lecture 10 Flashcards
what are platelets? are they true cells?
- not true cells
- cytoplasmic fragments of giant cells called megakaryocytes
platelets and megakaryocytes do not contain a nucleus
(T/F)
false,
platelets have no nucleus while megakaryocytes have a huge nucleus
why are the megakaryocytic cells unsual?
- their nuclei can undergo several mitotic divisions without cytoplasmic divisions
- generating giant multinucleated or polyploid cells
what are the characteristics of the multiple nuclei & the chromosomes in a megakaryocytic cell?
- they usually remain attached to each other
- often are superimposed
- give a multilobulated appearance
- multiple nb of chromosomes (polyploid)
- multiple of the original 2 sets of chromosomes (2n)
the divison that the megakaryocitic cells undergo is also referred to as:
endomitosis or endoreduplication
what are the different stages in megakaryopoeisis?
(shu asamiyon lal shramit)
- megakaryoblast
- promegakaryocyte
- megakaryocyte (mature)
what is the megakaryoblast?
what are the characteristics of its nucleus, cytoplasm…
d=?
- earliest recognizable cell
- large (d=15-50 micro m)
- irregularly shaped
- single nucleus or severel round/oval nuclei
- blue, non-granular cytoplasm
- may have bluish pseudopods or protrusions from the cytoplasmic membrane
what is the promegakaryocyte?
what are the characteristics of its nucleus, cytoplasm…
d=?
- nucleus has divided 1 or more times
- cell has increased in size (d=20-80 micro)
- cytoplasmic protrusions
- cytoplasm has bluish granules
- cytoplasm rich in RNA –> basophilic
with maturation. cytoplasmic basophilia increases
(T/F)
false, basophilia fades and cytoplasm becomes filled with reddish-purple granules
what is the mature megakaryocyte?
what are the characteristics of its nucleus, cytoplasm…
- huge cell (d= ~150 micro)
- segmented nucleus
- dense nuclear chromatin
- cytoplasm appears pinkish
- highly granular
- abundant
- irregular peripheral border (shape)
the megakaryoblast has few bluish granules
(T/F)
false, its non-granulated
how are platelets formed/ produced?
- platelets are produced directly from the megakaryocyte cytoplasm
- megakaryocyte puts out pseudopods into the BM sinosoids
- detach and fragment into individual platelets
what happens to the cytoplasm and nucleus of the megakaryocyte after the production of platelets?
- entire cytoplasm is broken away
- leaving the nucleus naken in the BM
- nucleus without cytoplasm will not survive
- nucleus will undergo degeneration
- nucleus will be taken by the macrophages
how many platelets are produced by megakaryocytes?
between 2000-4000 platelets
in what case do we have higher production of platelets?
(shu lezim ykun fiyo l megakaryocyte aktar)
- the more nuclear lobes
- the larger the cytoplasmic mass
- the more platelets are produced
what stimulates platelet production?
thrombopoeitin
what happens to the megakaryocytes when thrombopoiesis is activated?
- nb & size of megakaryocytes increase
- maturation time decrease
- younger & larger platelets are seen in PB
- they are metabolically more active and effective in hemostasis
how long is the platelet’s life span?
in circulation
9-11 days
after circulating, where do platelets go to?
spleen
are all platelets trapped in the spleen?
no, only 30%
the remaining 70% circulate
platelets in the spleen are interchangeable with those found in the blood
(T/F)
true
platelet structure:
(shape-color-diameter-consist of..) BALA L MEMBRANE W CYTOPLASM
- discoid structure
- d=1-3 micro m
- appears light blue to purple in color
- very granular
- consists of 2 parts: chromomere & hyalomere
what is the chromomere?
- part of a plaetelet
- granular
- located centrally
what is the hyalomere?
