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BLOOD

Dr. Emad I H Shaqoura, M.D, M.Sc.

Lecturer of Anatomy & Histology

Faculty of Medicine, Islamic University-Gaza

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BLOOD COMPOSITION

Blood is a specialized connective tissue in which the cells are suspended in fluid extracellular material called plasma.
Normal blood volume in an adult is about 5 Lit.
Centrifugation of a non-clotted blood sample results in separation of blood into three layers:
Erythrocytes: about 45% of the total blood volume in healthy adults, & is called the hematocrit.
Plasma: about 55% of the total blood volume.
Buffy coat: less than 1% of the total blood volume & composed of leukocytes & platelets.

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FIGURE 12-1

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BLOOD FUNCTIONS

Blood is a distributing vehicle, transporting O2, CO2, nutrients, metabolites, hormones, and other substances to cells throughout the body.
Blood also participates in heat distribution, the regulation of body temperature, and the maintenance of acid-base and osmotic balance.
Leukocytes are one of the body’s chief defenses against infection.

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FIGURE 12-2

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BLOOD COMPOSITION

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Blood

Plasma 55%

Formed Elements 45%

- Erythrocytes.

- WBCs.

- Platelets.

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COMPOSITION OF PLASMA

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Plasma

Water 92%

Plasma Proteins 7%

Other Solutes 1%

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COMPOSITION OF PLASMA

The major plasma proteins include the following:
Albumin, the most abundant plasma protein, is made in the liver and serves primarily to maintain the osmotic pressure of the blood.
α-Globulins and ß-globulins, made by liver and other cells, include transferrin and other transport factors; fibronectin; prothrombin and other coagulation factors; lipoproteins and other proteins entering blood from tissues.
γ-Globulins, which are immunoglobulins (antibodies) secreted by plasma cells in many locations.

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COMPOSITION OF PLASMA

Fibrinogen, the largest plasma protein, also made in the liver, which, during clotting, polymerizes as insoluble, cross-linked fibers of fibrin that block blood loss from small vessels.

Complement proteins, a system of factors important in inflammation and destruction of microorganisms.

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BLOOD CELLS

Blood cells can be studied histologically in smears.
Blood smears are routinely stained with special mixtures of acidic (eosin) and basic (methylene blue) dyes.
These mixtures may also contain dyes called azures that are more useful in staining cytoplasmic granules containing charged proteins and proteoglycans.
Azurophilic granules produce metachromasia in stained leukocytes.
Some of these special stains are Giemsa and Wright stain.

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FIGURE 12-3

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ERYTHROCYTES

Erythrocytes (red blood cells or RBCs) are terminally differentiated structures lacking nuclei and completely filled with hemoglobin.
RBCs are the only blood cells whose function does not require them to leave the vasculature.
Human erythrocytes suspended in an isotonic medium�are flexible biconcave discs.
They are approximately 7.5 μm in diameter, 2.6 μm thick at the rim, but only 0.75 μm thick in the center.

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FIGURE 12-4

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FIGURE 12-4

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ERYTHROCYTES

Because of their uniform diameters and their presence in most tissue sections, RBCs can often be used by histologists as an internal standard to estimate the size of other cells or structures.
The biconcave shape provides a large surface-to-volume ratio and facilitates gas exchange.
Being flexible, erythrocytes can bend and adapt to the irregular small diameters of capillaries frequently assuming a cup-like shape.
In larger blood vessels RBCs often adhere to one another loosely in stacks called rouleaux.

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FIGURE 12-4

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ERYTHROCYTES

The plasmalemma consists of about 40% lipid, 10% carbohydrate, and 50% protein (mostly integral membrane proteins), including ion channels, the anion transporter called band 3 protein, and glycophorin A.
The extracellular polysaccharides attached to the latter proteins include antigenic sites that form the basis for the ABO blood typing system.
Several peripheral proteins are associated with the inner surface of the membrane, including spectrin, dimers of which form a lattice bound to underlying actin filaments, and ankyrin, which anchors the lattice to the�glycophorins and band 3 proteins.
This submembranous meshwork stabilizes the membrane, maintains the cell shape, and provides the cell elasticity required for passage through capillaries.

