appunti del dott. Claudio Italiano
One of the first theories concerning hematopoiesis consists in the fact that
the blood cells would be derived from a single progenitor element, the
hemocytoblast. This has been described as a cell with a diameter of about 16
microns with a large nucleus with a chromatin structure consisting of finely
interwoven strands, 1-2 nucleoli, weakly basophilic homogeneous cytoplasm
without granulations. In turn, the hemocytoblast, derived from an
undifferentiated mesenchymal element, the 'emohistioblast, is a cell able to
give rise to all the cells of the blood and the connective tissue. This theory
has been revisited in the following decades, but still retains its validity,
even if the terminology, referred to today, no longer speaks of hemocytoblast
but of "multipotent stem cell" or "uncommitted Stem cell" to followed by
differentiated or (committed) stem cells.
The current nomenclature is based on functional rather than morphological
criteria. Very useful in this regard were the experiments of hemopoietic cell
grafting in animals (mainly Nymphium nude so-called NOD-SCID) whose
hemocytoformative parenchyma had previously been destroyed by ionizing radiation.
The inoculated cells are able to completely repopulate the hematopoietic tissues
of the host, thus demonstrating that among them there are elements capable of
giving rise to all the cells of the blood. The functional properties of the
blood cells have been studied more comprehensively using the haematopoietic cell
grafting method in the spleen of rodents: under these conditions the appearance
of cell colonies composed of all the blood cells and of elements can be observed.
in turn capable of giving rise to new colonies.
Colony-forming cells (C.F.U. Colony Forming Units) for their proven evolutionary
pluripotency and self-replicating ability are currently identified with the
primitive precursor elements of blood cells and defined stem cells. In the adult
organism the blood cells are produced at the bone marrow level starting from a
progenitor cell called "stem cell" (stem cell), which is endowed with 2
fundamental characteristics, self-renewal (self-renewal) ) and the ability to
differentiate along the two main haematopoietic proliferative chains: myeloid (understood
in the pan-Imohyoid exception, including both the granulo-monocyte and erythroid
and megakaryocyte series) and lymphoid series. The need for a continuous supply
of hematopoietic cells derives from the fact that the blood cells have a limited
life span and therefore must be continuously replaced during the entire life
span. Recent studies of telomere length measurement have however shown in murine
models that the capacity of self-replication of bone marrow stem cells is not
infinite but limited to about 50 replication cycles. The haematopoietic bone
marrow accounts for about 5% of the total body weight, and is distributed mainly
in flat and long bones in pediatric and juvenile age, while in adulthood it is
available at the ribs, the sternum vertebrae, the pelvis.
Multipotent stem cells lurk in niches bounded by the spongiosa trabeculae where
they find an ideal micro-environment for their growth and maturation. The
medullary microenvironment (stroma) is made up of reticular cells, sinusoids (without
basal membrane and delimited by endothelial cells), parasinusoidal adipocytes,
reticular fibers, and extracellular matrix consisting of proteoglycans, collagen,
vitronectin, thrombus-spondin, hemonectin, hyaluronate, fibronectin and laminin
(adhesive proteins).
The hematopoiesis process is also under the control of several growth factors produced by medullary cells belonging mostly to the stroma or the immune system (T lymphocytes, monocytes-macrophages and to a lesser extent B lymphocytes). Among these we mention innanzitutt the so-called CSF ("colony stimulating factors"): multi-CSF (also known as interleukin 3, stimulating the growth of GEMM progenitors: granulo-erythroid-monocyte-megakaryocyte), GM-CSF (which stimulates the maturation of "committed" granulo-monocytic progenitors), G-CSF (specific for granulocytic precursors), M-C5F (monocytic progenitors), CSF-MEG (megakaryocyte precursors), erythropoietin (specific for the differentiation of commissioned progenitors in erythroid direction), thrombopoietin (specific for megakaryocyte progenitors), "stem cell factor" also called c-kit receptor), interleukins IL) of different types: IL6, ILI, IL4, IL7, IL5, IL2, IL9, IL 11, and others. Difficulties still exist for the morphological recognition of this alleged stem cell, whose identification is still based on observations of an embryological, kinetic-functional and cytofluorimetric order. It is believed that the most undifferentiated stem cells are morphologically similar to the small lymphocyte, from which they would deviate due to the presence of a slightly looser chromatin structure that sometimes reveals a small nucleolus.
