What is hemapoiesis?
Hemapoiesis is a process of blood cell formation, differentiation and development. In fetus the process called extramedullary hemapoiesis but in adult it was called medullary hemapoiesis.
Where the Hemapoiesis occurs?
When in fetus 0-2 month years old baby hemapoiesis in the yolk sac after 2 month it will change the sites at the liver and spleen until 7 month. Lastly in fetus from the 5-9 month the sites of hemapoesis occurs in the bone marrow.
In Fetus it was begins at 19th day of life producing the red blood cell which is the pool of blood island at yolk sac. After 2 months the cell will migrate to liver and spleen to produce rbc and after 4 month the medullary hemapoiesis begins in bone marrow which are the rbc , wbc and platelets develop.
After the birth to 3 years old the hemapoiesis occurs in red bone marrow of almost every bone. Hemapoiesis occurs in red bone marrow of almost every bone at this stage. Beginning at 4 years the skeleton growth exceed the demand of the rbc production. Thus the hemapoietic activity moves to axial skeleton and proximal ends of long bones completed at 18 years old.
The marrow cavities are replaced with fat or known as yellow bone marrow at age 40, the composition of fat & haemopoietic tissue are equal. Extramedullary haemopoiesis is the hemopoiesis that occur only in fetus. But it can be extramedullary haemopoiesis can also occur in adults if the bone marrow is no longer functional and if the bone marrow cannot keep up with the demand for blood cells. When it happens the liver and spleen will be enlarged.
References
Adamo L., García-Cardeña G. (2012). The vascular origin of hematopoietic cells. Dev. Biol. 362, 1-10 [PubMed] [Google Scholar]
Bertrand J. Y., Kim A. D., Violette E. P., Stachura D. L., Cisson J. L., Traver D. (2007). Definitive hematopoiesis initiates through a committed erythromyeloid progenitor in the zebrafish embryo. Development 134, 4147-4156 [PMC free article] [PubMed] [Google Scholar]
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