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ERYTHROCYTES

Erythrocyte cytoplasm lacks all organelles but is densely filled with hemoglobin, that accounts for the cells’ uniform acidophilia.
Erythrocyte differentiation includes loss of the nucleus and organelles, shortly before the cells are released by bone marrow into the circulation.
Lacking mitochondria, erythrocytes rely on anaerobic glycolysis for their minimal energy needs.
Lacking nuclei, RBCs cannot replace defective proteins.

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ERYTHROCYTES

Human erythrocytes normally survive in the circulation for about 120 days.
By this time defects in the membrane’s cytoskeletal lattice or ion transport systems begin to produce swelling or other shape abnormalities, as well as changes in the cells’ surface oligosaccharide complexes.
Senescent or worn-out RBCs displaying such changes are removed from the circulation, mainly by macrophages of the spleen, liver, and bone marrow.

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MEDICAL APPLICATION

Anemia is a decreased concentration of erythrocytes below the normal range leading to inability of tissues to receive adequate O2.

Symptoms of anemia include: lethargy, shortness of breath, fatigue, skin pallor, and heart palpitations.
Anemia may result from: insufficient red cell production, e.g., due to iron deficiency, or from blood loss with a stomach ulcer or excessive menses.

Erythrocytosis, or polycythemia: is an increased concentration of erythrocytes in blood may be a physiologic adaptation as at high altitudes, where O2 tension is low.

Elevated hematocrit increases blood viscosity, putting strain on the heart, and, if severe, can impair circulation through the capillaries.

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MEDICAL APPLICATION

Sickle cell disease is caused by a mutation of one nucleotide (a point mutation) in the gene for the hemoglobin β chain.
When the altered hemoglobin (called HbS) is deoxygenated in capillaries,�it polymerizes and forms rigid aggregates that cause a characteristic sickle shape.
The sickled erythrocyte is less flexible and more fragile and has a shortened life span that can lead to anemia.
It increases the blood viscosity and can damage the wall of blood vessels, promoting blood coagulation.
Sickle cells can block capillaries, restricting O2 delivery to tissues and leading to varying degrees of ischemia or anoxia and organ damage.

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FIGURE 12-5

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LEUKOCYTES

Leukocytes (WBCs) leave the blood and migrate to the tissues where they become functional.
According to the type of cytoplasmic granules and their nuclear morphology, leukocytes are divided into: granulocytes and agranulocytes.
Both types are spherical in blood plasma, but they become amoeboid and motile after leaving the blood vessels and invading the tissues.
Their estimated sizes appear slightly larger than they are in the circulation.

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LEUKOCYTES

Granulocytes possess two major types of cytoplasmic granules:
Lysosomes (often called azurophilic granules in blood cells).
Specific granules that bind neutral, basic, or acidic stains and have specific functions.
Granulocytes have polymorphic nuclei with two or more distinct lobes and include the neutrophils, eosinophils, and basophils.
All granulocytes are terminally differentiated with a life span of only a few days.
Their Golgi complexes and rough ER are poorly developed.
They have few mitochondria and depend largely on glycolysis.
Granulocytes normally die by apoptosis in the connective tissue & the resulting cellular debris is removed by macrophages.

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LEUKOCYTES

Agranulocytes do not have specific granules, but they do contain azurophilic granules (lysosomes), with affinity for the basic stain azure A.
The nucleus is spherical or indented but not lobulated.
This group includes lymphocytes and monocytes.
All leukocytes are key players in the defense against invading microorganisms, and in the repair of injured tissues, specifically leaving the microvasculature in injured or infected tissues.
The number of leukocytes in the blood varies according to�age, sex, and physiologic conditions.

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FIGURE 12-6

Figure 12–6 Diagram of events involving leukocytes in a postcapillary venule at sites of inflammation.

