Histology Mentor

Blood, Questions 60-64

60. Question - Define plasma, serum, hematocrit, myeloid tissue, stem cell, hemopoiesis, formed elements.

Plasma is the extracellular fluid component of blood. Serum is the fluid that remains after blood clots. It is different from plasma in that it does not contain fibrinogen and several proteins involved in the clotting reaction have been altered. Albumin and immunoglobulin are the major proteins in serum and plasma.

Hematocrit is a measure of the fraction of the blood volume occupied by erythrocytes. When whole blood is centrifuged with anticoagulant a thin creamy white layer is seen between the packed erythrocytes and the plasma. This "buffy coat" consists of leukocytes and platelets.

Myeloid tissue is the tissue where blood cells are differentiating. It is not always in the bone marrow and for that reason is a more precise way of referring to it. In the fetus it is in the yolk sac. Later it is in the spleen and liver but ultimately it is almost entirely in the bone marrow. In some severe cases of anemia the spleen and even the liver may be called back into action.

Stem cells are pluripotent cells which can become many different kinds of cells - not just blood cells - when appropriately stimulated.

Hemopoiesis is the term used to refer to the processes involved in the formation of all blood cells. Erythropoiesis refers to the formation of erythrocytes and leukopoiesis to leukocytes.

Formed elements include all membrane-bound structures in blood. Erythrocytes and platelets are not true cells.

61. Question - Describe the mature erythrocyte and its concentration in peripheral blood. Diagram its formation. 4-5 million per microliter

The mature erythrocyte is a biconcave disk with little evidence of organelles. Special proteins associate with the inner surface of the cell membrane forming a cytoskelton. They contribute to its biconcave shape but allow the cell to be greatly deformed when passing through capillaries. Hemoglobin comprises about 30% of the cytoplasmic proteins. Stem cells, capable of producing all blood cells differentiate to become the proerythroblast.

62. Question - What is the life span and the normal fate of erythrocytes? How does the body respond to the loss of 500 ml of blood?

Erythrocytes function in the circulation for about 120 days. As erythrocytes age molecules on their surface change until macrophages in the spleen identify them as aged and remove them from the circulation. The salvaged iron is either stored as ferritin or transported back to the myeloid tissue by a blood protein called transferrin. It takes about 5 days for a reticulocyte to arise from an erythroblast. Since there is little storage of erythrocytes in the myeloid tissue (bone marrow) immediately after losing 500ml. of blood, fluid is drawn into the vascular system and there is a drop in the hematocrit. This increases the production of erythropoietin by the kidneys which stimulates the proliferation of erythroblasts. It takes about 5 days for the increased rate of production of erythrocytes to be evident in the circulation.

63. Question - Diagram the structure of each of the blood granulocytes. Indicate their function and concentration in blood. Outline the stages of their differentiation.

Neutrophils (PMN, polymorphonuclear neutrophilic leukocyte) or "Polys" are the most abundant of the leukocytes, 60-75%. They are called microphages because of their role in destroying microorganisms. Eosinophils contribute 2-4% to the leukocytes in the blood. They are increased in parasitic infections and allergies. They destroy antigen-antibody complexes. Basophils make up less than 1% of the circulating leukocytes. While they are similar to mast cells and augment the function of mast cells they are not the same.

In the formation of granulocytes myeloblasts differentiate from totipotent stem cells and are capable of becoming any of the granulocytes. They first accumulate many azurophilic granules (lysosomes containing myeloperoxidase) and are then classified as promyelocytes. When specific granules (neutrophilic, eosinophilic and basophilic) are recognizable in their cytoplasm they are called neutrophilic, eosinophilic or basophilic myelocytes. The nucleus becomes progressively more condensed and lobulated until they reach their fully differentiated state. In infections when there is a great demand for neutrophils there will be an increased percentage of band cells (young neutrophils) in the peripheral blood.

64. Question - Illustrate the morphology of lymphocytes, monocytes and platelets. What are their concentrations in blood and their functions?

Lymphocytes make up about 30% of the circulating leukocytes and 90% of them are T-lymphocytes. There are small numbers of medium and large lymphocytes which are activated lymphocytes and then even fewer small cells that look like lymphocytes but which are actually universal stem cells. Lymphocytes function in the immune system, B-cells becoming plasma cells and producing antibodies; T-cells serving in cell mediated immunity. Monocytes make up about 5% of circulating leukocytes. They function to maintain the body's population of macrophages (histiocytes) and antigen presenting cells. There are 200,000 to 400,000 platelets per microliter of blood. These fragments of cytoplasm arise in myeloid tissue from megakaryocytes and are also called thrombocytes. They have a central basophilic part, the granulomere, and a lighter cytoplasmic part, the hyalomere. They are important in blood clotting and wound healing.

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