Check out the formation of Blood Cells in human body

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What is the hematopoiesis ?

Hematopoiesis is the production, Development and the maturation of various cellular components of the blood

The cellular components of blood are developed from the Hematopoietic stem cells also known as Pluripotent Stem cells or totipotent stem cells which further differentiate and mature into typical blood cells.

The Pluripotent stem cells give rise to two types of Stem cells or lineages –

Myeloid stem cells/Lineage – Differentiate in the Redbone marrow and form Erythrocytes, Granulocytes (Neutrophils, Eosinophils, and Basophils), Monocytes and Thrombocytes
Lymphoid stem cells/Lineage – differentiates in the Redbone marrow and then migrates to the lymphoid tissue. They form T- and B- Lymphocytes.

The production of erythrocytes is called as Erythropoiesis; the leukocytes are called as Leucopoiesis and that of platelets is Thrombopoiesis.

Leucopoiesis is further subdivided into –

Granulopoiesis – formation of granulocytes
Monopoiesis – formation of monocytes
Lymphopoiesis – formation of lymphocytes

What are the Sites of Hematopoiesis ?

The site of Hematopoiesis depends on the age of the person as follows –
In human embryo – Yolk sac is the main site of Hematopoiesis
By the 3^rd to 7^th month of Intra-uterine life – Hematopoiesis occurs in liver and spleen.
By the 7^th month till birth – it starts in the bone marrow.
From birth till maturity – occurs in bone marrow, spleen, and liver.
In adults – Hematopoiesis occurs in the Central skeleton (Vertebrae, Sternum, Ribs, Skull, Sacrum, and pelvis), proximal ends of the long bones like Femur, Tibia, and Humerus.

What is Erythropoiesis?

It is the process of formation, development, and maturation of Red blood cells in the bone marrow.

Sites of production –

The red bone marrow is the prime site for the production of RBC’s from birth till age 20
After the age of 20 years, RBCs are mainly produced in the marrow of Vertebrae, Sternum, Ribs, and Pelvis
After the gradual structural and functional maturation – the cells are released in peripheral circulation.

Regulation of Erythropoiesis – Erythropoietin is the hormone produced mainly by the kidneys, helps to regulate the process of Erythropoiesis so that the number of RBCs is sufficient to sustain adequate tissue oxygen levels.

STAGES OF ERYTHROPOIESIS	NAME OF STAGE	CELL SIZE	CYTOPLASM	NUCLEUS
1.)	Proerythroblast	15-20 microns	Deeply basophilic because of high RNA content.	Large, Central and contains multiple Nucleoli and thick strand of chromatin are present.
2.)	Early Normoblast	12-16 microns	More basophilic.	Gets smaller, coarse and granular chromatin are present, nucleoli may be absent.
3.)	Intermediate Normoblast	10-14 microns	Polychromatic – it contains mixture of basophilic RNA and Acidophilic Hemoglobin – Pinkish- blue.	Nuclear chromatin – condensed, nucleoli absent.
4.)	Late Normoblast	8-10 microns	Acidophilic – pink color.	Small and pushed to one side of the cell.
5.)	Reticulocytes	7-9 microns	Light pink, RNA strands are present.	Degenerates.
6.)	Erythrocytes	7.2-7.5 microns	Pink – due to hemoglobin.	Absent.

Reticulocytes are called as the juvenile RBCs or Immature RBCs.

Reticulocytes spend 1-2 days in the marrow than 1-2 days in circulation where it gets mature into Erythrocytes and attains biconcave shape.

Normal range of Reticulocytes –

Adults – 0.5 – 2.5%
Infants – 2 – 6%

What is Leukopoiesis?

It is the process of formation or production of leucocytes which normally occurs in the blood-forming tissue of the bone marrow. There are 3 independent series, which leads to the formation of 3 different types of white cells –

Granulocytes – Granulopoiesis
Lymphocytes – Lymphopoiesis
Monocytes – Monopoiesis/ Monocytopoiesis

The formation of White Blood cells is regulated by –

Stimulation by cytokines and Colony stimulating factor.
Produced by Macrophages & T – lymphocytes.
Activated by coming in contact with foreign organisms.

What is Granulopoiesis?

Granulopoiesis is the process of formation or production of Granulocytes that are the type of White blood cells that contains the granules in their cytoplasm, which begins at the Myeloblast and passes through the stages of a Promyelocyte, Myelocyte, Metamyelocyte and Band cells and ultimately results in the formation of 3 types of Granulocytes – Neutrophils, Eosinophils & the Basophils.

