Lecture #11: The Thrombocytes

Cells of the megakaryocytic system are peculiar in that the nucleus undergoes multiple mitotic divisions without cytoplasmic separation, thus producing giant polyploid cells. All the nuclei in a given cell undergo mitosis at the same time producing 2,4,8 and in rare instances 16 or 32 nuclei. The multiple nuclei usually remain attached to each other are often superimposed giving a lobular appearance. The dividing nuclei maintain the distinct linear chromatin pattern of young cells while the cytoplasm undergoes maturation changes characterized by the development of granules and membranes, culminating in platelet differentiation and liberation.

Well–defined platelet masses usually appear at the margin of megakaryocytes in the 4 – 8 nucleated stages of development, but in some cells, platelets form in cells with single or double nuclei.

Megakaryoblast – are large, irregularly shaped cells with a single or several round or oval nuclei and with a blue, non–granular cytoplasm. There may be blunt pseudopods which stain various shades of blue and which may contain multiple chromophobic globules. The nuclear chromatin strands in megakaryoblasts are distinct. Nucleoli usually are demonstrable.

Promegakaryocyte – differ from megakaryoblasts in that there are bluish granules in the cytoplasm adjacent to the nucleus. The nucleus in this second stage of maturation has usually divided one or more times and the cell has increased size.

Megakaryocyte (megakaryocyte without thrombocyte) – megakaryocytic cells in the third sage of maturation are large cells with relatively large amounts of cytoplasm, round shapes, even margins and multiple nuclei. The chromatin pattern of the nuclei is linear and coarse with distinct spaces between the chromatin strands. The cytoplasm contains numerous small, rather uniformly distributed granules which have reddish – blue hue.

Metamegakaryocyte (megakaryocyte without thrombocyte) – megakaryocytic cells in the fourth stage of maturation are characterized by the aggregation of granular cytoplasmic material into masses which are separated from each other by relatively clear spaces (demarcation membrane vesicles). These units of granular cytoplasm tend aggregate near the periphery of the cell. Megakaryocytes in the more advanced stages of maturation are slowly amoeboid. They extend portions of their cytoplasm through the basement membranes and between the endothelial cells of the sinusoids of the bone marrow. From these cytoplasmic protrusions, the differentiated and membrane–bond platelets separate and are swept into the flowing blood stream.

Thrombocyte (Platelet) – thrombocytes are derived from the selective fragmentation of the cytoplasm of mature megakaryocytes. The distinctive feature is that it has no nucleus, a feature matched only by mature erythrocyte. Normal platelets vary in diameter from 2 – 5 micrometers. As a rule, thrmobocytes have multiple pointed filaments or tentacle like protrusions. Round, oval, spindle and discoid shapes with smooth margins are also observed. On morphological examination, the cytoplasm stains a light blue color and contains variable numbers of small blue granules which tend to aggregate in the center – called the granulomere or chronomere as contrasted with marginal zone which is hyaline with sky blue color called hyalomere. The border is irregular.

Morphology of thrombocytes

1. Wet preparations

Thrombocytes appear colorless, moderately refractile bodies that are discoid or elliptical in shape. Under darkfield illumination, they are translucent and reveal a sharp contour. A few immobile granules are present in the center of the cell. When observed by phase contrast microscopy, contractile vacuoles and vacuoles of pinocytosis (drinking by cell) have been noted.

2. Stained preparations

In smear stained by Romanowsky dyes, they appear round, oval or rod–shaped. Azurophilic granules are seen in hyaline, light blue cytoplasm. Platelets tend to adhere to each other. Individual platelets and clumps of platelets are most numerous at the distal (feather) ends of blood smear.

Life span of thrombocytes

Platelet survives for 8 – 11 days in the vascular system. The normal platelet count is

                        150,000 – 400,000 or 150 – 400 x 10⁹/L

Chemistry of thrombocytes

More than 80 enzymes have been associated with platelet. It is composed of 60% protein, 15% lipids and 8% carbohydrates – mainly glycogen and sulfate mucopolysaccharides.

Platelet proteins include thrombosthenin, fibrinogen and some coagulation factors found in the plasma.

Amino acids and enzymes include glutamic acid, aspartic acid and alkaline phosphatases and transaminases.

Lipid includes phosphatidylethanol amine and phosphatidylserine, potassium, 5–hydroxytryptamine (serotonin) and the granules present are chiefly polysaccharide.

As soon as platelet contacts a wetable foreign surface, their hyalomere spreads over the platelet granules aggregate to form a loose mass. This physiological property is termed “viscous metamorphosis.”

Functions of platelets

1. Endothelial support – platelet act to maintain or support the integrity (leak–free state) of the blood vessel by attaching to gaps which normally develop in the endothelium.

2. Aid in stopping flow from injured blood vessels by forming aggregates or platelet thrombi which mechanically plug the ends or holes in blood vessels and aid in closing the wound.

3. Play an important role in the initiation of clot formation, accelerate the clotting process in all stages and participate in clot retraction.

4. Favor hemostasis by liberating serotonin, a vaso–constricting factor.

5. Help in the localization of bacteria and other small objects by adhering to these objects and producing aggregates too large to pass through capillaries.

The platelet role in hemostasis

Various proteins or lipoprotein substances derived from the platelet have come to be designated as platelet “factors” because of their apparent function in blood coagulation. These are designated by Arabic numerals, in distinction to the Roman numerals used in referring to the coagulation factors.

Platelet factors                    Platelet factor activity

                                                 Theorized action

PF₁                               Accelerates the conversion of prothrombin to thrombin

PF₂                               Accelerates the clotting of purified fibrinogen by thrombin

PF₃                               Phospholipid needed in the intrinsic coagulation pathway

PF₄                               Antiheparin

PF₅                               Believed to be necessary for normal fibrin formation

PF₆                               Antifibrinolysis

PF₇                               Believed to be necessary in the formation of intrinsic

                                                thromboplastin

Note:   Most of the platelet factors are of minor consequence except PF₂,PF₃ and PF₄

Platelet disorders 

1. Quantitative platelet disorder

a. Thrombocytosis or thrombocythemia – increase in platelet count

b. Thrombocytopenia – decrease in platelet count

2. Inherited disorders of platelet function

a. Surface membrane abnormalities

b. Storage granule abnormalities

c. Deficiencies of thromboxane generation

3. Acquired disorder of platelet function

Patients with previously normal hemostasis may acquire a variety of disorders of platelet function. The most frequent cause is ingestion of drugs having inhibitory effects upon platelets, but a number of other disease states may also adversely affect platelet function.