voiceover: i'm gonna startby drawing a blood vessel. inside the blood vessel i'm gonna draw in some of the cells that you'dexpect to see in normal blood. i'm gonna put in a few red blood cells. i'm gonna draw in a couple of platelets which are just fragments of cells and then i'm gonna put ina few white blood cells. even though all of thesecells are found in the blood they're actually not made there.
the cells in the bloodare made inside the bone. if you were to take across section of bone, if you were to take a slice like this and you looked at it, you'd see that on the inside of bone there's a central cavity. this is the outside part of the bone. the hard part and thisis the central cavity. inside that cavity there'sthis red spongy tissue.
that stuff is called bone marrow. bone marrow. bone marrow is where all ofthese blood cells are made. it's the site of hematopoiesis. hemato means blood and poiesis means to form or to make. hematopoiesis is just a scary greek word that means to make blood. let's take a look at what goeson inside the bone marrow.
what's interesting is that as different as all of the blood cells are they all actually originatefrom the same cell and that cell is called ahematopoietic stem cell. this stem cells gives rise to all of the different cellsthat you see in the blood. it gives rise first to twodifferent cell lineages or two different cell groups. first there is the myeloid group.
myeloid group. which is different fromthe lymphoid group. the lymphoid group. all of the cells that you see in the blood belong to either the myeloid group or the lymphoid group. the lymphoid group includes two different types of blood cells. first there is the t cell
or actually that's too big because this is a very little cell. that seems about right. there's a t cell andthe majority of the cell is taken up by the cell's nucleus, and that's what i'm shading in over here. so much so that this cell has a nickname. it's often referred to as a naked nucleus because it looks like the nucleus
isn't surrounded by very much cytoplasm. this is a t cell or a t-lymphocyte. t-lymphocyte. it's very similar in appearance to the other type of lymphoid cell. this cell also has a nucleus that takes up the majority of the cell and this cell is a bcell or a b-lymphocyte. b-lymphocyte, lymphocyte.
those are the two differenttypes of lymphoid cells. what about the differenttypes of myeloid cells. for starters we have a red blood cell. i guess we should draw that in red. this is also very small cell. i'm gonna shade in this cell so that i can show you thatthe center of this cell is much lighter than the edges or the periphery of the cell.
this is not the cell's nucleus, guys because we know that red blood cells don't have nuclei. what i'm trying to show isthat the center of the cell is much lighter than theperiphery of the cell. i guess i could do a better job of that if i showed you what thiscell looks like on its side. this is what a red blood cell looks like when it's laid on its side.
it kind of looks like a dumbbell where the edges are much thicker or much wider than the center. the edges would since they're thicker they would be much moredensely packed with hemoglobin. since we know that hemoglobin is what gives red bloodcells their red color, the edges would then bedarker than the center. since the center is much thinner
it would have a lot less hemoglobin. it would be a lot paler in comparison. we said that this is a red blood cell but since in science we never use a plain andordinary name of things we call this an erythrocyte. erythrocyte which is just a fancy name for a plain old red blood cell. what are some of the othertypes of myeloid cells?
we have this one cell thati'm drawing in over here. you might say well, thatlooks nothing like a cell and you'd be totally right. this is a very odd-looking,very large cell and it's called a megakaryocyte. even if you never heardof a megakaryocyte before you may have heard ofwhat it gives rise to because a megakaryocyte gives off little blebs of its cytoplasm
to form this small cell fragments and these fragmentsare known as platelets. you may have heard ofthe platelets before. the rest of the myeloid cells are actually differenttypes of white blood cells. for example you have thisone type of white blood cell and the rest of these white blood cells are about twice thesize of a red blood cell so that looks like it's twice the size
of a red blood cell to me. this cell is called a monocyte. it's known for having this nucleus that's in the shape of a kidney bean. that kind of looks likea kidney bean to me. this is a really cool white blood cell because it protects us from bacteria and viruses and other invading organisms just like any other white blood cell does
but it does so in a very interesting way. let's say that this was a bacterium or actually, no i like the color pink so let's use a color i don't like. let's say that this was a bacterium. the monocyte would defend us against it. it would attack this bacterium by engulfing itself around the bacterium. when it does that it kind of looks like
it's eating the bacteria. it looks like its eating the bacterium. that's a very interesting way of dealing with these invading organisms. let's clear out that. mono actually refers to the fact that this cell has a nucleusthat's in one piece and i guess a lot ofthese cells have nuclei that are in one piece.
it makes us wonder what types of cells have nuclei that are not in one piece and that would bring us to a neutrophil. this is a neutrophil. a neutrophil. it has a nucleus like i'm drawing in that is broken up into severaldifferent pieces or segment. the segments are still held together by little pieces of string.
this is called a multi segmented nucleus. even if you've never heardof a neutrophil before i'm sure you've seen them before and how do i know that? it's because neutrophils are actually the main component of pus. if you like anybody elsehas ever popped a pimple you've looked at a bunch of neutrophils. what are the other types of myeloid cells?
well, we have this one cellthat's called a basophil. basophil. it looks something like this. you might say, well, that looks an awful lot like a monocyte and you'd be completely correct except this cell has a very unique feature that the monocyte doesn't have, and that is that it hasthese bright blue granules
in its cell's cytoplasm. that make it stick out. the way i like to thinkof a basophil is like a basophil is like a blueberry muffin and that reminds that it has these bright blue granules inits cell's cytoplasm. for the last type of myeloidcell we talked about. whoops. for the last type of myeloidcell that we talked about
is an eosinophil. it's an eosin-o-p-h,p-h-i-l, eosinophil, okay? it has a nucleus thatcan sometimes be found in two pieces. this is the first piece andthis is the second piece but that's not what makes it so unique. what it's really well known for is the fact that it has these bright, beautiful red granulesin its cell cytoplasm.
those are all of the different blood cells with all the lymphoid cells on this side and all the myeloid cells on this side. it turns out that all of these cells don't directly develop fromhematopoietic stem cell. instead they undergo multiplestages of development to mature into their adult form, but don't worry guyswe're not gonna hash out each of those differentforms and their names
and physical characteristics. instead we're gonna makea very general statement. i guess we could put that up here. we're gonna say that the immature forms. the immature forms of all these cells are called blasts. when the blast mature, they mature they are called cytes. let me show you what i mean.
to spare you guys the agony of having to watch me draw out all of those different forms i just pasted them in here. we can see here that what we said about the immature forms being called blasts and the more mature formbeing called cytes is true. the t-lymphocyte developsfrom a t lymphoblast just like an erythrocyte develops
from an erythroblast and a megakaryocyte developsfrom a megakaryoblast. you guys get the pattern. the only wrench i'm gonnathrow into this pattern is with the neutrophil,basophil and eosinophil which all actuallydevelop from the same cell and that cell is called a myeloblast. i think that that's a reasonable place to end our discussion on hematopoiesis.
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