Answer
The sunken center is the position of the nucleus which is lost in he process of development of erythroblast through reticulocyte to mature erythrocyte.
Work Step by Step
Erythropoietin --a kidney hormone-- convert some cells of erythrocyte colony forming units (ECFUs) to erythroblastss. While still in the bone marrow, erythroblasts multiply and synthesize hemoglobin. Once enough hemoglobin is synthesized, the nucleus shrivels and dies,and is ejected from the cell. The potential RBC, now in the reticulocyte stage, leaves the bone marrow and enters the circulation. Soon, the reticulocyte loses its polyribosomes ( residue of its protein sythethetic apparatus) and becomes a mature erythrocyte (RBC).
The loss of the nucleus results in the erythrocyte having a biconcave shape with a depression where the nucleus used to be. This so-called biconcave shape enables the RBC to fold, and pass safely through very narrow vascular channels far from the large blood vessels.