The cytoplasm is poor in organelles, due to the reduced protein synthesis and the low mitotic index of these elements. About 90% of these cells are permanently dormant (G0 / G1), outside the reproductive cycle, and do not respond to the differentiating stimuli of hematopoietic regulatory substances of the eitropoietin type, GM-CSF, CFS-Meg etc. Instead, these regulatory factors respond to other cells, no longer totipotent but already differentiated as "committed" or commissioned progenitor cells, "committed CFU", oligo-uni-powerful progenitors. From these elements, recognizable only in in vitro growth systems (the so-called "colony forming units" - CFU), would derive cells already differentiated and recognizable morphologically as immediate precursors of the circulating elements (megacaryoblast, proeritroblast, myeloblast, monoblast). The cells of the already differentiated stem cells are larger than the true multipotent stem cell, more abundant and basophilic cytoplasm (rich in organelles), laseous chromatin, 1-2 nucleoli, and are in the active phase of the cell cycle (blastic morphology). The progenitor stem cells are recognizable by the presence of the CD34 antigen (surface marker expressed by the multi- and uni-potent stem / progenitor cells of tulle), which is expressed in a decreasing manner as these elements proceed when ripe in the bone marrow. But while the most undifferentiated stem cell does not co-execute other chain or activation markers, the progenitor cells already in some way commissioned express class 2 antibodies of the major histocompatibility system (HLA-DR), CD38, incidentally the CD90, CD133, CD117, and others.
Within this series, at least two classes of erythropoietic progenitors have been identified, connected in a progressive sense, designated as Burst Forming Unit (s) (BFU-E) and, respectively, Colony Forming Unit (s) (CFU-E) ). In the longitudinal direction, the colonies derived from the growth of BFU-E are closest to the pluripotent stem cell (CFU-S) and give rise, in vitro, to the formation of cellular macroaggregates ("Bursts"). The BFU-E complex can further recognize the presence in its context of two sub-populations with different degrees of maturity (as regards sensitivity to humoral stimuli and replicative activity), the Primitives BFU-E (P-BFU- E) and the "Mature" BFU-E (M-BFU-E). The latter would represent a third category of erythropoietic precursors acting as a link between the real BFU-E (P-BFU-E) and the CFU-E. The differentiation steps between these various groups of progenitor cells are regulated by some humoral factors such as erythropoietin (Epo).
Erythropoietin (a glycoprotein
containing sialic acid, exosamine and hexose (PM 39,000 daltons) which increases
after bleeding and decreases or disappears, eg in animals rendered polyglobulins
by transfusions or with hypoxia), is produced in large part in the kidney: its
production is regulated by the oxygen tension of the tissues. Erythropoietin
stimulates the differentiation and maturation of erythroid stem cells and
exhibits a humoral effect at the level of late progenitor elements such as CFU-E
and to a lesser extent on intermediate elements such as M-BFU-E. In particular,
it induces the differentiation of CFU-E in the direction of the erythroblastic
compartment, accelerates the maturation of erythroblasts and influences the
introduction of reticulocytes into the circulation. The transition from CFU-E to
proherrinblasts initiates the erythroid cell component most commonly known for
its easy identification, at the level of hematopoietic tissues, and in
particular bone marrow.
In conditions of disturbed and slowed DNA synthesis, as it is especially in
pernicious anemia for vitamin B12 and folic acid deficiency, there is a change
in chromatin maturation, and while cell volume becomes higher than normal, the
chromatinic reticulum it becomes finer and thinner; this type of evolution is
described as a erythropoietic series in itself (megaloblastic series), starting
from promegaloblast, a large cell with a very fine chromatin nucleus, through
the stages of basophilic, polychromatophile, orthochromatic megalo-blast, up to
megalocytes (more voluminous erythrocytes and more intensely colored than the
common normocytes.
The erythrocytes or red blood cells, observed in the fresh, appear as small
citrine-yellow discs with a clearer central part; in the fixed and colored
smears, they appear bright pink with a less colored central part: these aspects
are due to the particular biconcave disc shape of the erythrocyte. Red blood
cells are nucleus-free: they average 7.2-7.7 microns. In the blood there is a
small amount (about 1%) of erythrocytes whose diameter is slightly higher than
the average, slightly polychromatophilous, which, over-invigorated with basic
dyes (brillant cresyl blau, methylene blue, etc.) show up inside the
granulations blue joined by thin filaments arranged to form a more or less
compact lattice (so-called "granulofilamentosa reaction"): to them, due to their
properties, we give the name of reticulocytes (pro-erythrocytes,
granulofilocytes). The number of reticulocytes, which should be considered as
juvenile blood cells, increases considerably during accelerated erythropoiesis.
