Now let's just consider the environment in which
this haemopoiesis takes place and specifically
consider the stem cell niche.
Blood is made in the bone marrow.
We don't know why this evolved, but it seems to be
a highly effective strategy for the generation of blood.
Now haemopoietic bone marrow or bone marrow
that's making blood is present in almost all bones in
children, but as we get older, it gets more restricted.
Its in our vertebra or pelvis and in our proximal
long bones, and stem cells are supported within
the bone marrow in a niche on microenvironment.
We can have a closer look of that on the right.
On the top right in dark green, we see the cortical bone
and up against it on the top right of the stem cells,
the progenitor cells and bone cells such as osteoblasts
directly support the survival of haemopoietic stem cells.
As these stem cells give rise to haemopoiesis, we see
waves of differentiated haemopoietic cells, which flow
through the bone marrow and into that grey central
longitudinal vein and you will see in that diagram,
haemopoietic islands and erythropoietic
islands for haemopoiesis.
You'll see just to the bottom there, a megakaryocyte
a very large cell with multiple nuclei that's
releasing platelets into the central longitudinal vein,
On the right, we will see some of the cells that
support bone marrow function - adipocytes or fat cells.
They comprise a great proportion of the bone marrow
and if you take a biopsy of bone marrow, you'll see
a lot of empty space occupied by fat cells.
And just at the bottom right the extracellular matrix
fibres, which provide the structural support for the
bone marrow and also bind molecules such as
chemokines which are critical in guiding the
migration of cells in and out of the bone marrow.
I want to finish this lecture by talking about the
structure of the spleen which is a fascinating organ.
Normally under your left rib, you shouldn't feel your
spleen, it is well tucked away.
This consists of two major types of tissue: the red
pulp and the white pulp.
The red pulp is where the red cells in our blood undergo
a quality control and I will explain that in a minute.
Whereas the white pulp is part of our immune system
and the spleen has a very important role
in scanning the blood for evidence of
The blood system in the spleen is really quite
fascinating because it has a unique open system
which is not contained within capillaries.
Let me explain.
Look at that diagram on the right.
At the bottom, we have the blood coming in through the
trabecular artery and into the central artery and around
it in grey and green,you will see lots of lymphocytes.
It is coded there as PALS, the periarteriolar
That is the massive lymphoid tissue which is screening
the blood for the presence of bacteria and
starting and initiating antibody responses against those.
That is a good example of the white pulp activity of spleen.
But those arteries go into capillaries, which break
down and lose their wall structure
and red cells end up in things called venous sinuses,
which you can see represented higher up in the structure.
Here red cells very slowly migrate in very
large sinuses in the presence of many macrophages.
The macrophages scan the red cells for any
evidence of dysfunction or residual bits of iron
from their release from the bone marrow, and they
remove old and damaged red cells from the blood.
Finally these red cells, now clear and the healthy
ones are released back into the splenic vein
and enter the blood system.
It's a very remarkable open system of
blood circulation within the spleen.
So In summary, we have seen in this lecture, blood
cells are derived from rare stem cells
within the bone marrow.
A wide range of different cell types are made,
which help to carry oxygen, fight infection
and repair damaged blood vessels.
As we shall see, inherited or acquired disorders
can affect any of these different cells
and lead to a wide range of different
I hope you have enjoyed this introduction to blood.