Hematopoiesis: The stages of blood cell formation.

Hematopoiesis refers to the process of production or formation of the cellular components of blood which include the white blood cells, red blood cells, and platelets. The hematopoietic system, therefore, refers to the tissues responsible for the production of cellular components of blood.

A group of blood-forming cells is generated during embryonic development. the process occurs first in the York sac then the fetal liver at 6 weeks, the spleen at 12 weeks, and in the bone marrow at 20 weeks of gestation and in adult life. Under some circumstances, the process of hematopoiesis may occur in the spleen of an adult, a process known as extramedullary hematopoiesis.

The undifferentiated stem cells give rise to precursor cells that are committed to either the lymphoid or the myeloid lineage pathway. The former gives rise to the lymphocytes while the latter gives rise to the mononuclear phagocytes, polymorphonuclear lymphocytes, blood cells and megakaryocytes which develop into platelets as shown below:

Disorders of the systems involved in hematopoiesis

Just like any other process in the body, the hematopoietic process is affected by disorders that prevent the formation of fully functional blood cells. some of them are treatable while others are not. For instance, cancers of the bone, lymph nodes, thymus, liver, and spleen are the most notorious in affecting the process of blood cell formation, with lymphoma being the leading tumor affecting the hemopoietic system.

These disorders result from a wide range of sources including exposure to ionizing radiations, insecticides, birth defects, exposure to heavy metals, nitrites, nitrates among others.

The process of hematopoiesis is important in that it maintains the balance in cellular components of the blood as some are cleared out due to maturity, Others that are still young enter the circulation. This must be done in a regulated manner, failure of which can be problematic. The regulation of the hematopoietic processes is through cytokines which are chemicals produced by specific cells to stimulate or regulate the activity of other cells.