- part of a platelet
- agranular
- surrounds the chromomere
- light blue
platelet membrane comes from:
the plasma membrane of megakaryocyte
platelet membrane characteristics + constituents:
briefly
- has a trilaminar unit structure (3 layers)
- surface coat –> glycocalyx
- membrane & glycocalyx are smooth
- contain pore-like indentations
- nb of glycoproteins
- contractile protein (thrombosthenin)
- phospholipid constituents & other fatty acid pools
why does the platelet membrane contain pore-like indentations?
opens communication channels between the platelet cytoplasm and its surroundings
glycoproteins found on the platelet membrane are important for:
- adhesion
- aggregation
- coagulation (provides a surface to which coagulation factors may adhere)
contractile protein found on the platelet membrane is also called:
thrombosthenin
thrombosthenin is important for:
- maintain platelet shape
- responsible for changes in platelet shapes
phospholipid constituents & other fattu acid pools found on the platelet membrane are required for:
fattu acid metabolism
platelet cytoplasm constituents:
briefly
- few mitochondria
- glycogen deposits
- 3 types of granules
what are the 3 types of granules present inside the platelets cytoplasm?
only their names
- alpha granules
- electron dense granules
- lysosomes
what are the alpha granules rich in?
various substances of biological importance
* clotting factors (fibrinogen, factor V)
* PF4
* PDGF (platelet derived growth factor)
what are the electron dense granules rich in?
- Ca2+
- Mg2+
- serotonin (contracts blood vessels)
- ADP
- ATP
what are the lysosomes rich in?
digestive enzymes
platelets are not capable of phagocytosis
(T/F)
false, they are capable
they contain lysosomes rich in digestive enzymes (microphagocytosis)
what is hemostasis?
process by which bleeding from an injured blood vessel is controlled and stopped
what factors are involved in hemostasis?
- blood vessels
- platelets
- coagulation factors
hemostasis occurs in 2 phases, what are they:
- primary hemostasis: involves the vascular and platelet response to vessel injury
- secondary hemostasis: includes the response of coagulation process leading to the formation of a stable fibrin - platelet plug
=> vessel healing
what are the basic events in primary & secondary hemostasis after vessel injury?
briefly name the steps
- vasoconstriction
- platelet adhesion
- platelet aggregation
- fibrin-platelet plug formation
what happens during the 1st event of hemostasis?
what causes it?
- vasoconstriction
* first response
* damaged blood vessel constricts
* decreasing the blood flow through the injured area
* serotonin causes vasoconstriction
what happens during the 2nd event of hemostasis?
briefly
- platelet adhesion
* when there is an injury, platelets adhere to exposed collagen
* activation & change in platelet shape (round to spiny)
* series of biochemical reactions take place
in the circulation platelets appear as a relatively active cell
(T/F)
false, inactive
show little tendency to interact with other platelets/ endothelial cells
platelet surface contains receptors for a nb of substances, important among these is the substance called:
collagen
when do platelets change shape from round to spiny?
following pseudopodia formation
what is the exposed collagen?
fibrous tissue underneath the endothelium
what are the series of biochemical reactions that take place during platelet adherence?
- when platelet is activated, membrane phospholipases are activated
- in turn liberate arachidonic acid (fatty acid) from platelet phospholipid stores
- enzyme cyclooxygenase acts on arachidonic acid to liberate thromboxane A2
- thromboxane A2 produced –> stimulates granules to release their content, important = ADP
what happens during the 3rd event of hemostasis?
- platelet aggregation
* ADP has a receptor for it on the platelet membrane
* once ADP is released, it comes to the receptor
* causes platelet aggregation
* platelets will not only stick to the collagen surface but will also stick to each other
* form the initial plug that inhibits bleeding
serotonin will contract blood vessels
what happens during the 4th event of hemostasis?
- fibrin-platelet plug formation
coagulation factors interact on the platelet surface to produce fibrin (clot)
what are the 2 pathways that lead to the formation of a fibrin clot?
- intrinsic pathway
- extrinsic pathway
the 2 converge in a common pathway that leads to clot formation
what happens during the clotting cascades?
- platelets stuck by aggregation
- we get a strong block which will obstruct the vessels leading to stoppage of bleeding
*
