Final answer:
Extramedullary hematopoiesis occurs primarily in the liver and spleen in adults, especially under pathological conditions that impair the bone marrow. It is part of the broader process of hematopoiesis, the formation of blood cells from stem cells, which prior to birth, takes place in multiple tissues.
Step-by-step explanation:
Extramedullary hematopoiesis is the process of blood cell formation occurring outside the bone marrow. This process can occur in various tissues throughout the body, but in the adult human, it predominantly occurs in the liver and spleen, especially during pathological conditions such as bone marrow failure due to diseases like bone cancer. Hematopoiesis involves the differentiation of blood cells from hematopoietic stem cells located in the red bone marrow. Initially, stem cells divide and give rise to more specialized cells which then differentiate into all the various types of blood cells including erythrocytes, leukocytes, and platelets.
Prior to birth, hematopoiesis occurs in several tissues beginning with the yolk sac and continuing in the liver, spleen, lymphatic tissue and red bone marrow. After birth, the primary location of hematopoiesis is within the red marrow of certain bones, yet the capability for extramedullary hematopoiesis remains latent in organs like the spleen and liver. The importance of this process is underscored when there is a failure in the normal sites of hematopoiesis, which can prompt the liver and spleen to resume their blood-forming function.