In pathological conditions the erythrocytes can undergo modifications:
- of the dimensions (microcytes, macrocytes);
- of the form (poichilocytes)
- of the hemoglobin content (hypo-hyperchromia).
Particular forms are the spherocytes (red blood cells with a large size and
increased thickness), sickle cells or sickle red blood cells, ovalocytes,
leptocytes (flattened hematodes), anulocytes (hypo-comical forms with hemoglobin
located in the periphery), and the stomatocytes (with a light elongated central
area), the target cells (with a dark center and periphery separated by a light
ring), the schizocytes or fragments of a red blood cell and the red blood cells
with spicua variously called 'burr cells' , "Spur cells", acantocytes .
Granule-monocytopoietic series
The
identification of the precursors of the granulomonocytopoietic series was
carried out on the basis of the in vitro cultures in semi-solid soil. They have
shown that the proliferation of the stem cells of the granulomonocyte series is
regulated by soluble factors produced by cellular elements present in various
organs "Colony Stimulating Activity" (CSA). (Metcalf, Van Furth). Cells that
give rise to colonies of granulocyte elements and monocytes and macrophages at
the same time, have been defined as Colony Forming Units in Cultures (CFU-C). In
relation to the observation that the potential to give rise to granulo-monocytic
colonies at the same time, or only granulocyte or monocytic, has been
demonstrated the existence of progenitor cells "engaged" selectively along these
specific pathways (M-CFU, GM -CFU, M-CFU).
The data referred to above confirm that since the initial stages the
granulopoietic and monocytopoietic proliferative lines appear closely linked in
their genesis and evolution.
As far as the granulopoietic series is concerned, it recognizes a series of
well-defined steps on the morphological side that go from the myeloblast, to the
promyelocyte, to the myelocyte, to the metamielocyte, to the segmented mature
granulocyte.
The maturation of these elements takes place over a period of about 13 days and
is marked by nuclear modifications (increase in the degree of chromatinic
compactness, progressive cavitation and final lobulation of the nucleus) and
cytoplasmic (appearance of granules respectively blue-violet or primary or
aspecific - in the early stages of maturation and secondary or specific in the
most advanced stages of maturation evolution). The latter mark the granulocytic
evolutionary line in the neutrophilic, basophilic and eosinophilic direction
according to their characteristic conformation.
The elements of the granuloblastic series recognize in their maturation a
strictly medullary phase in which the granuloblasto that reaches the mature
neutrophil element (which becomes part of the so-called "marginal quota" ready
to be mobilized for defensive or reactive needs of various kinds) , a transit
phase in the circulating blood (in which the granulocytic elements are arranged
in two compartments, the one margined and the other circulating), and finally a
tissue phase where the leukocyte elements, migrated by vessel diapedesis, exert
their functions (chemotaxis phagocytosis).
The progenitor element is the myeloblast, a round cell with a diameter of 20-25
microns, with a large, round nucleus, with chromatin arranged in delicate
filaments or in the form of granules. The cytoplasm is light blue, inhomogeneous,
with a clear perinuclear area; it contains granules that are colored
methacomatically in violet red with Giemsa (azurophile granulations).
From the myeloblast derives the promyelocyte, an element with a diameter of
about 20 microns, provided with an oval nucleus; between the chromatin meshes it
is sometimes possible to notice clear areas that suggest the presence of
nucleoli. Cytoplasm tends to turn from basophilia to mild acidophilia: it
contains the specific properties of the mature element (neutrophils, acidophils,
basophils) next to the azurophile granulations.
The following maturative stage is represented by the myelocyte, a cell with a
diameter of 15-18 microns, with a smaller nucleus than the previous form and
chromatin arranged in very distinct masses. The cytoplasm is acidophilic and
contains almost exclusively specific granulations. This cell evolves to
metamielocyte, which differs only in the shape of the nucleus, which appears to
be reniform or horseshoe, with the concavity corresponding to the centrosome.
