Lineages and Stem Cells:
B-cell development
The Circulatory
System
The circulatory system consists of the heart (to pump
blood),the blood vessels (to transport blood) and the
blood itself. All of these must develop concurrently in
the early embryo to allow a functioning circulatory
system
What is blood?
Mammalian blood is 45% red blood cells, 1% white
blood cells and the remainder is plasma (the liquid
portion containing various dissolved proteins).
Why do we need blood cells?
1.) Red blood cells carry oxygen to tissues and carbon
dioxide to lungs (or to the placenta prior to birth)
2.) Platelets are crucial for blood clotting, which
prevents excess bleeding if there is a breach in the
integrity of a blood vessel
3.) White blood cells fight infection.
The first two functions need to be peformed at an early
stage of embryonic development (as soon as blood vessels
form), but the last one is only needed mainly after birth.
So not all blood cell types are produced at the same time
in the embryo
When is blood first made in an embryo?
Early human (before 16 days gestation) and mouse embryos
(before 7 days gestation) do not have blood. Instead the
embryo obtains all the required nutrients and oxygen from
simple diffusion. Waste products also diffuse out of the
embryo into the mother’s circulation.
Once the embryo reaches a large enough size, simple diffusion
is not sufficient to bring nutrients or remove waste products
necessitating the development of a circulatory system and blood.
5 days
6 days
7 days
8 days
12 days
Hematopoiesis
Hematopoiesis means forming blood
Hematopoiesis has discrete stages in embryonic
development
In mammals, blood cell development takes place in
four main organs - yolk sac, placenta, fetal liver
and bone marrow
and two discrete stages - primitive and definitive
Places of Hematopoiesis
Note: The AGM is part of the early embryonic body, described later
Primitive Versus Definitive Erythropoiesis
Primitive erythropoiesis –
Red blood cells have nuclei
Primitive erythropoiesis
occurs in early embryos
Definitive erythropoiesis
Red blood cells DO NOT have nuclei
Definitive erythropoiesis
occurs in later embryos and
adults
The Hemangioblast
Studies have demonstrated that there are cells in the
early embryo that have the potential to develop into both
blood cells and vascular endothelial cells. These cells are
termed hemangioblasts and are derived from mesodermal
cells.
Hemangioblasts give rise to hematopoietic stem cells that
then form all the different types of blood cells
Hematopoietic Stem Cells
The HSC
All blood cells are
derived from hematopoietic stem cells (HSC).
Stem cell can proliferate to replace themselves and to give rise
to more committed
progeny.
Lots of red
blood cells and
platelets are
needed. Many
fewer of the
other cell types
are required
Yolk Sac Hematopoiesis
Blood and a circulatory system
need to start forming
beginning around day 18-20 of
human development and around
day 7-8 of mouse development
At this stage, the embryos are
still very tiny, so there is not
much room to generate blood
Therefore, the first blood
cells to form are made outside
the embryo in a membrane
Yolk Sac
called the yolk sac
Embryo
Yolk Sac Hematopoiesis (blood islands)
In the yolk sac, the first place of
hematopoiesis, undifferentiated
mesenchymal cells differentiate to
clusters of hemangioblast cells. The
hemangioblasts further differentiate
into endothelial cells and primitive blood
cells. This is the first “blood vessel”
like structure in the embryo.
Fetal Liver Hematopoiesis
When the embryo is bigger
and more developed,
hematopoiesis switches
from the yolk sac to the
fetal liver. This occurs
around day 30-40 of human
development and around day
10-11 of mouse development.
The AGM region
Derived from mesoderm that is associated with the
internal organs of the body. This mesoderm lies close to
and contributes to the development of the aorta, the
early kidney (mesonephros) and the gonad.
Stem cells in the AGM
Hematopoietic stem cells from the yolk sac or the
aorta-gonad-mesonephros (AGM) region of the
embryo migrate to the fetal liver to give rise to fetal
liver hematopoiesis
Another view of AGM Blood Formation
Fetal Liver Hematopoiesis
Colonized by definitive hematopoietic stem cells
derived from the AGM and possibly also from the
yolk sac blood islands.
All blood cell types (except T cells) can differentiate
in the fetal liver.
Fetal liver cells
Neutrophil (n)
Placental hematopoiesis
The discovery of the placenta as a major source of
embryonic blood cells came later than the other tissues
and is less well-understood than yolk sac, fetal liver or
bone marrow hematopoiesis.
Hematopoiesis in the placenta overlaps with yolk sac and
fetal liver hematopoiesis.
