Hematopoiesis is the continuous process of blood cell formation that begins in embryonic development and persists throughout life, originating from hematopoietic stem cells that can differentiate into various blood cell types. In adults, this process mainly takes place in the bone marrow, while in infants and children, it may also occur in the spleen and liver. Hematopoietic stem cell transplantation is a medical procedure for treating certain blood cancers and other diseases, involving risks such as infection and graft-versus-host disease.

1. Haematopoieticsystem
Introduction:
Hematopoiesis is the process through which the body manufactures blood cells. It begins early in
the development of an embryo, well before birth, and continues for the life of an individual.
Hematopoiesis begins during the first weeks of embryonic development. All blood cells and
plasma develop from a stem cell that can develop into any other cell.
The blood is made up of more than 10 different cell types. Each of these cell types falls into one
of three broad categories:
1. Red blood cells (erythrocytes): These transport oxygen and hemoglobin throughout the
body.
2. White blood cells (leukocytes): These support the immune system. There are several
different types of white blood cells:
a) Lymphocytes: Including T cells and B cells, which help fight some viruses and tumors.
b) Neutrophils: These help fight bacterial and fungal infections.
c) Eosinophils: These play a role in the inflammatory response, and help fight some parasites.
d) Basophils: These release the histamines necessary for the inflammatory response.
e) Macrophages: These engulf and digest debris, including bacteria.
3. Platelets (thrombocytes): These help the blood to clot.

2. Current research endorses a theory of hematopoiesis called the monophyletic theory. This theory
says that one type of stem cell produces all types of blood cells.
Sometimes called primitive hematopoiesis, hematopoiesis in the embryo produces only red blood
cells that can provide developing organs with oxygen. At this stage in development, the yolk sac,
which nourishes the embryo until the placenta is fully developed, controls hematopoiesis. As the
embryo continues to develop, the hematopoiesis process moves to the liver, the spleen, and bone
marrow, and begins producing other types of blood cells. In adults, hematopoiesis of red blood
cells and platelets occurs primarily in the bone marrow. In infants and children, it may also
continue in the spleen and liver. The lymph system, particularly the spleen, lymph nodes, and
thymus, produces a type of white blood cell called lymphocytes. Tissue in the liver, spleen, lymph
nodes and some other organs produce another type of white blood cells, called monocytes.
The process of hematopoiesis
The rate of hematopoiesis depends on the body’s needs. The body continually manufactures new
blood cells to replace old ones. About 1 percent of the body’s blood cells must be replaced every

3. day. White blood cells have the shortest life span, sometimes surviving just a few hours to a few
days, while red blood cells can last up to 120 days or so. The process of hematopoiesis begins with
an unspecialized stem cell. This stem cell multiplies, and some of these new cells transform into
precursor cells. These are cells that are destined to become a particular type of blood cell but are
not yet fully developed. However, these immature cells soon divide and mature into blood
components, such as red and white blood cells, or platelets.
Stem cells:
Haematopoietic stem cells (HSCs) reside in the medulla of the bone (bone marrow) and have the
unique ability to give rise to all of the different mature blood cell types and tissues. HSCs are self-
renewing cells: when they differentiate, at least some of their daughter cells remain as HSCs, so
the pool of stem cells is not depleted. This phenomenon is called asymmetric division. The other
daughters of HSCs (myeloid and lymphoid progenitor cells) can follow any of the other
differentiation pathways that lead to the production of one or more specific types of blood cell, but
cannot renew themselves. The pool of progenitors is heterogeneous and can be divided into two
groups; long-term self-renewing HSC and only transiently selfrenewing HSC, also called short-
terms. This is one of the main vital processes in the body.
Types:
Each type of blood cell follows a slightly different path of hematopoiesis. All begin as stem cells
called multipotent hematopoietic stem cells (HSC). From there, hematopoiesis follows two
distinct pathways.
Trilineage hematopoiesis refers to the production of three types of blood cells: platelets, red blood
cells, and white blood cells. Each of these cells begins with the transformation of HSC into cells
called common myeloid progenitors (CMP).
After that, the process varies slightly. At each stage of the process, the precursor cells become
more organized:
Red blood cells and platelets
Redblood cells: CMP cells change five times before finally becoming red blood cells, also known
as erythrocytes.

