Blood Cells
Precursor proerythroblasts (pronormoblast, normoblast, or rubriblast) produce erythroglasts. Erythroglasts produce reticulocytes. After about four days of differentiation and hemoglobin production, the erythroglast sheds its nucleus and becomes a reticulocyte. After spending two more days in the bone marrow, the reticulocyte enters the circulation where, twenty-four hours later, they complete their maturation and become indistinguishable from other mature RBCs. An elevated reticulocyte count indicates bleeding. Normal range is from 0-1.5%.
Red blood cells (RBCs) (erythrocytes, corpuscles)
Mature erythrocytes have no nuclei, and consist mainly of hemoglobin in a supporting framework called stroma. RBC formation takes place in the red bone marrow of the adult and in the liver, spleen, and bone marrow of the fetus. This formation requires ample supplies of such dietary elements as iron, cobalt, copper, amino acids, and certain vitamins.
The main function of RBCs is to transport oxygen and carbon dioxide as well as the maintenance of a normal acid/base balance. Since they also help to determine the viscosity (a tendency to resist flow) of the blood, RBCs influence its specific gravity.
During its 120-day life span, an RBC makes about 75,000 round trips between the lungs and tissues. At the end of its life, it returns to the bone marrow where it is removed by the reticuloendothelial system (particularly the liver, bone marrow, and spleen). Despite the constant destruction and production of RBCs (about 300 billion are destroyed and replaced each day), the body is able to maintain a fairly constant number. A decreased number usually indicates some form of anemia.
White blood cells (WBCs) (immune cells)
WBCs are also known as leukocytes (leuko meaning white and cyte meaning cell). Their lifespan averages 13-21 days after which they are destroyed by the lymphatic system. Their numbers change with age and during pregnancy. During the first two weeks after birth, WBC numbers will be high.
WBCs are classified according to whether or not they have granules in their cytoplasm. Those that contain granules are called granulocytes and those that do not have granules are called agranulocytes.
There are five different types of WBCs.
- Three kinds of granulocytes
- basophils
- neutrophils
- eosinophils
- Two kinds of agranulocytes
- lymphocytes
- monocytes
- Granulocytes
Myeloblasts are the immature and most primitive precursor of granulocytes. Myeloblasts are cells of the bone marrow not normally found in peripheral blood. They develop into promyelocytes (progranulocytes) which go on to produce the three cell types collectively known as granulocytes.When granulocytes are stained, their granules are distinctly coloured. Using Wright's blood stain (named after James H. Wright, 1871-1928, an American pathologist who developed the process):
- basophils have large, dark blue granules that often obscure the nucleus;
- neutrophils show lavender granules;
- eosinophils have beadlike, bright pink granules.
Basophils make up less than 1% of the WBCs and are somewhat smaller than other granulocytes. Their main functions are to release histamine and to play a primary role in the inflammatory response. Basophils migrate to sites of injury, crossing the capillary endothelium to accumulate within damaged tissues where they discharge their granules into the interstitial fluids. These granules release heparin, an anticoagulant, which stops the bleeding and begins the process of tissue repair. Because basophils are found in large numbers in areas where there are larger amounts of blood, as in the lungs and liver, the release of heparin is thought to reduce the formation of tiny blood clots. Other chemicals released by the stimulated basophils attract eosinophils and other basophils to the area, thereby reducing inflammation more quickly.
Neutrophils make up 50-70% of the circulating WBCs. The name indicates that they are chemically neutral and thus difficult to stain with either an acid or base dye. A mature neutrophil has a very dense, contorted nucleus that may be condensed into a series of lobes resembling beads on a chain.
Neutrophils are known by many names. Because the nucleus of a neutrophil can have many shapes and sizes, it is often referred to as a polymorph or simply, polys. For the same reason, neutrophils are also called "polymorphonuclear leukocytes" or PMNs. Neutrophils are referred to as segs when the nucleus appears segmented or having many lobes. Sometimes, they are called band cells because the immature neutrophil looks like a thick, curved band. If the band resembles a staff, the cells are then called staff cells or stab cells (from the German word meaning "staff").
No matter what they are called, the function of a neutrophil is to engulf pathogens or debris in damaged or infected tissues. They can survive minutes or days, depending on the tissue activity; but the average life span is about ten to twelve hours. After engulfing up to two dozen bacteria, for example, a neutrophil dies. Its breakdown releases chemicals that attract other neutrophils to the site to carry on where it left off. The collection of dead neutrophils, cell parts, and fluid left at a site of infection is called pus.
Eosinophils make up about 1-3% of the total WBCs. They are so named because their granules darkly stain an orange-pink with the red dye eosin. Although they are phagocytes, eosinophils generally ignore bacteria and cellular debris and, instead, are attracted to foreign compounds that have reacted with circulating antibodies. They are involved in dissolving clots and in the inflammatory response, secreting chemicals that destroy certain parasites. Their numbers also increase during allergy attacks.
Microphages are neutrophils and eosinophils, which are small enough to enter peripheral tissues when there is an injury or infection.
- Agranulocytes
Lymphocytes are usually the most important and the most numerous of the agranulocytes, making up about 20-25% of the WBCs. Until the age of about eight years, lymphocytes are more predominant than neutrophils. Pregnancy will also cause a slight rise in lymphocytes.Lymphocytes are often identified as being small, medium, or large, with the large ones found mainly outside the circulation in lymphatic organs and thus the name. Involved in immune responses, lymphocytes are further divided into T and B lymphocytes: T-cells for cell-mediated immune reactions and B-cells for humoral immunity. Morphologically, they are indistinguishable. Differences can only be seen by serological tests. During staining, the nucleus will be very densely stained with the cell appearing round but sometimes with a very slight indentation. The cytoplasm is a thin band to one side of the nucleus, and is stained a dusky blue color.
Monocytes are larger than lymphocytes and have a kidney-shaped nucleus. Monocytes make up about 4-8% of the total WBC population. They function as phagocytes and develop into macrophages. The nucleus varies in shape: kidney-shaped, bean-shaped, or horseshoe-shaped, with a deep indentation. Monocytes do not stain as deeply as lymphocytes do.
Platelets
Platelets are developed through the following process:
- Megakaryoblasts produce megakaryocytes which produce platelets (thrombocytes).
Platelets are the tiniest formed elements of the blood. Normally, each microliter of blood contains between 150,000 and 450,000 platelets. A platelet is not a complete cell, but a fragment of the megakaryocyte, which develops fissures in its cytoplasm and literally falls apart. Even though a platelet does not contain DNA or a nucleus, it does contain a cytoplasm with mitochondria and various enzymes surrounded by a cell membrane. Platelets have a life span of 5-9 days.
Responsible for initiating the clotting process, their only function is to prevent blood loss from injured blood vessels. Failure of the bone marrow to replace platelets at an adequate rate results in a deficiency called thrombocytopenia, which is characterized by pinpoint hemorrhages under the skin (petechiae) and abnormal bleeding episodes.
This page was updated in November 2005.