S. No.	STAGES OF GRANULOPOIESIS	CELL SIZE	CYTOPLASM	NUCLEUS
1.)	*Myeloblast*	Large, 15-20 microns	Thin rim of cytoplasm, Devoid of granules	Large, Round or Oval nearly filling the cell.
2.)	*Pro - Myelocyte*	Slightly larger than Myeloblast, 15-25 microns.	Contains primary or Azurophilic granules, are Peroxidase positive.	Round, contains nucleoli - less prominent
3.)	*Myelocyte*	Small, 12 – 18 microns	Specific or Secondary granules appears in cytoplasm	Round or Oval, Nucleoli are absent.
4.)	*Meta – Myelocyte*	Small, 10 – 15 microns	Both primary and secondary granules.	Horse shoe or Kidney shapes, Nucleoli absent.
5.)	*Band cells*	Very small, 9 – 15 microns	Pink with purple colored granules	Band like nucleus, Condensed nuclear chromatin, further constricts to form lobes
6.)	Segmented forms – I) Neutrophil granulocytes II) Eosinophil Granulocytes III) Basophil Graulocytes	10-14 microns 10-14 microns 10-14 microns	Bluish pink with Fine, Light pink granules. Pink colored with Coarse granules do not cover the nucleus. Bluish color with coarse granules, covers the nucleus.	Deep purple with 2-5 lobes, condensed chromatin Purple, Bilobed or Trilobed Purple, Bilobed

What is Lymphopoiesis?

Lymphopoiesis is the process of formation or production of Lymphocytes, a type of white blood cells that lack the granules in their cytoplasm, which begins at the Lymphoblast and passes through the stage of Prolymphocyte and ultimately results in the formation of mature Lymphocytes.

S. No.	STAGES OF LYMPHOPOIESIS	CELL SIZE	CYTOPLASM	NUCLEUS
1.)	*Lymphoblast*	Large, 10-20 microns	Agranular & Scanty	Large, Round or Oval with slightly condensed nuclear chromatin & few nucleoli.
2.)	*Prolymphocyte*	Slightly small, 10-18 microns	Medium to deep homogeneous blue	Centrally placed, Round or Indented which may have 0-1 nucleoli but is usually not seen.
3.)	*Lymphocyte*	i.) Large, 9-15 microns ii.) Small, 7-8 microns	i.) Plentiful, colorless to light blue. ii.) Narrow edge, Light or dark blue.	i.) Oval / bulging, Central, Nucleoli absent. ii.) Round/ Kidney shaped, compact chromatin.

What is Monopoiesis / Monocytopoiesis?

Monopoiesis or Monocytopoiesis is the process of formation of production of Monocytes, a type of White blood cells that lack the granules in their cytoplasm, which begins at the Monoblast, passes through the stage of Promonocyte and ultimately results in the formation of mature Monocytes.

S. No.	STAGES OF MONOPOIESIS	CELL SIZE	CYTOPLASM	NUCLEUS
1.)	*Monoblast*	Large, 12-20 microns	Agranular, Light blue.	Round / Oval, has fine chromatin, 1-4 nuclei, Central or eccentric.
2.)	*Promonocyte*	Large, 12-20 microns	Scanty, Gray to pale blue & with rare or no granules. Sometimes Vacuoles may be present.	Round, Indented nucleus with immature chromatin.
3.)	*Monocytes*	Large, 15-20 microns	Dull, Gray-blue	Round or Kidney shaped.

What is Thrombopoiesis?

Thrombopoiesis is the formation or production of Thrombocytes or Platelets in the bone marrow which is formed by fragmentation of mature Megakaryocyte membrane projections, begins at Megakaryoblast and passes through the stages of Promegakaryocyte and then the Megakaryocytes ultimately results in the formation of mature Thrombocytes or Platelets.

S. No.	STAGES OF THROMBOPOIESIS	CELL SIZE	CYTOPLASM	NUCLEUS
1.)	*Megakaryoblast*	Large, 20-30 microns	Scanty, Agranular cytoplasm	Large, Oval or Kidney shaped with several nucleoli
2.)	*Promegakaryocyte*	Large, 25-30 microns	Intensely basophilic, Agranular	Large, Indented & may show mild lobulation
3.)	*Megakaryocyte*	Very Large, 50-100 microns	Breaks up to form platelets (into tiny fragments)	Multi lobed, Very large
4.)	*Platelets*	Small, 1-4 microns	Basophilic, purple- reddish granules are present	Anucleated cells

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