The metamielocyte, due to the appearance of nuclear bottlenecks that determine
the segmentation of the carioplasm, evolves to a circulating granulocyte.
Under the electron microscope, the myeloblast is provided with numerous bags of
granular endoplasmic reticulum and abundant ribosomes; the Golgi apparatus is
well developed. The promyelocyte always has one nucleoli and a cytoplasm rich in
heterogeneous granulations in shape and size. In the subsequent stages of
maturation there is a progressive decrease of the endoplasmic reticulum,
ribosomes and mitochondria.
The manner in which the granulogenesis is carried out have not yet been
clarified, but it is believed that two successive generations of granules take
place: the formation of the azurophilous granulations and of the specific ones
would occur independently at the Golgi apparatus level.
- Neutrophil granulocytes are 9-12 micron d-meter cells, easily recognizable by
the shape of the nucleus, which appears segmented into lobes (under normal
conditions no more than 5) joined by slender carioplasmic bridges. The cytoplasm,
pink to Giemsa, contains a variable number of granulations, small, fine, which
are colored with the mixtures: neutral reaction. It is believed that the extent
of nuclear segmentation depends on the age of the cell and tends to increase
with it: a quantitative evaluation of this phenomenon (nuclear formula) has been
attributed to diagnostic and prognostic importance (Arneth formula).
Under an electron microscope, neutrophil granulations appear to be heterogeneous
in shape and size: in addition to specific oval granulations, larger, spherical
granulations are found, which represent the residue of the azurephilous ones of
the promyelocyte. The granulations are rich in hydrocarbon enzymes and should be
considered as lysosomes: in the neutrophilous granules there are, among other
things, lysozyme, and alkaline phosphatase.
The neutrophil granulocytes are cells with amoeboidal mobility and phagocytosis
capacity: their migration is determined at least in part by the attraction of
various substances (chemotaxis), especially bacterial substances and complement
components.
These cells intervene in the body's defense processes by engulfing and digesting
microorganisms: they also play an important role in the dynamics of the
inflammatory process, releasing substances that accelerate inflammation,
vasoactive peptides, histamine and various other active substances.
- Eosinophil granulocytes measure in mids; 14-20 microns: the nucleus is mostly
bilobed, with the two lobes joined by a carioplasmic bridge that sometimes
thickens in its central part to form a third small lobe. The chromatinic
structure is roughly that of neutrophils. The cytoplasm is crammed with
red-orange granules, larger than those of neutrophils (average diameter 0.5-1.5
micron) relatively uniform in size within a single cell.
Eosinophilic granulations appear under the electron microscope as formations
provided with: a membrane, consisting of a homogeneous or granular dense matrix,
and an internal crystalloid structure, perhaps identifiable with a specific
myeloperoxidase.
Eosinophilic granulocytes are mobile cells, endowed with phagocytic power and
endowed with chemotactic properties. Among the various substances that determine
its migration (soluble bacterial factors, foreign antigens, antigen-antibody
complexes), the main role seems to be due to histamine. These cells are believed
to intervene in the body's defense processes against foreign bodies: the
presence of substances able to antagonize the action of histamine,
5-hydroxy-tryptamine and bradykinin has been demonstrated.
The basophilic granulocytes are the smallest of the granulocytes,
measuring 10 to 14 microns in diameter.
The cytoplasm is crammed with large irregular-shaped blackish granules, which
can be colored methacomatically with the basic water-soluble aniline dyes, which
extend to cover the nucleus. This appears homogeneous, not clearly segmented,
with an approximately clover-like appearance.
Under an electron microscope, the granulations appear to be composed of lamellae
arranged in blocks or fascicles. Cytochemical studies indicate that histamine,
chemically related substances with heparin, serotonin, hyaluronic acid and
various enzymes are contained in the granules.
1 basophilic granulocytes are mobile cells with phagocytic capacity, but in
these functions they are overall less active than other granulocytes. It is
believed that they can play an important role in allergic reactions: they are
the blood cells that most bind IgE antibodies and, upon contact with the antigen,
they degranulate releasing histamine. They are distinguished from tissue
basophil granulocytes (or mast cells): the former are called basophils with
soluble granulations and the second basophils with insoluble granulations.
