(83)

(82)

The erythrocyte is a highly specialized end cell whose function is to supply oxygen to all body cells. Packed with the protein-iron compound hemoglobin, they lack nuclei and most of their cytoplasmic organelles. Its hallmark biconcave shape serves to increase the surface area for gas exchange.

Erythrocytes have a short lifespan so there is a continual need for regulated production. They are phagocytosed by the reticuloendothelial system every 50 to 70 days in the fetus and every 120 days in the adult. Because of the constant need for new erthrocytes, it is especially important for its stem cells to strike a balance between maintaining a reserve of stem cells and providing cells for differentiation according to demand. (77)

Many intermediate progenitor stages occur before maturation of the final erythrocyte. In in vitro experiments, progenitors can be identified based on the morphological appearance, size, and cellular composition of the colonies. The different morphological stages of the progenitors are induced by a variety of signaling molecules such as interleukin-3, burst-promoting activity (BPA) hormone, and erythropoietin. (1)