Source of blood cells in the placenta
Appear to derive from cluster of hematopoietic stem cells
associated with the fetal vessels on the fetal side of the
placenta
The placenta may have more early blood cell
progenitors than the fetal liver
Source of GEMM
GEMM are types of blood cell colonies derived from early
hematopoietic progenitors, while BFU-E and GM are colonies derived
from more committed progenitors
Bone Marrow Hematopoiesis
In adult humans and mice, hematopoiesis mainly takes
place in the bone marrow. The figure above shows the
main sites of hematopoiesis in adult humans (red bones
are the ones with highest blood cell production)
Timing of bone marrow hematopoiesis
Bone marrow hematopoiesis begins in late in
gestation (day 17-18 until birth in mice, week 22birth in humans), because the bones have not
developed sufficiently before this time to support
hematopoiesis
Bone marrow hematopoiesis continues throughout
adult life, but hematopoiesis can also occur in the
liver or spleen if the bone marrow is not producing
enough blood cells
The Bone Marrow Hematopoiesis
Colonized late in embryogenesis by definitive hematopoietic stem cells derived from the fetal liver (and
placenta?). All blood cell types (except T cells) can
differentiate in the bone marrow.
Generalities of Hematopoiesis
No matter which organ (yolk sac, placenta, fetal liver or
bone marrow, hematopoietic stem cells and their more
differentiated progeny need a proper environment to
grow and differentiate.
This includes proper cell-cell contacts, proper cytokines
and growth factors and other additional requirements for
particular blood cell lineages
These specific requirements can explain, at least in part,
why blood cannot form anywhere in an embryo, but is
localized to very specific regions
Various
types of
cytokines
and growth
factors are
also required
for hematopoiesis to
occur
Many Different Transcription Factors
Are Required for Hematopoiesis
Differentiation of a specific blood cell typethe B lymphocyte
As an example of hematopoiesis, we will discuss the
development of one type of white blood cell, the B
cell, in the adult bone marrow.
To better understand their development, it is
important to know what B cells do – they produce
antibodies to fight infection
Antibody genes are not encoded in our DNA in an
assemble form, but rather need to be generated by
DNA rearrangements that must take place during B
cell development
B Cell Development
Hematopoietic stem cells give rise to a lymphoid
progenitor cell, which is thought to be able to
differentiate into both B cells and T cells as well
as natural killer (NK) cells.
This lymphoid progenitor can migrate to the thymus
and initiate T cell development or remain in the bone
marrow to initiate B cell development.
Bone marrow B cell development can be subdivided
into various stages: pro-B, pre-B, immature B and
mature B cells.
B Cell Development is Dependent
on Several Factors
A.) The presence of bone marrow stromal cells
(a kind of fibroblast) providing cellmediated contacts and secreting the cytokine IL-7.
B.) The rearrangement of the immunoglobulin
(antibody) genes. In the common lymphoid
progenitor both the immunoglobulin heavy and light
chains are in their germ-line configuration (the
unrearranged state) and must be rearranged (so
they can give rise to an in-frame antibody protein)
for B cell development to progress.
C.) Expression of particular cytokine receptors and
transcription factors by the developing B cell
Bone Marrow Stromal Cell-Cell Contacts
Bone marrow B lymphoid
progenitors adhering to
bone marrow stromal cells
in culture.
Bone marrow stromal cells provide at least two important
contacts to B cell progenitors: an adhesive interaction
through the integrin VLA-4 binding to VCAM-1 and a
signaling one through surface bound SCF interacting with
the Kit receptor on the lymphoid progenitor.
Bone Marrow Stromal Cell Derived IL-7
IL-7 secreted by bone marrow stromal cells binds to
the IL-7 receptor expressed on pro- and pre-B cells
and stimulates the survival and proliferation of the
B cells.
Rearrangement of the Immunoglobulin Genes
B cells exist to produce
antibodies (immunoglobulins)
Thus, B cell progenitors in the
bone marrow will only survive
and proliferate if they are able
to rearrange the immunoglobulin
genes to generate functional
Immunoglobulins.
How can a
developing B cell
tell if it has a
functionallyrearranged
immunoglobulin
gene?
The cells make a
surface receptor
out of the
rearranged
immunoglobulin and
see if it signals!
Surface Immunoglobulin Receptors
on B Cell Progenitors
Bone Marrow
pro-B
pre-B
Heavy
Chain
Rearrangement
Periphery
immature B mature B activated B
Light
Chain
Rearrangement
Antibody
Production
In pre-B cells, the rearranged immunoglobulin
heavy chain pairs with two other proteins l5 and
V-preB, the surrogate light chains. This forms
the pre-B cell receptor, which signals the cell to
proliferate and further differentiate.
Summary of B Cell Development
Stromal Cell Dependence? Yes
IL-7 Dependence?
No
Yes
Yes
Yes
Yes
No
Yes
No
Yes
No
No
No
No