Locations in connective tissue with injuries or infection require the rapid immigration of various leukocytes to initiate cellular events for tissue repair and removal of the invading microorganisms. The cytokines and cell binding proteins target various leukocytes and are best known for neutrophils. The major initial events of neutrophil migration during inflammation are summarized here:

1. Local macrophages activated by bacteria or tissue damage release proinflammatory cytokines such as interleukin-1 (IL-1) or tumor necrosis factor-α (TNF-α) that signal endothelial cells of nearby postcapillary venules to rapidly insert glycoprotein selectins on the luminal cell surfaces.

2. Passing neutrophils with appropriate cell surface glycoproteins bind the selectins, which causes such cells to adhere loosely to the endothelium and “roll” slowly along its surface.

3. Exposure to these and other cytokines causes expression of new integrins on the rolling leukocytes and expression of the integrin ligand ICAM-1 (intercellular adhesion molecule-1) on the endothelial cells. Junctional complexes between the endothelial cells are selectively downregulated, loosening these cells.

4. Integrins and their ligands provide firm endothelial adhesion of neutrophils to the endothelium, allowing the leukocytes to receive further stimulation from the local cytokines.

5. Neutrophils become motile, probe the endothelium with pseudopodia, and, being attracted by other local injury-related factors called chemokines, finally migrate by diapedesis between the loosened cells of the venule. Rapid transendothelial migration of neutrophils may be facilitated by the cells’ thin, segmented nuclei. All leukocytes first become functional in the ECM after emerging from the circulation by this process.

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NEUTROPHILS (PMNL)

Mature neutrophils constitute 50% - 70% of circulating WBCs; circulating immature forms raise this value by 3% to 5%.
Neutrophils are 12-15 μm in diameter in blood smears, with nuclei having two to five lobes linked by thin nuclear extensions.
In females, the inactive X chromosome may appear as a drumstick-like appendage on one of the lobes of the nucleus.
Neutrophils are inactive and spherical while circulating but become actively amoeboid during diapedesis and upon adhering to solid substrates such as collagen in the ECM.

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FIGURE 12-7

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NEUTROPHILS (PMNL)

Neutrophils are active phagocytes and are usually the first leukocytes to arrive at sites of infection, where they actively pursue bacterial cells using chemotaxis.
The cytoplasmic granules of neutrophils provide the cells’ functional activities and are of two main types:

Azurophilic primary granules.
Specific secondary granules.

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FIGURE 12-8

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NEUTROPHILS (PMNL)

Azurophilic primary granules or lysosomes are large, dense vesicles and have a major role in both killing and degrading engulfed microorganisms.

They contain proteases and antibacterial proteins, including the following:
Myeloperoxidase (MPO), which generates hypochlorite and other agents toxic to bacteria.
Lysozyme, which degrades components of bacterial cell walls.
Defensins, small cysteine-rich proteins that bind and disrupt the cell membranes of many types of bacteria.

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MEDICAL APPLICATION

Several kinds of neutrophil defects, often genetic in origin, can affect function of these cells, for example by:

decreasing adhesion to the wall of venules,
causing the absence of specific granules,
deficits in certain factors of the azurophilic granules.

Individuals with such disorders typically experience more frequent and more persistent bacterial infections, although macrophages and other leukocytes may substitute for certain neutrophil functions.

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NEUTROPHILS (PMNL)

Specific secondary granules are smaller and less dense, stain faintly pink, and have diverse functions, including secretion of various ECM-degrading enzymes such as collagenases, delivery of additional bactericidal proteins to the phagolysosomes, and insertion of new cell membrane components.
Activated neutrophils at infected or injured sites release chemokines that attract other leukocytes and cytokines that direct activities of these and local cells of the tissue.
Important lipid mediators of inflammation are also released from neutrophils.

