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© Dirk Biddle
Although arising from the essentially same stem cell lineage in the bone marrow,red blood cells (erythrocytes) and platelets (thrombocytes) are not of the leucocyte (white blood cell) family. Erythrocytes make up the largest population of blood cells, numbering between 4 to 5 million per cubic millimetre of blood in women and 5 to 6 million in men. Erythrocytes are continuously being produced from stem cells in the red bone marrow of large bones and develop in about 7 days. As they mature, the erythrocytes lose their nuclei (thus they contain no DNA, become bioconcave disk-shaped, and begin to produce haemoglobin, which fills the entire cytoplasm. They carry out the exchange of oxygen and carbon dioxide between the lungs and the body tissues. To effectively combine with oxygen, the erythrocytes must contain a normal amount of the red protein pigment haemoglobin, the amount of which in turn depends on the iron level in the body. A deficiency of iron (and therefore of haemoglobin) leads to anaemia and poor oxygenation of the body tissues. After circulating for about 100-120 days, erythrocytes swell up to sphere shaped and undergo destruction by phagocytosis in the liver and spleen, where their constituents are released back into the bloodstream. The heme constituent of haemoglobin is eventually excreted as bilirubin. Although all red blood cells are essentially similar, certain glycoproteins on their surfaces vary from person to person and these serve as the basis for the classification into the four main blood groups. Erythrocytes play no direct role in immunological activities.
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Stem Cell: An unspecialised progenitor cell in the bone marrow that gives rise to a lineage of cells. Particularly used to describe the most primitive cells in the bone marrow from which all the various types of blood cell are derived.
More commonly used of a cell that, upon division , produces dissimilar daughters , one replacing the original stem cell, the other differentiating further (e.g. Stem cells in basal layers of skin , in haematopoetic tissue and in meristems ) . (OMD)
Red Blood Cell: Cell specialised for oxygen transport (erythrocyte), having a high concentration of haemoglobin in the cytoplasm (and little else). Biconcave, anucleate discs, 7nm diameter in mammals, nucleus contracted and chromatin condensed in other vertebrates. (OMD)
Platelet: A particle found in the bloodstream that binds to fibrinogen at the site of a wound to begin the blood clotting process. Platelets are formed in bone marrow, where they arise from cells called megakaryocytes. (OMD)
DNA: Deoxyribonucleic acid.
The molecule that encodes genetic information in the nucleus of cells. It determines the structure, function and behaviour of the cell.
DNA is a double-stranded (double helix) molecule (structured from alternate links of deoxyribose and phosphate) held together by weak hydrogen bonds between base pairs of nucleotides (a purine and a pyrimidine) which project inward from two chains. The four nucleotides in DNA contain the bases: adenine (A), guanine (G), cytosine (C), and thymine (T).
In nature, base pairs form only between A and T and between G and C, thus the base sequence of each single strand can be deduced from that of its partner.
Anaemia: 1. a: A condition in which the blood is deficient in red blood cells, haemoglobin, or total volume.
b. Ischaemia.
2. lack of vitality.
Bilirubin: A reddish yellow pigment (C33H36N4O6) that occurs especially in bile and blood and causes jaundice if accumulated in excess. (M+)
A pigment produced when the liver processes waste products. A high bilirubin level causes yellowing of the skin. (OMD)