The monocyte series evolves from CFU-GM according to the stages represented
by the monoblast, to the promonocyte, to the mature promonocyte. The
morphological aspects of these passages are marked by a progressive bowing of
the nucleus to assume a reniform appearance, with nuclear chromatinic
condensation, and by cytoplasmic modifications characterized above all by the
development of particularly important enzymatic enzymes (a-naphthyl
acetate-esterase, phosphatase alkaline, lysozyme).
Their maturational evolution of the monocyte elements includes a medullary phase
(from monoblast to monocyte) that includes two replicative cycles, a transit
phase in very short circulating blood (1/2 hour) and a tissue phase, where the
migration of mononuclear elements gives place for the formation of fixed
macrophage cells.
Depending on the settlement organ, the migrated monoclonal elements take
different functional adaptations, with transformation into cells endowed with
particular characteristics (Kupffer cells at the level of the hepatic tissue,
alveolar macrophages at the pulmonary level, macrophage elements in the strict
sense at the level of the organs hematopoietic, glial cells in the CNS,
osteoclasts in the bone tissue)
The other term coined by Foucar in 1990 to indicate this cellular system is
M-PIRES (Mononuclear Phagocyte and Immunoregulatory System), which has the
advantage of including in this group not only macrophage cells but also
immunologically active cells such as dendritic cells.
However we wish to name it, the histiocytic cellular system (mononuclear
phagocyte system), as it is currently conceived, is made up of a progressively
differentiating line which originates in correspondence of the bone marrow from
relatively differentiated stem elements (monoblasts and promonocytes). ), and
that, after leaving this site, they migrate as monocytes through the blood to
the tissues where they acquire the characters of the macrophages. In this phase
the macrophages assume particular configurations according to the place where
they are to settle (Kupffer cells, alveolar macrophages, microglia cells), or
they accumulate at the sites of inflammation or immunological reaction such as
macrophages, epithelioid cells or cells giants. Macrophages are further
distinguished in this classification, in two types (free and fixed, A and B)
whose functional biochemical characteristics are quite different.
To this system are now also included the so-called "antigen presenting cells" (APC),
also called accessory cells, which play a fundamental role in the immune
response and in particular in the phases of antigen presentation. These elements
are devoid of phagocytic activity in the classical sense, poor in lysosomal
enzymes, and have a characteristic morphology, possessing long and thin
cytoplasmic projections that allow an adequate interaction with the immune cells;
their action is synergistic with the other cells of the monocyte-macrophage
system. The APC cell population is constituted by dendritic reticular elements (found
in all the tissues of the human body except the cerebral organism) subdivided
into
a) dendritic follicular cells, with antigen presentation function to the B
lymphocyte class (they are located at the level of the germinal center, and of
the lymphatic follicle mantle of the lymph nodes);
b) interdigitated cells, with function of antigen presentation to the lymphocyte
class (located in the T dependent areas of the body's lymphatic structure);
c) Langerhans cells, located at the level of the epidermis and characterized by
a peculiar morphology (coffee bean core, eosinophilic cytoplasm endowed with
numerous fine projections). Because of their migratory capacity they can be
located in various tissues, particularly in the lymph nodes.
The functions that can be attributed to diseases are divided into locations
of dislocation of the same (splenic, lymph node, medullary). In any case it is
common to all cells of the base to express phagocytane properties in respect of
different substances. In this sense, the function of "compensation" is that it
is aimed at the ingestion of germs or parasites or inert particles such as
silica and coal, represents an operation of conservation of the pathogenetic
factors defending the organism. More in general foreign bodies (inert powders)
and germs and parasites, also, in the immunological field, implementing those
processes that are the basis of the presentation of the antigens, their
processing, and the transmission of information to the lymphoid cells. Closely
connected to this function is the activity of cyto-architecture, which is of
particular importance, as has already been said, as death is senescent.
Less known, but pathophysiologically important are two other functions performed
by the histiocyte cell system, such as trophic and metabolic. The structure is a
very particular structure (reticulo-erythroblastic islets, reticulo-lymphocytic
islands). This is the reason why the information is contained in the network of
nutritious materials (rophocytosis of ferritin molecules in the case of
reticulo-erythroblastic islets).
The metabolic function is linked to the processing by macrophages of many
chemicals that intervene in the processes of the inflammatory response.
