Biology, embryology, histopathology, and pathology of the tissues that constitute multicellular organisms.


Blood is a highly specialized tissue produced in the bone marrow in a process called hematopoiesis. Blood contains red blood cells (a=erythrocytes) and white cells (leukocytes, b=neutrophil, c=eosinophil, d=lymphocyte) circulating in plasma accompanied by platelets, plasma proteins, and other dissolved substances.

[] Hemopoiesis [] Hematopoiesis - high-resolution version (837 KB) [] tem basophil [] tem eosinophil [] tem eosinophil [] tem small lymphocyte [] tem monocyte [] tem neutrophil engulfing Candida []

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bone marrow

Hematopoiesis (production of blood) occurs in the bone marrow, which is located within the hollow interior of bones, predominantly in flat bones and long bones. []Gray's, labeled photomicrograph, diagram []

The tissue within bone marrow is differentiated as 'red' marrow (erythropoiesis, thrombopoiesis, production of WBC) and 'yellow' marrow, which contains fat cells. The proportion of 'yellow marrow' increases with the accumulation of fat cells during the individual's life span, though reversion to red marrow is possible in response to blood loss.

normal [] bone marrow section [] bone marrow section 2 [] bone marrow smear []

In addition to hematopoietic stem cells (HSC), mesenchymal stem cells (hMSC) are located around the central sinus of the bone marrow and have the potential to differentiate into osteoblasts, chondrocytes, myocytes, and other cell types. Bone marrow stem cells have also been demonstrated to aid in the regeneration of adipocytes, hepatocytes, lung, alimentary canal, skin, and endothelial cells. Side population cells (SP's) are believed to be present in skeletal muscle, and thus scientists hypothesize that SP's express plasticity by contributing to hematopoiesis as well as skeletal muscle tissue.

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fibrinolytic system

The fibrinolytic system dissolves fibrin blood clots, acting in reverse to the coagulation system.

ε-aminocaproic acid : antifibrinolytics : fibrin clots : fibrin degradation products : fibrinolysis : kringle domains : PAI-1, PAI-2 : plasmin : plasminogen : plasminogen activator inhibitors : serine proteases : serpins : thrombolytic agents : tissue plasminogen activator (PLAT, tPA) : tranexamic acid : uPA : urokinase : urokinase receptor : zymogen

Plasminogen, the inactive, zymogen form of plasmin, is incorporated into fibrin clots as they form. Cleavage of plasminogen by tissue plasminogen activator (PLAT, tPA) and urokinase converts plasminogen to the active serine protease plasmin form. Acting as a serine protease, the kringle domains of plasmin bind to arginine and lysine residues and cleave fibrin into fibrin degradation products (fibrinolysis).

Tissue plasminogen activator (PLAT, tPA) is a serine protease secreted by cells of the arteriolar endothelium. Urokinase, also termed urokinase-type plasminogen activator (uPA), is also a serine protease that contains a serine protease domain, a kringle domain, and a growth factor domain. The serpins, plasminogen activator inhibitor-1 (PAI-1) and plasminogen activator inhibitor-2 (PAI-2) irreversibly inhibit the protease (peptidase) activity of tPA and uPA.

In the extracellular matrix, urokinase binds to the urokinase receptor, tethering urokinase to the cell membrane. Through its interaction with the urokinase receptor, urokinase participates in cell adhesion, migration, and cellular mitotic pathways. It appears that tissue degradation following plasminogen activation facilitates tissue invasion and contributes to establishment of tumor metastasis, making urokinase an attractive potential target for anticancer inhibitors. Urokinase is employed as a thrombolytic agent in the treatment of deep venous thrombosis (DVT) and pulmonary embolism (PE). Both urokinase and recombinant tissue plasminogen activator (PLAT, tPA) are employed in treatment of myocardial infarction (MI), and recombinant PLAT is used in treatment of acute stroke (CVA, cerebrovascular accident).

Conversely, antifibrinolytics, such as aminocaproic acid (ε-aminocaproic acid) and the more potent tranexamic acid are employed as inhibitors of fibrinolysis. They act by blocking the lysine-binding site in the kringle domains on plasminogen, and are employed in treatment of menorrhagia, excessive post-operative bleeding, and bleeding dyscrasias.



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