Blood cells
The blood is a mixture of cellular elements, fluid, proteins & metabolites.The blood has 4 major elements:Erythrocytes (RBC): function mainly to transport oxygen from the lung to the tissues.Leukocytes (WBC): have defensive role destroying the infective organisms such as bacteria & viruses as well as assisting in the removal of dead or damaged tissues Thrombocytes (platelets): are the first line of defense against any damage to blood vessel they adhere to the defect & share in the formation of blood clot.plasma: the solution in which the above cells circulate, also it carries nutrients, metabolites, antibodies, hormones…….etc. the main plasma proteins are albumins, alpha, beta & gamma globulins, lipoproteinsErythrocytes:
RBCs have a short life span of only 100-120 days in the circulation, with aging RBCs become less deformable until they cannot pass through the splinic microcirculation and so they will be removed by phagocytosis. The younger erythrocytes released recently into the blood stream from the bone marrow are called Reticulocytes. They contain a few granules in their cytoplasm & they usually form about 1% of the total number of circulating erythrocytes. Increase in retics count in the blood indicate an increase in oxygen demand as for example in hemorrhage or living in high altitude.Leukocytes:
Those are spherical cells that circulate in the blood until they migrate to the tissues. So the majority of functions of WBCs took place when they leave the circulation to enter the tissues. According to the presence or absence of granules in their cytoplasm & according to the shape of the nucleus, the WBCs are classified into two groups: Granulocytes: (polymorphonuclear leukocytes) because their cytoplasm contain prominent granules. They are Neutrophils, Eosinophils & basophils. Agranulocytes: (mononuclear leukocytes) they do not contain specific granules. They are Lymphocytes & Monocytes.Neutrophils:
Are called also polymorphonuclear leukocytes, they constitute about 60-70% of the circulating leukocytes, their nucleus consist of 2-5 lobes (usually 3) linked together by fine chromatine thread. The cytoplasm of the neutrophils are filled with specific granules that are small in size.
These cells circulate in the blood in a resting state but with appropriate activation they leave the blood and enter the tissues where they become highly motile, phagocytic cells and their primary function is to ingest and destroy the invading organisms. In females the inactive X chromosome (barr body) appear as a drum stick like appendage on one of the lobes of the nucleus. (this feature is found in only 3% of nutrophils)
Eosinophils:
Are less numerous then neutrophils, they form only 2-4% of total WBC count. They have a characteristic bilobed nucleus. Their main feature is the presence of many large & elongated granules (about 200/cell) that are stained by eosin. Number of eosinophils increase greatly in many types of parasitic infestations & the protection against the parasitic disease is one of their major functions. They also increase in allergic states.Basophils:
Constitute less then 1% of the total WBC count, their nucleus is divided into irregular lobes but this division is not so obvious. They have a cytoplasmic granules that are large and intensely basophilic they are irregular in size & shape. Basophils might be the precursors of tissue mast cells with which they have many structural & functional similarities.Lymphocytes:
Are groups of spherical cells with similar morphological characters, they have an ovoid or kidney shaped nucleus, the cytoplasm of the cell is scanty and can be seen as a thin rim around the nucleus.
there are two main types of lymphocytes: T- lymphocytes B- lymphocytes those lymphocytes are responsible for immune surveillance to detect any foreign particle in the tissues. Activation of B lymphocytes after an immune response to a foreign particle leads to their differentiation into plasma cells. Plasma cells are large cells with eccentric rounded nucleus, and they are responsible for active synthesis of immunoglobulins. In health plasma cells are not found in the blood but they are seen in small population in lymphoid organs.
Monocytes:
Are spherical cells with oval or kidney shaped nucleus which is often placed eccentrically. Their cytoplasm is basophilic. Blood monocytes are the precursor cells of tissue macrophages and other cells of the mononucleal phagocytic system such as kupffer cells in the liver and pulmonary and alveolar macrophages.Hematopoiesis
Mature blood cells have a short life span & they must be continuously replaced by proliferation of stem cells that are found in the hemopoietic organs. In the earliest stages of development blood cells arise from the yolk sac mesoderm. Later on the liver and spleen will act as a temporary hematopoietic organs. From the second month of the intrauterine life the bone marrow will become an increasingly important hematopoietic tissue. At birth the bone marrow will be the main site of hematopoiesis.The origin and process of maturation of blood cells in the bone marrow are termed according to the type of cell which will be formed so we have: Erythropoiesis: formation of erythrocytes Granulopoiesis: formation of granulocytes. Monocytopoiesis: formation of monocytes Megakariocytopoiesis: formation of megakariocytes
Stem cells:
All blood cells arise from a single type of cells found in the bone marrow called stem cells and because these cells are able to produce all blood cell types they are called pluripotential stem cells. These cells will divide continuously to maintain their number within the bone marrow & some of the daughter cells will differentiate and mature into various types of blood cells. Stem cells will proliferate and differentiate in two ways: where the cells will eventually become lymphocytes (lymphoid cells) where the stem cells will form the myeloid cells that will develop in the bone marrow leading eventually to the formation of erythrocytes, granulocytes, monocytes & megakariocytes.Progenitor cells:
Stem cells will proliferate to produce the progenitor cells, however the progenitor cells are still so similar to stem cells so they cannot be distinguished morphologically & both are similar to lymphocytes in shape The progenitor cells will be named after the cell which will be formed eventually. So we have: Lymphocyte forming cells Erythrocyte forming cells Megakariocyte forming cells Monocyte forming cells Granulocyte forming cells (neutrophile forming cells, eosinophiles forming cells & basophiles forming cells)Precursor cells: (blast cells) In these cells the morphologic characteristics will differentiate for the first time suggesting the cell types they will become
Bone marrow:
Is one of the largest organs in the body it is the main site for hematopoiesis it is found in the medullary region of long bones & in cavities of cancellous bones. We have two types of bone marrow: red bone marrow: whose color is due to the presence of blood & blood forming cells 2. yellow bone marrow: whose color is due to the presence of large number of adipose tissue.Red bone marrow:
Also called hematopoietic bone marrow, is formed of stroma which is a meshwork of reticular fibers containing the hematopoietic cords & macrophages. Between the hematopoietic cords are very large number of sinusoidal capillaries. The main function of red bone marrow is the production of blood cells & storage of iron derived from breakdown of Hemoglobin.Erythropoiesis:
During formation of erythrocytes several changes will occur in the precursor cells. the cell volume will decrease nucleolus will decrease in size until it is invisible nucleus also decrease in size and finally extruded from the cell at the same time there will be increase in the amount oh hemoglobin within the cytoplasm.So the formation of erythrocyte involve the formation of: proerythroblast basophilic erythroblast Polychromatophilic erythroblasts Orthochromatophilic erythroblasts reticulocytes Erythrocytes the process of development of RBC from the proerythroblasts into the release of reticulocytes into the takes about 7 days & it is under control of the hormone erythropoietine