4. Platelets: CMP cells transform into three different cell types before becoming platelets.
White blood cells
There are several types of white blood cells, each following an individual path during
hematopoiesis. All white blood cells initially transform from CMP cells into to myeoblasts. After
that, the process is as follows:
Before becoming a neutrophil, eosinophil, or basophil, a myeoblast goes through four further
stages of development.To become a macrophage, a myeoblast has to transform three more times.
A second pathway of hematopoiesis produces T and B cells.
T cells and B cells
To produce lymphocytes, MHCs transform into cells called common lymphoid progenitors, which
then become lymphoblasts. Lymphoblasts differentiate into infection-fighting T cells and B cells.
Some B cells differentiate into plasma cells after exposure to infection.
Development:
In developing embryos, blood formation occurs in aggregates of blood cells in the yolk sac,
called blood islands. As development progresses, blood formation occurs in the spleen, liver and
lymph nodes. When bone marrow develops, it eventually assumes the task of forming most of the
blood cells for the entire organism. However, maturation, activation, and some proliferation of
lymphoid cells occurs in the spleen, thymus, and lymph nodes. In children, haematopoiesis occurs
in the marrow of the long bones such as the femur and tibia. In adults, it occurs mainly in the
pelvis, cranium, vertebrae, and sternum.
Early haematopoiesis
In utero, the production of blood cells begins in the embryonic yolk sac around 14-19 days.
The predominant site of haematopoiesis in the foetus during the second trimester is the liver. From
seven months, the bone marrow takes over and continues to be the predominant site in adults. In
the first four years of life, almost all marrow cavities contain haematopoietic tissue, termed red
marrow. As we age, the quantity of this haematopoietic tissue decreases and is replaced by fat
(yellow marrow). In adults red marrow is limited to the axial skeleton and long bones. Early

5. haematopoiesis has limited leucopoietic activity. Maternally derived antibodies are therefore key
to the immune system early in life.
Adult haematopoiesis
Normal adult haematopoiesis occurs within the bone marrow and at a steady state of to
match the loss of mature blood cells. The increase in yellow marrow during ageing restricts sites
of haematopoiesis to the axial skeleton and proximal ends of long bones (e.g. femur and humerus).
In the presence of defective or insufficient haematopoiesis in the bone marrow, expansion of the
red marrow may occur. Haematopoiesis can occur in organs such as the liver and spleen, termed
extra-medullary haematopoiesis. All blood cells are derived from the most primitive
haematopoietic cell, referred to as a multipotent haemopoietic stem cell. From these cells, all cell
lineages can be formed. Successful haematopoiesis is dependent on both cell proliferation and
cellular maturation (e.g. development of structural and functional characteristics).
Clinical Significance:
A stem cell transplant is a transplant intended to replace the progenitor hematopoietic stem cells
Hematopoietic stem cell transplantation (HSCT) is the transplantation of multipotent
hematopoietic stem cells, usually derived from bone marrow, peripheral blood, or umbilical cord
blood. It may be autologous (the patient's own stem cells are used), allogeneic (the stem cells come
from a donor) or syngeneic (from an identical twin). It is most often performed for patients with
certain cancers of the blood or bone marrow, such as multiple myeloma or leukemia. In these
cases, the recipient's immune system is usually destroyed with radiation or chemotherapy before
the transplantation. Infection and graft-versus-host disease are major complications of allogeneic
HSCT. Hematopoietic stem cell transplantation remains a dangerous procedure with many
possible complications; it is reserved for patients with life-threatening diseases. As survival
following the procedure has increased, its use has expanded beyond cancer to autoimmune diseases
and hereditary skeletal dysplasias; notably malignant infantile osteopetrosis and
mucopolysaccharidosis.
References:
https://en.wikipedia.org/wiki/Haematopoietic_system

6. https://app.pulsenotes.com/medicine/haematology/notes/haematopoiesis
https://www.medicalnewstoday.com/articles/319544