Hematopoiesis (Part two)
Granulocytes
These cells are on
the spectrum of white blood cells and in fact they are the most abundant type
of white cells. Granulocytes are characterized by the appearance of granules in
their cytoplasm which get different colors according to each subtype. Everything
starts off with HSC which will later evolve into a multipotent progenitor cell which even though has a less strong proliferation
ability than HSC, it has the ability to differentiate into any type of cell in
our body. (For example, we saw in the previous article, how MPP with EPO and
other types of molecules will eventually develop into erythrocytes). Here MPP
will give off a common progenitor cell of the myeloid cell line (CMP). Furthermore,
this progenitor cell will also differentiate into another progenitor cell, the
granulocyte/macrophage cell (GMP). To continue, GMP will differentiate into a
progenitor of eosinophils (EoP) and a progenitor of basophiles/ mast cells
(BMCP). From BMCP other progenitors will be developed, which are the progenitor
of mast cells (MCP) and the progenitor of basophiles (BaP). In the end, myeloblast will be developed.
Myeloblast
Its nucleus occupies
most of the cell’s place. The cytoplasm seems light bluish without granules. Three
to four nucleoli can be seen. Its size can vary from 15-20 μm. Eventually, myeloblast will differentiate
into promyelocyte.
http://histology-world.com/photoalbum/displayimage.php?album=65&pid=3478
Promyelocyte
We can see more promyelocytes
than myelocytes in the bone marrow. In addition, its cytoplasm has gained more
space than its progenitors. Also, some primary granules are able to be seen. There
still can be observed some nucleoli more abstractly though. Its size surprisingly
can vary from 15 to30 μm and it is the only exception of the rule. (Generally, the produced cells
are smaller than their progenitors). Promyelocyte will differentiate into
myelocyte.
https://medical-dictionary.thefreedictionary.com/promyelocyte
Myelocyte
Here we can observe primary
granules as well as secondary (secondary granules might appear to be more).
Primary granules contain peroxidase, lysozyme, and hydrolytic
enzymes while secondary granules contain collagenase, lactoferrin and lysozyme.
There are no nucleoli and the nucleus has become even smaller. Lastly,
myelocyte is the last cell that can undergo mitosis.
http://education.med.nyu.edu/Histology/courseware/modules/b-hematopoiesis/bloodrev26.html
Metamyelocyte
What characterizes this cell is the shape of its nucleus, which looks like a C shape. Some have described it as a kidney look alike shaped nucleus. The cytoplasm gets colors pink-blue and there appear to be even more secondary granules than primary (some say that there cannot be seen any primary granules). At this stage, the granules will indicate what type of granulocyte this cell will be.
hematologyatlas.com/ https://gr.pinterest.com/pin/531706299747358455/
Band Form
We can see the
nucleus forming a C thin shape with lobes. The secondary granules that we are
able to observe will be colored according to what type of cell it is going to
be (basophil, neutrophil or eosinophil). Also, tertiary granules will be
produced at that stage, which consist of gelatinase, leucolysin and lysozyme. After
this stage we will have the production of the final mature granulocytes
according with what kinds of granules their progenitors had.
https://xenia.sote.hu/depts/pathophysiology/hematology/e/morphology/norm-per/neutro.html
https://www.slideshare.net/sufyanakram/haemopoiesis-69383196
http://histology-world.com/photoalbum/displayimage.php?pid=3479
Eo(b) : eosinophilic
band cell
Ne(b) : neutrophilic
band cell
http://courses.md.huji.ac.il/histology/blood/VI-5b.html
Important stages of myelopoiesis
As it has been said
before, many molecules have an important role in the differentiation of each cell
type. Some factors such as Pu.1 and CEBPa are of crucial importance, since
without them the whole process and production of granulocytes could have not been
developed. In fact, GMP is common for both granulocytes and macrophages, in
order for the progenitor to start developing towards the granulocytes is highly
related to the existence of Pu.1, a transcription factor. In other words, high
levels of Pu.1 will activate macrophage pathway while the complete deactivation
of Pu.1 will lead to the granulocyte’s development.
Also, CEBPa has the ability to start the differentiation and to deactivate the
proliferation. That has been proved from the fact that, many mutations on this
molecule which deactivate it can eventually contribute to the appearance of
myeloid leukemia and myelodysplastic syndromes.
Neutrophils
Τhe
neutrophil or polymorphonuclear leukocyte has a nucleus with many lobes (3-4) and
a granular cytoplasm with both primary and secondary granules, and it gets
colored with both acid and basic substances. Also, they can appear with
pseudopods. These cells are produced in the bone marrow and enter the
bloodstream and circulate for 7-10 hours before migrating to the tissues needed,
where they remain for only a few days. What “drives” them to those tissues are
specific chemotactic agents that are produced during inflammation. To put it
differently, those agents appear in the area that neutrophils should be called
in. During cases of inflammation or infection, because of the signals produced,
the bone marrow produces more neutrophils. Neutrophils are able to perform
phagocytosis and their primary and secondary granules enable them to do it. In
fact, both primary and secondary granules fuse with phagosomes, and the
contents of them are digested and destroyed. Furthermore, neutrophils produce a
variety of antimicrobial agents.
https://www.verywellhealth.com/polymorphonuclear-leukocyte-2252099
Basophiles
These cells have a nucleus with lobes and a highly
granular cytoplasm (very big granules), which is colored with the basic
methylene blue dye (blue). Basophils are not able to undergo phagocytosis. Their ability is to release from their
granules many substances which have an important role allergic reactions.
https://www.blutwert.net/granulozyten/basophil/
Eosinophiles
Their nucleus appears
with two lobes and a cytoplasm filled with granules which gets colored with the
acid red pigment eosin. Just like neutrophils, they have the ability to undergo
phagocytosis while either being immovable or mobile in the tissues. They also migrate
from the blood to the tissues, however their phagocytic role is much less
important than that of neutrophils. Additionally, and they are considered to be
actively involved in the defense against parasitic organisms by secreting the
contents of their eosinophilic granules, which causes damage to the structure
of the parasitic membrane.
https://medschool.co/tests/blood-film/eosinophils
Mast cells
Mast cells are considered
to be granulocytes, but they develop in a different hematopoietic line than the
other three granulocytes. They emerge from the common myeloid progenitor and it
is believed that mast cells are developed from the granulocyte/megakaryocyte
progenitor. In fact, they are developed
in the bone marrow, but they get released in the blood before their maturation
process is finished. This results in them, undergo their differentiation after
their set up into the tissues. This means that mast cells progenitors are the
ones migrating from marrow to the tissue and this happens after the appearance
of special factors, triggered by inflammation. Moreover, they are found in a
wide variety of tissues, including skin, connective tissue of various organs
and mucosal epithelial tissues of the respiratory. They have a lot of granules
in their cytoplasm, and they contain histamine and various other
pharmacologically active substances. Mast cells play an important role in the
development of allergies since the chemicals released by them can trigger
allergy symptoms. Many abilities of them as well as their origin is still under
research.
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