Final answer:
The primary site for blood cell development during the second trimester of pregnancy is the bone marrow. Initially, the yolk sac, and then the liver, are responsible for hematopoiesis, but this function is taken over by the bone marrow by weeks 9-12 of fetal development.
Step-by-step explanation:
During the second trimester of pregnancy, the primary site for blood cell development shifts from the liver and spleen to the bone marrow. Early in development, structures like the yolk sac and later the liver play pivotal roles in hematopoiesis. By weeks 9-12 of fetal development, the bone marrow begins to take over the process of erythrocyte production, also known as red blood cell formation, which is a significant change from the embryonic period where the liver was mainly responsible for this task.
The bone marrow, especially the red bone marrow, becomes a highly active site for the production of blood cells, while the yellow bone marrow serves primarily as an energy storage site. The red bone marrow is comprised of a loose collection of cells where most hematopoietic (blood-forming) activities occur, including the development of B cells, whereas the maturation of T cells mainly happens in the thymus gland after an immature version, called a thymocyte, leaves the bone marrow.
Overall, the critical role of the bone marrow in blood cell production during fetal development underscores its importance in the growing human body, ensuring that adequate blood supply is available to meet the metabolic needs of developing tissues and organs.