Embryonic Haemopoiesis

What is the primary source of new blood cells during normal childhood and adult life?

At which stage of fetal development does the bone marrow become the most important site of haemopoiesis?

What is the term for the process by which the liver and spleen resume their fetal haemopoietic role in certain disease states?

What is the percentage of fat in the marrow of middle-aged adults?

What is the region of the developing embryo where the stem cells that give rise to definitive haemopoiesis are first observed?

What is the term for the common precursors of endothelial and haemopoietic cells?

During which stage of fetal development do the liver and spleen continue to produce blood cells?

What is the fate of the marrow in the long bones during childhood and beyond?

Haemopoiesis During Gestation

Haemopoiesis starts with the embryonic yolk sac through a process called 'primitive haemopoiesis'
However, the main source of haemopoiesis comes from stem cells in the aorta-gonads-mesonephros (AGM) region
These stem cells, called haemangioblasts, give rise to endothelial and haemopoietic cells that colonize the liver, spleen, and bone marrow

Haemopoietic Sites During Fetal Development

From 6 weeks to 6-7 months, the liver and spleen are the primary haemopoietic organs
The placenta also contributes to fetal haemopoiesis during this period
From 6-7 months, the bone marrow becomes the main site of haemopoiesis

Haemopoiesis in Childhood and Adult Life

In childhood and adult life, the bone marrow is the sole source of new blood cells
Developing cells are located outside bone marrow sinuses, and mature cells are released into the sinus spaces and general circulation
In infancy, all bone marrow is haemopoietic, but in childhood and adulthood, fatty replacement of marrow occurs in long bones
In adults, haemopoietic marrow is limited to the central skeleton and proximal ends of the femurs and humeri

Haemopoiesis in Diseased States

In certain diseases, haemopoiesis can expand down the long bones
The liver and spleen can also resume their fetal haemopoietic role, known as 'extramedullary haemopoiesis'
Even in active haemopoietic areas, approximately 50% of the marrow consists of fat in middle-aged adults
Remaining fatty marrow can revert to haemopoiesis in some diseases

Learn about the development of haemopoiesis in the embryonic stage, from primitive to definitive haemopoiesis and the role of haemangioblasts. Understand how the liver, spleen, and bone marrow are involved in this process.