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, they appear as plate-like structures with no nuclei. The number of blood platelets ranges from 200-400 × 10⁹/L of blood. 

Each platelet has 2 main regions; a granulomere and a hyalomere. 

Granulomere is a basophilic zone, centrally located that contains granules which contain clotting factors and serotonin. Granulomere contains a small Golgi complex, elements of RER, a few scattered mitochondria, glycogen deposits, & lysosomes. 

Hyalomere, is pale blue and contains ribosomes, vacuoles, microtubules and filaments. These cytoskeletal elements help maintain the platelet shape and involved in movement. Actin and myosin in this region are involved in contraction during blood clot formation. 

The function of thrombocytes is clotting of blood and they have a lifetime of 10-12 days in the blood. Thrombocytosis is the increase of thrombocytes and thrombopenia is the decrease in thrombocytes. 

Thrombocytopoiesis occurs in bone marrow and begins with precursor stem cell hemocytoblast. This pluripotent stem cell differentiates into myeloid lineage which the thrombocyte belongs to. This myeloid stem cell then differentiates into a megakaryoblast, which is about 50μm in diameter and with a lobulated nucleus and many nucleoli. 

Megakaryoblast, is then transformed into a large megakaryocyte, varying from 30-100μm in diameter.

Megakaryocytes are cells where their cytoplasm is pinched off as blood platelets. platelets are formed from megakaryocytes in bone marrow.

By LM, the homogeneous cytoplasm is lightly basophilic because there are lots of free ribosomes and many small azurophilic granules. 

By EM, megakaryocytes contain a network of platelet demarcation channels. Platelets are formed by fragmentation of megakaryocytic cytoplasm along demarcation channels. This process is controlled and regulated by thrombopoietin which is produced by the kidneys.