Thrombocytes and Platelet Hemostasis

Comprehensive vocabulary flashcards focusing on platelet (thrombocyte) maturation, ultrastructure zones, functional characteristics, counting methods, and clinical disorders.

Thrombocytes

Also known as platelets; tiny cell fragments that play a chief role in hemostasis and are highly metabolically active despite their size.

Hemostasis

The cessation of bleeding when there is injury to the blood vessels.

Platelet Diameter

The size of a platelet is approximately $2-4\,\mu m$.

Thrombosis

A condition that occurs when a blood clot forms either in the artery or in the vein, potentially obstructing organs like the heart and brain.

Metamegakaryocyte

The final stage in the platelet maturation series before cell fragmentation into platelets, characterized by a demarcating membrane system.

Demarcating Membrane System

A system in the metamegakaryocyte that functions as the future membrane system for its offspring, the platelets.

Erythropoietin (EPO)

The humoral factor or substance that regulates the production and maturation of Red Blood Cells (RBCs).

Renal Erythropoietic Factor (REF)

The factor produced by the kidneys in response to hypoxia that induces the release of Erythropoietin (EPO).

Hypoxia

A state of low oxygen, often existing in higher altitudes, that induces the kidneys to produce Renal Erythropoietic Factor (REF).

Thrombopoietin (TPO)

A specific hormone factor responsible for the commitment of stem cells to the megakaryocytic cell line and controlling platelet production.

Osteoclast

A cell found in the bone marrow involved in bone resorption that can sometimes be confused with young megakaryocytic cells.

Osteoblast

A cell found in the bone marrow involved in bone formation.

Circulating Pool

The portion of platelets found in circulation, accounting for $2/3$ of the total platelet number.

Splenic Pool

The portion of platelets found in the spleen, accounting for $1/3$ of the total, where old platelets undergo sequestration.

Platelet Lifespan

The duration platelets live before being destroyed, which is approximately $9-11$ days.

Adhesion

A reversible functional characteristic where platelets roll and cling to non-platelet surfaces like subendothelial collagen; requires Von Willebrand Factor (VWF).

Aggregation

An irreversible process where platelets adhere to each other to form platelet plugs; requires fibrinogen and receptors like GPIIb/IIIa.

Secretion

An irreversible functional characteristic where platelets discharge the contents of their granules; essential for coagulation.

$\alpha$-granules

Large platelet granules measuring $300-500\,nm$ containing proteins like VWF, Fibrinogen, and Platelet Factor 4.

Dense Granules

Also called $\beta$-granules; smaller granules measuring $250-350\,nm$ containing ADP, ATP, calcium, and serotonin.

Afibrinogenemia

A condition characterized by the depletion or total absence of fibrinogen (Plasma Factor I).

Platelet Factor 4 (Pf 4)

A platelet protein that acts as a heparin inhibitor, counteracting the effects of anticoagulants.

Serotonin

A substance found in platelet dense granules that acts as a vasoconstrictor.

Peripheral Zone

The outer layer of the platelet consisting of the glycocalyx and plasma membrane.

Glycocalyx

The thickest surface coat ($10-50\,nm$) of the platelet, containing glycoproteins that facilitate adhesion and aggregation.

Structural Zone

Also known as the sol-gel zone; the area containing microtubules and microfilaments that form the muscle and skeletal portion of the platelet.

Microtubules

Part of the structural zone that governs and influences the extent of the platelet contractile response.

Microfilaments

Filaments approximately $50\,nm$ in diameter containing actin and myosin that provide the contractile force after activation.

Organelle Zone

The major portion of the platelet cytoplasm containing granules, lysosomes, and mitochondria (the metabolic center).

Platelet Membrane System

An ultrastructural area divided into the canalicular system and the dense tubular system.

Platelet Factor 3

Also known as phosphatidylserine; contains Arachidonic Acid and serves as a coagulation factor.

Thromboxane

A substance released by platelets that enhances platelet aggregation.

Prostacyclin ($PGI_2$)

A prostaglandin that inhibits platelet aggregation and acts as a potent vasodilator.

Viscous Metamorphosis

An irreversible process where platelets lose their individuality and form large, hyaline-like clumps due to biochemical and morphologic changes.

Fonio Method

An indirect counting method using $14\%\,MgSO_4$ (Magnesium sulfate) to prevent platelet adhesion to glass during the procedure.

Rees and Ecker Method

The universal direct counting method of choice, using a diluting fluid containing Sodium Citrate, Formaldehyde, and Brilliant Cresyl Blue.

Brecher and Cronkite Method

A direct counting method that utilizes a phase contrast microscope and $1\%$ ammonium oxalate.

Thrombocytosis

A quantitative abnormality characterized by an increase in the platelet count, which can be primary or secondary.

Thrombocytopenia

A quantitative abnormality characterized by a decrease in the platelet count due to production defects, loss, or splenic sequestration.

Thrombocytopathy

A qualitative defect where platelets are normal in number but exhibit functional abnormalities.

Glanzmann's Thrombasthenia

An inherited aggregation defect where platelets fail to stick to each other despite being able to adhere to surfaces.

Thorn Test

An eosinophil response test measuring adrenal cortical function based on the decrease in eosinophil count after ACTH injection.

Hypoadrenalism

Indicated in the Thorn test when the eosinophil count decrease is $20\%$ or less after four hours.