FIGURE 10.13 • Electron and light micrographs of a megakaryocyte. This electron micrograph shows a portion of a megakaryocyte from a bone marrow section. Two lobes of the nucleus and the surrounding cytoplasm are visible. The cell border is indicated by the dotted line (upper right). The cytoplasm reveals evidence of platelet formation as indicated by the extensive platelet demarcation channels. 13,000. Left inset. Light micrograph showing an entire megakaryocyte from a marrow smear. Its nucleus is multilobed and folded on itself, giving an irregular outline. The “foamy” peripheral cytoplasm of the megakaryocyte represents areas in which segmentation to form platelets is occurring. The smaller surrounding cells are developing blood cells. 1,000. Right inset. Higher-power electron micrograph showing a section of cytoplasm that is almost fully partitioned by platelet demarcation channels (arrows). It also shows mitochondria (M), a very dense  granule, and glycogen particles. For comparison, Figure 10.14a shows a mature circulating platelet. 30,000.
several coagulation factors adsorbed from the plasma. The integral membrane glycoproteins function as receptors in platelet function.
 The structural zone comprises microtubules, actin filaments, myosin, and actin-binding proteins that form a network supporting the plasma membrane. From 8 to 24 microtubules reside as a bundle immediately below the actin filament network. They are circumferentially arranged and are responsible for maintaining the platelet’s disc shape.
 The organelle zone occupies the center of the platelet. It consists of mitochondria, peroxisomes, glycogen particles, and at least three types of granules dispersed within the cytoplasm. The most numerous granules are  granules (300 to 500 nm in diameter) that contain mainly fibrinogen, coagulation factors, plasminogen, plasminogen activator inhibitor, and platelet-derived growth factor. The contents of these granules play an important role in the initial phase of vessel repair, blood coagulation, and platelet aggregation. The smaller, denser, and less numerous  granules mainly contain adenosine diphosphate (ADP), adenosine triphosphate (ATP), serotonin, and histamine, which facilitate platelet adhesion and vasoconstriction in the area of the injured vessel. The  granules are similar to lysosomes found in other cells and contain several hydrolytic enzymes. The contents of granules function in clot resorption during the later stages of vessel repair.
 The membrane zone consists of two types of membrane channels. The open canalicular system (OCS) is the first type of membrane channel. The OCS is a developmental remnant of the platelet demarcation channels and is simply a membrane that did not participate in subdividing the megakaryocyte cytoplasm. In effect, open canaliculi are invaginations into the cytoplasm from the plasma membrane. The dense tubular system (DTS) is the
FIGURE 10.14 • Platelet electron micrograph and diagram. a. High-magnification electron micrograph of a platelet situated between an erythrocyte on the left and an endothelial cell on the right. Visible organelles include a mitochondrion, microtubules, a single profile of the surface-connected open canalicular system, profiles of the dense tubular system, the moderately dense  granules, a single very dense  granule, and glycogen particles. The microfilaments are not evident against the background matrix of the platelet. b. Diagram of a platelet showing the components of the four structural zones.