Hey y'all. Dr. Brown back with another histology video. In this one I really am
here to kind of simplify a process. So we're gonna be talking about
erythropoiesis or red blood cell production and hematopoiesis is making
blood cells, and it's complicated. This is a common chart that you'll see
in lots of places this one happens to come from Wikimedia Commons, so it's
something that I can use in this video, but if you look in your textbook you'll
see a very similar image, and there's a lot going on here. What I really want to
do is for this...for this video is really just focus in on how we make red blood
cells, because this sub module is about red blood cells and that's what we're
gonna be talking about here. So we're gonna ignore a lot of this stuff and
really focus on this part, ooh my drawing was bad, okay, so we're really focused on
this bit here. So let's go to the next slide. Let me switch to say a blue pen.
Cool. So all blood cells are gonna start out with the same stem cell, and this is
called a multi potential hematopoietic stem cell or we're gonna call it a
hemocytoblast, because that's just easier for me to remember. Hemo...cyto...blast. And the
hemocytoblast can differentiate into two different stem cells. So...so this is a
stem cell, and it can become two other stem cells. Okay? So it can become what we
call a lymphoid progenitor cell. A lot of these are gonna look kind of alike cuz, you
know, Dr. Brown and his art skills. So this is a lymphoid progenitor cell, and if we
were to diagram all of that out, that you saw on the last slide, this cell would
eventually be able to differentiate into T cells, and B cells, and natural killer
cells, and dendritic cells, but for today we're gonna leave it here. We're not
going to follow this lineage anymore. The other kind of stem cell that
hemocytoblast can become is a myeloid progenitor cell, which is gonna look a
lot like a lymphoid progenitor cell, because I'm drawing it. So this is a
myeloid progenitor cell, and these guys are going
to differentiate into all the other kinds of blood cells. So red blood cells,
all your white blood cells besides lymphocytes, your mast cells, thrombocytes,
those are all gonna come from these myeloid stem cells, okay. So we're not
interested in all of the descendants of the myeloid progenitor cell. So we have
one kind of cell can become that's gonna eventually be thrombocytes or platelets,
so that guy will go right there. So that's called a megakaryoblast. We're
not gonna mess with him right now. This guy can also become a type of cell
called a myeloblast. Here I'll make its nucleus extra big. There we go. And
this is the cell that's going to become all of your white blood cells that
aren't lymphocytes. We're interested in the progenitor cell that comes from the
myeloid progenitor cells called the proerythroblast. That's gonna be this guy
right here, and notice that this cell has a nucleus. Okay. So this is the
proerythroblast. This is the one we're interested in. All of this is happening
inside the bone marrow. Now hematopoiesis or blood cell
formation can take place to some degree in the liver in, to some degree in
the thymus gland and to some degree in the spleen, but the vast, vast, vast,
majority of blood cell production is going to be happening in your red bone
marrow, that is in cancellous or spongy bone. Now this proerythroblast has now
undergone differentiation to the point where it's gonna become a red blood cell.
It can't be anything else now, so this is almost its terminally differentiated, and
now it's just got to go undergo the morphological transition from being this
proerythroblast, this pre red blood cell, to becoming a red blood cell. And
it's going to undergo a series of transformations so it's gonna go become
a basophilic erythroblasts. So something like this. My art skills are
amazing guys. I don't see how you guys can stand how
amazing this is. And then from the baz... basophilic erythroblasts it's gonna go
to a polychromatic erythroblast, and this has to do with changes in its
biochemistry. So this cell is starting to produce hemoglobin. Okay, and it's
producing vast amounts of it, because that's the job of a red blood cell is
essentially to be a sack filled with hemoglobin. So we're going to go through
numerous steps, and what we're gonna eventually end up as is a cell called a
reticulocyte, and this is almost terminally differentiated. So we'll go
this way. So now don't concern yourselves with the
the basophilic erythroblasts and the polychromatic erythroblast, i'm not going
to ask you to remember those, but i am gonna ask you to remember the
reticulocyte. Re -tic-u-o-l-cyte. It's hard to write on these tablets. And the
reticulocyte is still going to have a little bit of a nucleus. So when it's
released into the blood it's gonna have a little bit of a nucleus. You'll still
see some cytoplasmic inclusions. Okay. So this is what's actually released into
the bloodstream from the bone marrow. So everything above this line, this is all
happening in the bone marrow,
but it's the reticulocyte that's released into the bloodstream. And these
only last for about three days, or so. So over the course of the three days
between when the reticulocyte is released, it will undergo its final
morphological changes, so it's going to shed its nucleus. So the nucleus gets
shed out here, and finally after about three days in circulation, the
reticulocyte has taken on the characteristic biconcave disk shape of a
mature red blood cell or erythrocytes. And this little guy can live in the
bloodstream for about 120 days.
Erythrocytes are anucleate. You see that the reticulocyte lost its nucleus. Now if for
some reason oxygen demand in the body goes up, you're going to produce a lot
more red blood cells. Right, these are produced on demand
so normally reticulocyte reticulocytes should be no more than 1% of the
circulating red blood cells. Okay. So for every hundred red blood cells you should
see one or fewer reticulocytes, but if this concentration goes up that's an
indicator that hematopoiesis, particularly erythropoiesis
or red blood cell production, has gone up rapidly in recent days, because these
will within three days become the mature erythrocytes. So we need three main
components in order for this process to take place. We need a hormone called
erythropoietin. Wow. Well I'm not gonna lie, it's really hard to ride on this
thing. Oh no, no, no, no. No i'm not done. Continued recording. back it up a little
bit. There we go. Sorry about that. So let's erase this, and
try to write a little more neatly. I probably should have pre typed this, but
it is what it is. Okay. So we need erythropoietin...my thumb was hitting a
button on my pen. One of these days I'm gonna get this word on the page for you
guys. Erythropoietin, so this is a hormone that triggers this whole process of red
blood cell production. Another vital factor is iron. Hemoglobin the main
protein in your red blood cells has four iron atoms per hemoglobin, and then
finally we need vitamin b12, and we need vitamin b12 to be able to produce folic
acid, and folic acid is needed to produce thymine, which is of course necessary for
RNA production. So the last thing I want to show you is kind of what's going on
inside this cell as we make our way from the myeloid
progenitor cell to the mature erythrocyte. And that has to do with the
amount of RNA and the amount of hemoglobin that's present. So if we just
make ourselves a little graph, it's a little crooked but it is what it is, and
so this is concentration in percent, and here's time. So as we progress from
myeloid progenitor cell, to pro-erythroblasts, to basophilic erythroblast, to
polychromatic erythroblasts, to reticulocyte, to erythrocyte. What we're
going to see is the amount of RNA in the cell is going to decline, and the amount
of hemoglobin in the cell is going to go way up. So the solid line is hemoglobin,
and the dashed line is RNA. And that mirrors the morphological process that's
taking places as we go from proerythroblast to erythrocyte, the cell is
becoming not only terminally differentiated but highly specialized to
transport hemoglobin, and thereby to transport gases, prominently oxygen and
carbon dioxide. So I know that seems like a lot, but when you compare it to that,
hopefully that makes a little bit more sense. So thanks for hanging in there.
Study hard. we'll see you later.