Among the most important products to be counted next to the lysozyme, the active
proteases a neutral pH (including the plasminogen activator that catalyzes the
formation of plasmin from the same). A difference in lysozyme secretion that is
intrinsically constitutive of macrophage activity, the synthesis of neutral
proteases can be induced and modulated by various extrinsic pathways. The
macrophages also synthesize and secrete numerous complement factors, bind the
complement and degrade it through protease action. In inflammatory conditions
they can also be criticized in groups of enzymes, and break the bonds at the
level of proteins, lipids, nucleic acids.
Finally, the activity of macrophages seems to extend to other fields of human
pathophysiology, such as the remodeling of tissues, the healing of wounds, the
development of atheromas in vascular diseases, the use of cells in the
senescence phase.
The genesis of megakaryocyte cells recognizes a stage at the level of the
multi-potency stem cell (CFU-S - Pre-H-C) and a subsequent one involving the "committed"
stem cell CFL-Meg. To a large extent, the formation of megakaryocytes appears to
be influenced by microenvironmental factors (as appears in experimental studies
demonstrating a prevalent growth of megakaryocyte-containing colonies at the
level of the splenic subcapular region) and conditioned in the formation of
platelets from the humoral activity of a substance called "trombopoietin".
In the phase of maturation of the morphologically recognizable megakaryocytes
three stages (I, II, III) corresponding to definite nuclear and cytoplasmic
modifications (megacaryoblast, megakaryocyte, megakaryocyte granulosum) and
equivalent to phases already identified in the past (megacaryoblast,
megacaryoblast lymphoid, megakaryocyte) were distinguished grainy). The
acquisitions of some kinetic data and the evaluation of the nuclear DNA content
have however recently modified some concepts inherent to the maturity
progression as it appears in the suprargerita sequence, which involved, together
with the platelet activity, also an increase in the ploidy degree. cell phone.
In fact, the concept that cellular polyploidization and maturation intervened
simultaneously was contradicted by the observation that the phenomenon of
polyploidization is confined predominantly to the most immature elements capable
of operating DNA synthesis, and that therefore polyploidization in general
precedes cytoplasmic differentiation ( Queisser).
According to Penington, cytoplasmic differentiation would not in particular be
related to the degree of cellular ploidy, but rather to the number and extent of
the demarcation membranes contained in the cell cytoplasm.
Mature megakaryocytes are polyploid elements: under normal conditions about two
thirds have a set of 16 nuclei DNA, one sixth about a set of 8 nuclei and
another sixth about 32 nuclei.
The formation of platelets is morphologically documentable from stage III of
maturation of megakaryocytes in cytoplasmic areas well delimited by the system
of demarcation membranes ("preformed platelet areas").
The passage of the platelets into the bloodstream is carried out by cytoplasmic
projections of the megakaryocytes in the parasinusoidal position which protrude
through the endothelium of the sinusoids themselves and give rise to the
formation of fragments that prelude the release of single platelets in the
peripheral blood.
Platelets are the smallest of the blood-formed elements: on the
May-Grunwald-Giemsa colored preparations they appear as round or oval
corpusculops with a diameter of 2-5 microns. In these conditions, a homogeneous
peripheral zone weakly colored in blue (hyalomer) and a granular part, intensely
colored in violet (chromomer or granulomer) can be distinguished in the
platelets. The platelets actively intervene in the hemostatic processes thanks
to their property of adhering to the collagen fibers of the subendothelial
tissue and then aggregating at the vessel's gap giving rise to the so-called
white or platelet thrombus. These elements further contribute to haemostasis by
releasing coagulation platelet factors and vasoconstrictive substances: they are
also indispensable for the retraction of the clot. During the aggregation, the
platelets undergo changes in shape (passage from the discoid to the spherical
shape, thickening of the granules in the center, emission of pseudopodia), known
in the past as "viscous metamorphoses" of the platelets.
With an electron microscope, the platelets appear to have a distinct membrane on
the outside of which there is an amorphous layer: plasma coagulation factors
would adhere to this layer. Immediately below the membrane there is a system of
fibers and microtubules that plays a role in maintaining the discoidal shape and
carrying out the contractile functions of the platelets. In the hyaloplasm there
are many organelles (mitochondria, lysosomes, granules containing serotonin,
catecholamines, platelet coagulation factors, nucleotides, occasionally
ribosomes and siderosomes). In the platelets are also residues of the Golgi
apparatus, glycogen particles, lipid inclusions, vacuoles and vesicles. In the
ialoplasma fibrils are present, which are believed to correspond to the
contractile protein of the platelets, the thrombostenin.
Hematology