HEMATOPOIESIS
Hematopoiesis (Hema = Blood, Poiesis= Formation) is the process of differentiation, proliferation and maturation of blood cells inhematopoietic tissue under the influence of growth factors and hormones.
SITE OF HEMATOPOIESIS
Site of hematopoiesis is depend on hematopoiesis phase, Hematopoiesis is divided into two phases:
1. Pre-natal Hematopoiesis (Intra-uterine Hematopoiesis)
2. Post-natal Hematopoiesis (After birth Hematopoiesis)
1. Prenatal Hematopoiesis (Intra-uterine Hematopoiesis)
○ First few weeks (19 days) of gestation yolk sac is a transient site of hemopoiesis.
○ Definitive hemopoiesis initiate from a population of stem cells first observed on the AGM (aorta‐gonads‐mesonephros).
○ These common precursors of endothelial and hemopoietic cells is called Hemangioblast.
○ Haemangioblast is responsible to produce liver, spleen and bone marrow
○ From 6 weeks to 6–7 months of fetal life, the liver and spleen are the major hemopoietic organs and continue to produce blood cells until about 2 weeks after birth.
○ The placenta also contributes to fetal hematopoiesis.
○ The bone marrow is the most important site from 6–7 months of fetal life.
2. Post-natal Hematopoiesis (after birth Hematopoiesis)
○ During infancy (up to 3 years) bone marrow is a major site of hematopoietic. In this age hematopoiesis take place in all bones.
○ After infancy red marrow is gradually replaced by yellow marrow.
○ In adult age hematopoiesis take place in flat bones i-e Vertebrae, ribs, sternum, skull, sacrum and pelvis, proximal ends of long bones.
ORIGEN OF HEMATOPOIESIS
○ All blood cells are originate from a single primitive cell called Pluripotent hematopoietic stem cells (PHSC)
○ PHSC is unique in nature having the capability of selfrenewal and differentiate into committed cell line
○ It is a rare cell found 1 in 20 millions of all nucleated cells of the bone marrow
○ Morphologically its look like a small lymphocyte, it can be differentiate with immunohistochemically (CD34 +ve and CD38 –ve)
○ PHSC differentiate into myeloid and lymphoid precursor stem cells
○ Myeloid progenitor stem cell differentiate into colony forming unit CFU-GEMM. It is further differentiae into erythroid , myeloid, monocytic and megakaryocytic committed stem cells
○ Lymphoid progenitor stem cell is differentiate into B or T or NK cell precursor
○ The spleen, lymph nodes and thymus are secondary sites of lymphocyte production
REGULATION OF HEMATOPOIESIS
○ Hematopoiesis is up regulated by certain growth factors/cytokines/ interleukins and hormones and down regulated with inhibitors
○ Combination of cytokines stimulates the proliferation and/or differentiation of various hematopoietic cells
