Final answer:
During the first trimester of fetal development, hemopoiesis occurs initially in the yolk sac and is later taken over by the fetal liver, spleen, and lymph nodes. Towards the end of the first trimester, the bone marrow begins to assume the role of blood cell production.
Step-by-step explanation:
During the first trimester of fetal development, hemopoiesis—the process of blood cell formation—begins in the yolk sac of the developing embryo. As the fetus grows, this critical function is taken over by the fetal liver, spleen, and lymph nodes. By the end of the first trimester, the bone marrow begins to take on the responsibility of erythrocyte production, a role initially performed by the liver. Hemopoietic stem cells, influenced by various growth factors, give rise to the vast array of blood cells required to sustain life.
Proper hematopoietic function is crucial during this period, as the fetal internal organs and body structures are rapidly developing. Exposure to toxins can interrupt this delicate process, leading to adverse effects on the fetus's development and survival. The transition of hemopoiesis from different embryonic sites to the bone marrow signifies the body's evolving needs as it prepares for autonomous function post-birth.