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NEUTROPHILS (PMNL)

Neutrophils contain glycogen, which is broken down into glucose to yield energy via the glycolytic pathway.
The ability of neutrophils to survive in an anaerobic environment is highly advantageous.
Neutrophils are short-lived cells with a half-life of 6-8 hours in blood and a life span of 1-4 days in connective tissues before dying by apoptosis.

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MEDICAL APPLICATION

Neutrophils look for bacteria to engulf by pseudopodia and internalize them in vacuoles called phagosomes.
Immediately thereafter, specific granules fuse with and discharge their contents into the phagosomes which are then acidified by proton pumps.
Azurophilic granules then discharge their enzymes into this acidified vesicle, killing and digesting the engulfed microorganisms.
During phagocytosis, free oxygen radicals are formed resulting in powerful microbial killing system inside the neutrophils.

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MEDICAL APPLICATION

Lysozyme cleaves cell wall to kill certain bacteria.
Lactoferrin protein avidly binds iron, a crucial element in bacterial nutrition whose lack of availability then causes bacteria to die.
A combination of these mechanisms will kill most microorganisms, which are then digested by lysosomal enzymes.
Apoptotic neutrophils, bacteria, semi-digested material, and tissue fluid form a viscous, usually yellow collection of fluid called pus.��

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EOSINOPHILS

Eosinophils constitute only 1% to 4% of leukocytes.
In blood smears, this cell is the same size as a neutrophil or slightly larger.
They have a characteristic bilobed nucleus.
The main identifying characteristic is the abundance of large, acidophilic specific granules typically staining pink or red.

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EOSINOPHILS

Ultrastructurally the eosinophilic specific granules are oval in shape, with flattened crystalloid cores containing major basic proteins (MBP), an arginine-rich factor that accounts for the granule’s acidophilia and constitutes up to 50% of the total granule protein.

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EOSINOPHILS

Functions of eosinophils:
MBPs, along with eosinophilic peroxidase, other enzymes and toxins, act to kill parasitic worms or helminths.
Eosinophils also modulate inflammatory responses by releasing chemokines, cytokines, and lipid mediators, with an important role in the inflammatory response triggered by allergies.
They also remove antigen-antibody complexes from interstitial fluid by phagocytosis.

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MEDICAL APPLICATION

An increase in the number of eosinophils in blood (eosinophilia) is associated with allergic reactions and helminthic infections.
In patients with such conditions, eosinophils are found in the connective tissues underlying epithelia of the bronchi, GIT, uterus, and vagina, and surrounding any parasitic worms present.
In addition, these cells produce substances that modulate inflammation by inactivating the leukotrienes and histamine produced by other cells.
Corticosteroids produce a rapid decrease in the number of blood eosinophils, probably by interfering with their release from the bone marrow into the bloodstream.

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BASOPHILS

Basophils are also 12 -15 μm in diameter but make up less�than 1% of blood leukocytes and are therefore difficult to find in normal blood smears.
The nucleus is divided into two irregular lobes, & usually obscured by the overlying large specific granules.
The specific granules (0.5 μm in diameter) typically stain purple with the basic dye of blood smear stains and are fewer, larger, and more irregularly shaped than the granules of other granulocytes.

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FIGURE 12-10

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BASOPHILS

The strong basophilia of the granules is due to the presence of heparin and other sulfated GAGs.
Basophilic specific granules also contain histamine and other mediators of inflammation, including platelet activating factor, eosinophil chemotactic factor, and phospholipase A that catalyzes an initial step in producing lipid-derived proinflmmatory factors called leukotrienes.
Basophils migrate into connective tissues and supplement the functions of mast cells.
Both of them have metachromatic granules containing heparin and histamine, have surface receptors for IgE, and secrete their mediators in response to certain antigens and allergens.

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LYMPHOCYTES

Lymphocytes are the most numerous type of agranulocyte in normal blood smears or CBCs.
They have spherical nuclei.
Lymphocytes are typically the smallest leukocytes.
Although they are morphologically similar, lymphocytes can be subdivided into functional groups by distinctive surface molecules (called “cluster of differentiation” or CD markers) that can be distinguished using antibodies with immunocytochemistry.

