Blood Platelets (or thrombocytes)

Blood Platelets (or thrombocytes)

Blood platelets do not contain a nucleus. Unlike erythrocytes, which also lack a nucleus, the blood platelets of mammals have never been nucleated cells. Instead, blood platelets are fragments of the cytoplasm of very large thrombocyte precursor cells, the megakaryocytes. Like other cells involved in the formation in blood cells, megakaryocytes are found in the bone marrow.

Platelets are about 3 µm long but appear somewhat smaller in the microscope. This is because their cytoplasm is divided into two zones: and outer hyalomere, which hardly stains, and an inner granulomere, which contains bluish staining granules. These granules are usually not individually visible with the highest magnification on your microscope, and the granulomere appears more or less homogeneously blue. In addition to different types of vesicles (i.e. the granules), mitochondria, ribosomes, lysosomes and a little ER are present in the thrombocyte granulomere. Different types of vesicles contain either serotonin (electron-dense delta granules; few) or compounds important for blood coagulation (alpha granules; they also contain platelet-derived growth factor (PDGF) which may play a role in the repair of damaged tissue). The hyalomere contains cytoskeletal fibres, which include actin and myosin.

Functions

Platelets assist in haemostasis, the arrest of bleeding. Serotonin is a potent vasoconstrictor. The release of serotonin from thrombocytes, which adhere to the walls of a damaged vessels, is sufficient to close even small arteries. Platelets, which come into contact with collagenous fibers in the walls of the vessel (which are not usually exposed to the blood stream), swell, become "sticky" and activate other platelets to undergo the same transformation. This cascade of events results in the formation of a platelet plug (or platelet thrombus). Finally, activating substances are released from the damaged vessel walls and from the platelets. These substances mediate the conversion of the plasma protein prothrombin into thrombin. Thrombin catalyzes the conversion of fibrinogen into fibrin, which polymerizes into fibrils and forms a fibrous net in the arising blood clot. Platelets captured in the fibrin net contract leading to clot retraction, which further assists in haemostasis.

Blood coagulation is a fairly complex process, which involves a large number of other proteins and messenger substances. Deficiencies in any one of them, either inherited or acquired, will lead to an impairment of haemostasis.

Suitable Slides

blood platelets (thrombocytes)
megakaryoblasts and megakaryocytes

Blood Smear, human - Leishman stain In lightly stained smears (B1), blood platelets will appear like light blue, fairly ill-defined specks between the other blood cells. In darker smears (B2), you will sometimes be able to see that the blue specks are formed by an accumulation of small bluish grains, the granules of the blood platelets. Identify and include a platelet in one of your other drawings. Red Bone Marrow, rabbit - H&E The marrow cavity of this bone is filled with red bone marrow. H&E is not the method of choice for looking at haemopoietic cells, but a few of the numerous named types or broader groups can actually be recognized. Precursors of platelets are the haemopoietic cells easiest to find in red bone marrow. The very dark and large megakaryoblast and the even larger but light megakaryocytes are clearly visible even at low magnifications. Note that adipocytes are present also in the red bone marrow. Identify and draw a megakaryocyte and megakaryoblast.