List of abbreviations
Every attempt was made to provide correct information and labelling, however any liability for eventual errors or incompleteness is rejected!
|detail 1: cross-sec-
tioned blood platelet
|detail 2: cytoplasm
|detail 3: cytoplasm
of one blood platelet
|detail 4: different
|detail 5: other
man blood platelets
|human blood platelets
showing long processes
|detail thereof||human blood
and longitudinal section
|vesicles of a human blood
|human blood platelet
in freeze fracture
|human blood platelet with
open canalicular system
and vesicles (freeze etching technique image)
|onset of aggregation of
human blood platelets
Platelets (thrombocytes; Terminologia histologica:
are no complete cells since they lack a nucleus.
The dish-like particles have a diameter of 2 - 4 µm
and aggregate to each other especially in case of a blood coagulation.
In normal human blood about 150,000
- 300,000 thrombocytes are present per mm³. If
the number is less we speak of a thrombocytopenia. The critical value is
about 40,000 since with lower values haemostasis is no longer ensured. The
of platelets is 5.7 - 8.9 femtolitres (µm³). The
(portion of the platelets related to the entire blood volume) is 13.7
- 26.9%; in children up to 45 % increasing with age. Thrombocytes are
demarked parts of cytoplasm from megakaryocytes
red bone marrow. Their average
life time is 5 - 10 days. They are removed from blood
by phagocytosis performed
by macrophages mainly of liver
(stellate macrophages, i.e. Kupffer
cells) and spleen. Platelets contain free
actin as well as a cytoskeleton
of actin filaments anchored to the
membrane. In this context filamin serves
to bind it to the cell membrane glycoprotein GPIb-IX
and talin for connection to integrin
alpha2b-beta3, the fibrinogen receptor.
Further, the cytoplasm shows mitochondria
of the crista-type and contains myosin, alpha-actinin
Granulomere (Terminologia histologica: Granulomerus) is the term for the central cytoplasm area of platelets with plenty of vesicles which in the light microscope appear to be granules therefore the incorrect term thrombocytic granules was established in the terminology (Terminologia histologica: Granula thrombocytica). The granulomere further contains some beta-glycogen granules. In Pappenheim's stain this area shows fine blue-purple azurophilic granules. However, the electron microscope without any doubt reveals membrane-bordered vesicles. The granulomere contains different kinds of vesicles with diameters about 200 nm which are classified in more or less electron-dense types:
alpha vesicles (Terminologia histologica: alpha granules; Granula alpha - should be corrected to alpha vesicles; Vesicula alpha). The less electron-dense alpha-vesicles are an inhomogeneous population of vesicles corresponding to lysosomes, peroxisomes and others which all contain factors for blood cell aggregation e.g., platelet factor 4 (antiheparin, a heparin binding chemokine), von-Willebrand factor (vWF), fibronectin, platelet derived growth factor (PDGF), thrombospondin, proaccelerin (factor V), factor VIII, beta-thromboglobuline, kallikrein, alpha2 antiplasmin and fibrinogen. The electron-dense vesicles are rare in human platelets, have a small excentrically located extremely dense core and contain serotonine, adenosin diphosphat (ADP), histamine,adrenaline, pyrophosphate, calcium ions (Ca++) and adenosin triphosphate (ATP). They are classified as
delta-vesicles (Terminologia histologica: delta granules; dense bodies; Granula deltae - should be corrected to delta vesicles; Vesicula deltae) which are more common or
lambda-vesicles (Terminologia histologica: lambda granules; Granula lambdae - should be corrected to lambda vesicles; Vesicula lambdae).
Hyalomere (Terminologia histologica: Hyalomerus) is the term for the region bordering the granulomere. The hyalomere contains slightly blue cytosol (in Pappenheim's stain) and a marginal microtubular bundle (Terminologia histologica: Fasciculus microtubularis marginalis) of 10 - 15 microtubules in circular orientation which is responsible for the discoid form of platelets. A protein-sugar layer, the glycocalyx 50 - 150 nm in thickness, is present on the surface membrane of platelets. A unique feature of thrombocytes is the open canalicular system (OCS; Terminologia histologica: Systema canaliculare apertum) with tubules about 50 nm in diameter consisting of tubules of invaginated cell membrane reaching deeply into the cell towards the centre of the granulomere. The tubules are similar to smooth endoplasmic reticulum but are in fact a cell membrane-bordered deep invaginations of the extracellular space 50 nm in diameter. Thus a tiny glycocalyx is present in its lumen. A second system of tubules with more electron-dense and less wide lumen termed dense tubular system (Terminologia histologica: Systema tubulare densum) is branching inside the thrombocytes as well. It corresponds to the remnants of the smooth endoplasmic reticulum of the megakaryocyte (from which the platelet originated). It concentrates calcium ions (Ca++) and thrombocyte specific peroxidase as well as enzymes for thromboxan-A2 synthesis in the lumen and plays an important role in activation of platelets.
An English page with further detailed information is available in the professional version of this atlas.
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Some images were kindly provided by PD Dr. Klinger; other images, page & copyright H. Jastrow.