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FIGURE 12-11

Figure 12–11 Lymphocytes.

Lymphocytes are agranulocytes and lack the specific granules characteristic of granulocytes. Lymphocytes circulating in blood generally range in size from 6 to 15 μm in diameter and are sometimes classified arbitrarily as small, medium, and large. (a) The most numerous small lymphocytes shown here are slightly larger than the neighboring erythrocytes and have only a thin rim of cytoplasm surrounding the spherical nucleus. X1500. Giemsa.

(b) Medium lymphocytes are distinctly larger than erythrocytes. X1500. Wright. (c) Large lymphocytes, much larger than erythrocytes, may represent activated cells that have returned to the circulation. X1500. Giemsa. (d) Ultrastructurally a medium-sized lymphocytes is seen to be mostly filled with a euchromatic nucleus (N) surrounded by cytoplasm containing mitochondria (M), free polysomes, and a few dark lysosomes (azurophilic granules). X22,000.

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LYMPHOCYTES

Major classes include:

B lymphocytes.
Helper and cytotoxic T lymphocytes (CD4+ & CD8+, respectively).
Natural killer (NK) cells.

These and other types of lymphocytes have diverse roles in immune defenses against invading microorganisms and certain parasites or abnormal cells.

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LYMPHOCYTES

Circulating lymphocytes have a wide range of sizes:

Small lymphocytes: newly released cells that have diameters similar to those of RBCs.
Medium lymphocytes.
Large lymphocytes:

represent activated lymphocytes or NK cells.

The small lymphocytes have spherical nuclei with highly condensed chromatin and only a thin surrounding rim of scant cytoplasm, making them easily distinguishable from granulocytes.

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9-18 μm in diameter

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LYMPHOCYTES

Larger lymphocytes have larger, slightly indented nuclei and more cytoplasm that is slightly basophilic, with a few azurophilic granules (lysosomes), mitochondria, free polysomes, and other organelles.
Lymphocytes vary in life span according to their specific functions; some live only a few days and others survive in the circulating blood or other tissues for many years.

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FIGURE 12-11

Figure 12–11 Lymphocytes.

Lymphocytes are agranulocytes and lack the specific granules characteristic of granulocytes. Lymphocytes circulating in blood generally range in size from 6 to 15 μm in diameter and are sometimes classified arbitrarily as small, medium, and large. (a) The most numerous small lymphocytes shown here are slightly larger than the neighboring erythrocytes and have only a thin rim of cytoplasm surrounding the spherical nucleus. X1500. Giemsa.

(b) Medium lymphocytes are distinctly larger than erythrocytes. X1500. Wright. (c) Large lymphocytes, much larger than erythrocytes, may represent activated cells that have returned to the circulation. X1500. Giemsa. (d) Ultrastructurally a medium-sized lymphocytes is seen to be mostly filled with a euchromatic nucleus (N) surrounded by cytoplasm containing mitochondria (M), free polysomes, and a few dark lysosomes (azurophilic granules). X22,000.

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MEDICAL APPLICATION

Lymphomas are a group of disorders involving neoplastic proliferation of lymphocytes or the failure of these cells to undergo apoptosis.

Although often slow-growing, all lymphomas are considered malignant because they can very easily become widely spread throughout the body.

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MONOCYTES

Monocytes are agranulocytes that are precursor cells of mononuclear phagocyte system.
All monocyte-derived cells are antigen-presenting cells and have important roles in immune defense of tissues.
Circulating monocytes have diameters of 12 to 15 μm, but macrophages are somewhat larger.
The nucleus is large and usually distinctly indented or C-shaped.
The chromatin is less condensed than in lymphocytes and typically stains lighter than that of large lymphocytes.

