rare in the blood. They have the same size as the neutrophils. Generally their nucleus is bi-lobed. But even nuclei with three or four lobes have been observed. The cytoplasm is full of granules which assume a characteristic pink-orange color. As for the neutrophil, the nucleus is still easily visible. Eosinophils attack parasites and phagocyte antigen-antibody complexes.

iii. Basophil:

Basophils are the rarest leukocytes: less than 1%. They are quite small: 9-10 µm in diameter. Cytoplasm is very rich in granules which take a dark purple color. The nucleus is bi- or tri-lobed, but it is hard to see because of the number of granules which hide it secrete anti-coagulant and vasodilatory substances as histamines and serotonin. Even if they have a phagocytory capability, their main function is secreting substances which mediate the hypersensitivity reaction.

iv. Lymphocyte:

a. Lymphocytes are quite common in the blood: 20-40%, 8-10 nm in diameter and generally they are smaller than the other leukocytes but they are still a few larger than red cells. The cytoplasm is transparent.

The nucleus is round and large in comparison to the cell and it occupies most of it. In any case, some of the cytoplasm remains visible, generally in a lateral position. According to the quantity of cytoplasm, lymphocytes are divided into small, medium and large.

With Giemsa stain, we cannot distinguish the different types of lymphocyte (B, T, NK), either in the blood because they are not activated, or because it would be necessary to perform special immunochemical staining.

b. Lymphocytes are cells which, besides being present in the blood, populate the lymphoid tissues and organs too, as well as the lymph circulating in the lymphatic vessel. The lymphoid organs include thymus, bone marrow (in bird’s bursa), spleen, lymphoid nodules, palatine tonsils, Peyer’s patches and lymphoid tissue of respiratory and gastrointestinal tracts.

c. Most lymphocytes circulating in the blood is in a resting state. They look like little cells with a compact round nucleus which occupies nearly all the cellular volume. As a consequence, the cytoplasm is very reduced.

The lymphocytes of the lymphoid tissues and organs can be activated in a different amount following antigenic stimulation. In the blood, lymphocytes are 20-40% of all leukocytes and are slight larger than red blood cells.

The lymphocytes are the main constituents of the immune system which is a defense against the attack of pathogenic micro-organisms such as viruses, bacteria, fungi and protista. Lymphocytes yield antibodies and arrange them on their membrane.

An antibody is a molecule able to bind itself to molecules of a complementary shape called antigens, and recognize them. As for all proteins, even the antibodies are coded by genes. On the basis of a recombination mechanism of some of these genes, every lymphocyte produces antibodies of a specific shape.

v. Monocyte:

Monocytes are the precursors of macrophages. They are larger blood cells, which after attaining maturity in the bone marrow, enter the blood circulation where they stay for 24-36 hours. Then they migrate into the connective tissue, where they become macrophages and move within the tissues.

In the presence of an inflammation site, monocytes quickly migrate from the blood vessel and start an intense phagocytory activity. The role of these cells is not solely in phagocytosis because they also have an intense secretory activity.

They produce substances which have defensive functions such as lysozime, interferons and other substances which modulate the functionality of other cells. Macrophages cooperate in the immune defense. They expose molecules of digested bodies on the membrane and present them to more specialized cells, such as B and Tlymphocytes.

Monocytes are the biggest leukocytes – 16-20 µm. They have a great reniform or horseshoe-shaped nucleus, in some cases even bi-lobed. The cytoplasm is transparent, but with an appearance of “ground glass”

Morphology of Platelet:

The main function of platelets, or thrombocytes, is to stop the loss of blood from wounds (hematostasis). To this purpose, they aggregate and release factors which promote the blood coagulation. Among them, there are the serotonins which, reduces the diameter of lesioned vessels and slows down the hematic flux, the fibrin which trap cells and forms the clotting.

Even if platelet appears roundish in shape, they are not real cells. In the smears stained by Giemsa, they have an intense purple color. Their diameter is 2-3 µm about; hence they are much smaller than erythrocytes. Their density in the blood is 200000 – 300000/mm³.