INTRODUCTION
The blood has been considered the life force in a living animal since ancient times. It has been described as liquid connective tissue, transparent connective tissue, or the connective tissue without fibers.
It flows through the body by the pumping action of the heart. However, the working of the musculature also contributes to circulation. The composition of blood can be divided into the fluid portion called the plasma and the cellular portion. The cellular portion comprises erythrocytes, various WBCs, and thrombocytes. They all have important functions in the body. The plasma is a complex liquid containing different proteins as colloids and various minerals and glucose in solution. The plasma proteins provide nutrition to the body, carry waste, maintain colloidal osmotic pressure, maintain blood pH, and provide immunity. Hormones and other biomolecules are also dissolved in the plasma. Most of the plasma proteins are produced by the liver. On the other hand, the cellular portion arises from the bone marrow. In some animals, hematopoiesis can also occur in other parts of the body called extramedullary hematopoiesis. The bone marrow can be divided into two portions of the red and yellow bone marrow. Red bone marrow is the active one, while the yellow bone marrow contains mostly fat cells.Formation of blood cells is a continuous process and is tightly controlled by various hormones and other chemicals. It requires a proper supply of nutrients, ironically provided by the blood itself. The common nutrients required are proteins, minerals (iron, copper, etc.), vitamins (B complex), fats, and glucose. A deficiency of nutrients can precipitate the common condition of anemia. Cell division is sensitive and can be easily suppressed by persistent inflammatory and neoplastic conditions or acute or chronic toxicity. Toxicity of heavy metals like lead can affect hematopoiesis.
Lesions like hematomas, abscesses, granulomas, cancers, and fibrosis can replace bone marrow, leading to loss of hematopoiesis. Poor hematopoiesis is clinically reflected by reduced total erythrocyte counts, total leucocyte count, hematocrit, and hemoglobin, however with considerable variation. Often, bone marrow malfunction is reflected by variation in shape and size of the erythrocytes. In veterinary science, this variation is of clinical significance in species like dogs and cats, while in cattle, the variation in size of RBCs, also called anisocytosis, is normal. The number of RBCs is more consistent than the leucocyte count, which keeps on varying with different diseases, inflammation, and even physiological conditions.Hematopoiesis also depends on the normal functioning of other organs, particularly liver and kidneys. Poor liver function can suppress bone marrow function by reducing the supply of plasma proteins and accumulation of wastes. Poor renal function, particularly reduced GFR, eventually leads to anemia due to chronic exposure to retained urinary components. Certain drugs like chloramphenicol are known to suppress bone marrow function.
Like any other cells, erythrocytes arise from nucleated stem cells. It is to be noted that erythropoiesis is not only the division and differentiation of the stem cells but also the maturation of the RBC into a functional cell. During the process of maturation, they lose their nucleus, all cell organelles, all nucleic acid and most of the cytoplasm except for some portion responsible for energy production for the survival of the cell. During this process, the RBC gets supersaturated with hemoglobin, the oxygen carrying pigment. Consequent to this the RBC’s are sensitive to osmatic imbalance as compared to other blood cells. Remnants of the nucleus or nucleic acids and even spindle fibers from improper cell division, are all clinical indicators of disturbance in hematopoiesis.
The Leucocytes are classified as granulocytes and agranulocytes.
The granulocytes are those cells which have easily discernable and functional granules present in their cytoplasm. On the other hand, agranulocytes have none such granules in the cytoplasm. Granulocytes also have a peculiarity in their nuclear morphology, that it is multilobulated. Consequently, they are called as polymorphonuclear cells (PMNC). The agranulocytes have a large nucleus with relatively consistent morphology and are called the Mononuclear cells (MNC). It is to be noted here that both the PMNC and MNC have only a single nucleus. The PMNC comprise of the neutrophils, eosinophils and basophils, classified according to the pH-based affinity of their cytoplasmic granules towards the dyes. They are synthesized in the bone marrow. The MNC comprise of the monocytes and lymphocytes. The monocytes are circulatory phagocytes and are synthesized in the bone marrow. The lymphocytes are synthesized in the bone marrow and also in the lymph nodes and spleen. In mammals they reach the normal blood circulation via the lymphatic flow.The cells produced by the bone marrow are not all present in the circulation. Many leucocytes particularly polymorphonuclear cells are stored in the bone marrow reserve and are released when the need arises. While a significant portion of RBCs may be present in spleen. The spleen is the site for removal of old and senile RBC’s. There is always a balance in the body in production of RBC’s in the bone marrow and their removal from the circulation by the spleen. If destruction is more than the production then a clinical condition called splenic anemia is resulted. In case of acute loss of RBC’s, the bone marrow cannot compensate the required number of cells needed immediately. So, the spleen releases the stored RBC’s. Some immature nucleated RBC’s are also released in the circulation by the bone marrow.
Thrombocytes are another important cell of blood arising from a special cell of the bone marrow called the megakaryocytes. They are required for preventing the hemorrhages. In birds they appear as nucleated cells while in mammals they are small fragments of original cell.
The process of hematopoiesis is a complex well-orchestrated function of the body. It is tightly controlled by various signaling molecules and hormones. It is robust and can show tremendous regeneration even after prolonged injury to the bone marrow. At the same time this process is also sensitive to injury especially toxicity, inflammation, systemic inflammatory cytokines and space occupying lesions like cancers.
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