Figure 914

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, x 13,000. Left inset. Light micrograph showing an entire megakaryocyte from a marrow smear. Its nucleus is multilobed and folded on itself, giving an irreg ular 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, x 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 5 granule, and glycogen particles. For comparison, Figure 9.15a shows a mature circulating platelet, x30,000.

Membrane Systems open canalicular system dense tubular system glycogen fj

. « granules canalicular

.system liiiiilii

5 granule

dense tubular.5

system lines these channels arises by invagination of the plasma membrane; therefore, the channels are in continuity with the extracellular space. The continued development and fusion of the platelet demarcation membranes result in the complete partitioning of cytoplasmic fragments to form individual platelets. Upon entry into the vascular system from the bone marrow, the platelets circulate as discoid structures about 2 to 3 /xm in diameter. Their life span is about 10 days.

Structurally, platelets may be divided into four zones based on organization and function

The TEM reveals a structural organization of the platelet cytoplasm that can be categorized into four zones (Fig. 9.15):

• Peripheral zone. This zone consists of the cell membrane covered by a thick surface coat of glycocalyx. The gly-

cocalyx consists of glycoproteins, glycosaminoglycans, and several coagulation factors adsorbed from the plasma. The integral membrane glycoproteins function as receptors in platelet function.

• Structural zone. This zone consists of microtubules, actin filaments, myosin, and actin-binding proteins that form a network supporting the plasma membrane. Approximately 8 to 24 microtubules reside as a bundle immediately below the actin filament network. They are circumferentially arranged and responsible for maintaining the platelet's disk shape.

• Organelle zone. This 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 agramiles (300 to 500 nm in diameter), which contain mainly fibrinogen, coagulation factors, plasminogen, plasminogen activator inhibitor, and platelet-

microtubules mitochondrion s • ■

Peripheral Zone plasma membrane glycocalyx

Organelle Zone

A. granule (lysosome)

glycogen a granules mitochond 8 granules

Structural Zone microtubules actin myosin II

Membrane Systems open canalicular system dense tubular system

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