Present-day hematopoietic system is based on the experimentally proved existence of stem cells.
The most immature form of stem cells is the pluripotential stem cell (PSC) that has the ability of differentiating into all kinds of stem cells, as well as the ability of self-renewing.
By activating hematopoietic hormones and through cells interaction PSC develops into:
The newly formed cells then develop into directed stem cells (DSC). DSC are incapable of self-renewing, possess restricted ability of differentiation, with a great ability of proliferation. All these cells are not morphologically recognizable.
Immature cells, i.e., a transient population of the cells morphologically and cytochemically recognizable, can arise from DSC. They develop into mature blood cells through consecutive divisions and parallel maturation. The process of maturation consists of a number of changes taking place in the cell. Finally, a mature end cell is formed and has capability of division. The process is characterized by perceptible morphological and functional changes within the specific cell population. During fetal life the hematopoietic cells are formed first in the yolk sac, then in the liver and spleen, and later their development is confined to the bone marrow. After birth, blood cells are formed principally in the bone marrow.
The cells of the reticuloendothelial system are the reticuloendothelial cell, hemohistioblast, and hemocytoblast. They can normally be formed in smears of hematopoietic organs (bone marrow, lymph gland, and spleen). Only in some diseases they may be rarely seen in peripheral blood smears.