● The process of formation of blood cells i.e. RBC’S, WBC’S and platelets is
called as haematopoiesis
● The sites where it occurs are known as hemopoietic tissues or organs
● Eg: Bone marrow,liver, spleen
● The physiologic process of formation of blood cells is called as
● During 3rd week of embryonic life, hematopoiesis begins in the yolk sac
● Around 3rd month of fetal life, some of the hematopoietic stem cells migrate
to the liver which then becomes the main site of hematopoiesis.
● Some blood cell formation also occurs in the spleen, lymph nodes, and
● Around 4 th month of fetal life bone marrow starts producing blood cells and
becomes the sole site of hematopoiesis after birth
● Bone marrow is the soft, gelatinous tissue that fills the intertrabecular
spaces in the cavities of the bones.
● There are two types of bone marrow:
○ Red marrow composed of hematopoietic tissue
○ Yellow marrow composed of fat cells (inactive marrow).
● Active sites of hematopoiesis are: pelvis, vertebra, skull, ribs, sternum, and
proximal ends of long bones.
● In children, 100% of the total marrow space is active, while in adults, 50% of
total marrow space is active
● All the blood cells are derived from the primitive mesenchymal cells called as
pleuripotent hematopoietic stem cells (PHSCs)
● Stem cells→ Become committed to a lineage and are called as progenitor
● Progenitor cells do not have capacity of self-renewal, and have restricted
● Two multipotential cell lines originate from the pleuripotent hematopoietic
○ Myeloid and
● Cells capable of asymmetrically dividing, one group of cell is responsible for
production of well differentiated products and another group of cell is
responsible to maintain the original population of stem cell and shows a
character called as self- renewal
Process of hematopoiesis
1. First step: Pleuripotent stem cell
a. Potency to convert into many different types of tissues
b. Pleuripotent stem cells divides into two different types of multipotent stem cells
2. Second step: Multipotent stem cells
a. Cells that have a capacity to self renew by dividing and to develop into multiple
specialised cell types present in a specific tissue or organ
3. 2 types of multipotent stem cells are:
a. Cells related to lymphoid system :Common Lymphoid Stem Cells
b. Cells related to myeloid system are known as Common Myeloid Stem Cells
Myeloid system refers to all blood cells other than lymphoid cells( lymphocytes)
● Hematopoiesis is regulated by two main factors:
○ Hematopoietic growth factors or cytokines, secreted by various cell types
○ Stromal cells in the microenvironment of the bone marrow.
- With each stage of development, cell size and nuclear size become smaller,
chromatin clumping increases, and ultimately nucleus is extruded.
- Color of cytoplasm changes from basophilic to pink due to hemoglobinization.
- A mature red cell or erythrocyte is a biconcave, non-nucleated disk of 7-8 μ size.
● Hemoglobin consists of
○ Heme (iron and protoporphyrin)
○ Globin (two polypeptide chains).
● A heme group is attached to each polypeptide chain.
● Normally, different types of hemoglobins are present during embryonic life,
fetal life, infancy, and adulthood
● The mature eosinophil (15- 16 μ) is slightly larger than a neutrophil
● Usually bilobed, and contains numerous bright orange-red granules.
● The eosinophil granules contain major basic protein that is toxic to many
● Basophils are 9-12 μ in size
● Cytoplasm is filled with coarse deep purple-black granules that obscure the
● Nucleus is segmented into 2-3 lobes.
● The basophil granules contain histamine and heparin.
● Basophils play a role in allergic and anaphylactic reactions.
● The maturation sequence is monoblast, promonocyte, and monocyte.
● Monocytes are the largest white blood cells in peripheral blood (15-20 μ)
● Irregular shape, kidney-shaped nucleus, and fine reticular chromatin.
● Cytoplasm is abundant, blue-gray with vacuoles.
● After migration to tissues, they are called as macrophages.
There are three main types:
● B lymphocytes: (10-20% of cells)
● T lymphocytes: (60-70%)
● natural killer (NK) cells: (10-15%) Attack and kill virus-infected cells
● Platelets are produced by cytoplasmic fragmentation of cells in bone marrow
called as megakaryocytes
● Megakaryoblast→ promegakaryocyte→ Megakaryocyte and platelets
● Each megakaryocyte produces about 4000 platelets during its lifespan
● The primary regulator of megakaryocyte differentiation is thrombopoietin
● The lifespan of platelets is about 7-10 days.