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MONOCYTES

The cytoplasm of the monocyte is basophilic and contains many azurophilic granules, some of which are at the limit of the light microscope’s resolution.
These granules are distributed through the cytoplasm, giving it a bluish-gray color in stained smears.
Mitochondria and small areas of rough ER are present, along with a Golgi apparatus involved in the formation of lysosomes.

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FIGURE 12-12

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MEDICAL APPLICATION

Extravasation of immigrating monocytes occurs in the early phase of inflammation following tissue injury.
Acute inflammation is usually short-lived as macrophages undergo apoptosis or leave the site, but chronic inflammation usually involves the continued recruitment of monocytes.
The resulting continuous presence of macrophages can lead to excessive tissue damage that is typical of chronic inflammation.��

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PLATELETS

Blood platelets (or thrombocytes) are very small non-nucleated, membrane-bound cell fragments only 2 to 4 μm in diameter.
Platelets originate from giant polyploid bone marrow cells called megakaryocytes.
Platelets promote blood clotting and help repair minor tears or leaks in the walls of small blood vessels, preventing blood loss.
Circulating platelets have a life span of about 10 days.

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PLATELETS

In blood smears, platelets often appear in clumps.
Each individual platelet is generally discoid, with a very lightly stained peripheral zone, the hyalomere, and a darker-staining central zone containing granules, called the granulomere.
A sparse glycocalyx surrounding the platelet plasmalemma is involved in adhesion and activation during blood coagulation.
Ultrastructural analysis reveals a peripheral marginal bundle of microtubules and microfilaments, which helps to maintain the platelet’s shape.

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FIGURE 12-13

Figure 12–13 Platelets.

Platelets are cell fragments 2-4 μm in diameter derived from megakaryocytes of bone marrow. Their primary function is to rapidly release the content of their granules upon contact with collagen (or other materials outside of the endothelium) to begin the process of clot formation and reduce blood loss from the vasculature.

(a) In a blood smear, platelets (arrows) are often found as aggregates. Individually they show a lightly stained hyalomere region surrounding a more darkly stained central granulomere containing membrane-enclosed granules. X1500. Wright.

(b) Ultrastructurally a platelet shows a system of microtubules and actin filaments near the periphery, called the marginal bundle (MB), which is formed as the platelet pinches off from megakaryocyte (Chapter 13), and helps maintain its shape. An open canalicular system (OCS) of invaginating membrane vesicles continuous with the plasmalemma facilitates rapid degranulation upon activation and Ca²⁺ release. The central granulomere region contains small dense delta granules (δG), larger and more numerous alpha granules (αG), and glycogen (G). X40,000.

(Figure 12-13b, with permission, from Dr M. J. G. Harrison, Middlesex Hospital and University College London, UK.)

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PLATELETS

Also in the hyalomere are two systems of membrane channels:
Open canalicular system of vesicles: connected to invaginations of the plasma membrane, which may facilitate platelets’ uptake of factors from plasma.
Dense tubular system: a much less prominent set of irregular tubular vesicles derived from the ER and stores Ca2 + ions.
Together, these two systems facilitate the extremely rapid exocytosis of proteins from platelets (degranulation) upon adhesion to collagen or other substrates outside the vascular endothelium.

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PLATELETS

The central granulomere has specific granules, and sparse mitochondria and glycogen particles.
Electron-dense delta granules (δG), 250-300 nm in diameter, contain ADP, ATP, and serotonin taken up from plasma.
Alpha granules (αG) are larger (300-500 nm in diameter) and contain platelet-derived growth factor (PDGF), platelet factor 4, and several other platelet-specific proteins.
Most of the stained granules seen in platelets with the light microscope are alpha granules.

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PLATELETS

The role of platelets in controlling hemorrhage and in wound healing can be summarized as follows:
Primary aggregation: Disruptions in the endothelium, which are very common, allow the platelet glycocalyx to adhere to collagen. Thus, a platelet plug is formed as a fist step to stop bleeding.
Secondary aggregation: Platelets in the plug release a specific adhesive glycoprotein and ADP, which induce further platelet aggregation and increase the size of the platelet plug.

