21. Blood platelets (thrombocytes) - structural, ultrastructural and functional characteristics. Thrombocytopoiesis.

Blood platelets are the smallest cells found in the peripheral blood, with a diameter of __________.

3-4μm

The number of blood platelets ranges from __________ of blood.

200-400 × 10^9/L

Each platelet has __________ main regions.

2

The central region of a platelet, known as __________, contains granules with clotting factors and serotonin.

granulomere

The pale blue region of a platelet, known as __________, contains ribosomes and microtubules.

hyalomere

The function of thrombocytes is __________ of blood.

clotting

The average lifetime of thrombocytes in the blood is __________ days.

10-12

Thrombocytosis refers to the __________ of thrombocytes.

increase

Thrombopenia refers to the __________ in thrombocytes.

decrease

Thrombocytopoiesis occurs in __________.

bone marrow

The precursor stem cell for thrombocytes is called __________.

hemocytoblast

The myeloid stem cell differentiates into a __________, which has a lobulated nucleus.

megakaryoblast

A megakaryoblast transforms into a large __________, from which blood platelets are formed.

megakaryocyte

Megakaryocytes vary from __________ in diameter.

30-100μm

Platelets are formed by __________ of megakaryocytic cytoplasm along demarcation channels.

fragmentation

The process of thrombocytopoiesis is regulated by __________, produced by the kidneys.

thrombopoietin

platelets

• smallest cells in the peripheral blood

• 3-4um

• plate-like

• no nucleus

• 200-400 × 10⁹/L of blood

• 2 main regions granulomere and hyalomere

granulomere

• basophilic zone

• centrally located conatining granules which have clotting factors and serotonin

• has small Golgi complex, RER, mitochondria, glycogen deposits, lysosomes

hyalomere

• pale blue with ribosomes, vacuoles, microtubules, filaments

• cytoskeletal elements help maintain the platelet shape involved in movement

• actin and myosin present here are involved in contraction during blood clotting

thrombocytes function

• clotting of blood

• lifespan of 10-12 days

• thrombopenia = decrease

• thrombocytosis = increase

thrombocytopoiesis

• occurs in the bone marrow begins with precursor stem cell hemocytoblast

• pluripotent stem cell differentiates into myeloid lineage which the thrombocyte belongs to

• myeloid stem cell differentiates into a megakaryoblast, 50um in diameter with lobulated nucleus and many nucleoli

• megakaryoblast becomes a megakaryocyte 30-100um

• megakaryocytes are cells where cytoplasm is pinched off as blood platelets which are in the bone marrow

megakaryocytes by LM

• homogenous cytoplasm lightly basophilic

• many free ribosomes

• many small azurophilic granules

megakaryocytes by EM

• contain a network of platelet demarcation channels

• platelets formed by fragmentation of megakaryocytic cytoplasm along demarcation channels, process is controlled and regulated by thrombopoietin which is produced by the kidneys