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PLATELETS

Blood coagulation: During platelet aggregation, fibrinogen from plasma, von Willebrand factor and other proteins released from the damaged endothelium, and platelet factor 4 from platelet granules promote the sequential interaction (cascade) of plasma proteins, giving rise to a fibrin polymer that forms a three dimensional network of fibers trapping red blood cells and more platelets to form a blood clot, or thrombus. Platelet factor 4 is a chemokine for monocytes, neutrophils, and fibroblasts and proliferation of the fibroblasts is stimulated by PDGF.

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FIGURE 12-14

Figure 12–14 Platelet aggregation, degranulation, and fibrin clot formation.

Minor trauma to vessels of the microvasculature is a routine occurrence in active individuals and quickly results in a fibrin clot, shown here by SEM (a). Upon contact with collagen in the vascular basement membrane, platelets (P) aggregate, swell, and release factors that trigger formation of a fibrin meshwork (F) that traps erythrocytes (E) and more degranulating platelets. Platelets in various states of degranulation are shown. Such a clot grows until blood loss from the vasculature stops. After repair of the vessel wall, fibrin clots are removed by proteolysis due primarily to locally generated plasmin, a nonspecific protease. X4100.

(b) Platelets aggregate at the onset of clot formation. This TEM section shows platelets in a platelet plug adhering to collagen (C). Upon adhering to collagen, platelets are activated and their granules undergo exocytosis into the open canalicular system, which facilitates extremely rapid release of factors involved in blood coagulation. When their contents are completely released, the swollen degranulated platelets (arrows) remain as part of the aggregate until the clot is removed. Several other key proteins for blood coagulation are released locally from adjacent endothelial cell processes (EP) and from the plasma. Part of an erythrocyte (E) is seen at the right. X7500.

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FIGURE 12-14

Figure 12–14 Platelet aggregation, degranulation, and fibrin clot formation.

Minor trauma to vessels of the microvasculature is a routine occurrence in active individuals and quickly results in a fibrin clot, shown here by SEM (a). Upon contact with collagen in the vascular basement membrane, platelets (P) aggregate, swell, and release factors that trigger formation of a fibrin meshwork (F) that traps erythrocytes (E) and more degranulating platelets. Platelets in various states of degranulation are shown. Such a clot grows until blood loss from the vasculature stops. After repair of the vessel wall, fibrin clots are removed by proteolysis due primarily to locally generated plasmin, a nonspecific protease. X4100.

(b) Platelets aggregate at the onset of clot formation. This TEM section shows platelets in a platelet plug adhering to collagen (C). Upon adhering to collagen, platelets are activated and their granules undergo exocytosis into the open canalicular system, which facilitates extremely rapid release of factors involved in blood coagulation. When their contents are completely released, the swollen degranulated platelets (arrows) remain as part of the aggregate until the clot is removed. Several other key proteins for blood coagulation are released locally from adjacent endothelial cell processes (EP) and from the plasma. Part of an erythrocyte (E) is seen at the right. X7500.

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PLATELETS

Clot retraction: The clot that initially bulges into the blood vessel lumen contracts slightly because of the interaction of platelet actin and myosin.
Clot removal: Protected by the clot, the endothelium and surrounding tunic are restored by new tissue, and the clot is then removed, mainly dissolved by the proteolytic enzyme plasmin, formed continuously through the local action of plasminogen activators from the endothelium on plasminogen from plasma.

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MEDICAL APPLICATION

Aspirin and other non-steroidal anti-inflammatory agents have an inhibitory effect on platelet function and blood coagulation because they block the local prostaglandin synthesis that is needed for platelet aggregation, contraction, and exocytosis at sites of injury.
Bleeding disorders result from abnormally slow blood clotting.
One such disease directly related to a defect in the platelets is a rare autosomal recessive glycoprotein Ib deficiency, involving a�factor on the platelet surface needed to bind subendothelial collagen and begin the cascade of events leading to clot formation.

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