Haematopoiesis Dr. Khadija

 PHASES OF HAEMOPOIESIS  SITES OF HAEMOPOIESIS  BONE MARROW

Haemopoiesis  A biological process in which new blood cells are formed, and is usually taking place in the bone marrow blood cellsbone marrowblood cellsbone marrow

Types of Cellular Elements   Red blood cells – Erythrocytes   White blood cells – Leukocytes   Neutrophils   Eosinophils   Basophils   Lymphocytes   Monocytes   Platelets – Thrombocytes (not true cells)

GRANULOCYTES AGRANULOCYTES

Sites of Hematopoiesis  In 2 week old embryo, hematopoiesis begins in yolk sac. Production of erythrocytes –first cells

 In 3rd-7th month, stem cells migrate to liver and spleen – major site of fetal liver and spleen – major site of fetal hematopoiesis. hematopoiesis. 

 At birth and into adulthood: confined to bone marrow (red marrow is active) bone marrow (red marrow is active)

Extramedullary haemopoiesis  Blood cell production in hematopoietic tissue other than bone marrow. Occurs when hyperplasia (increase in number of cells per volume of tissue) of marrow cannot meet physiologic blood needs of tissue. Principally occurs in liver and spleen

Blood vessels form in two ways: Vasculogenesis is the formation of new vascular channels by assembly of individual cell precursors called angioblasts. Angiogenesis is the formation of new vessels by budding and branching from preexisting vessels Angiogenesis is the formation of new vessels by budding and branching from preexisting vessels

Vasculogenesis Angiogenesis

 Mesenchymal cells (mesoderm derived) differentiate into endothelial cell precursors- angioblasts (vessel- forming cells).

 Angioblasts aggregate to form isolated angiogenic cell clusters called blood islands

 Small cavities appear within the blood islands and endothelial cords by confluence of intercellular clefts.

 Angioblasts flatten to form endothelial cells that arrange themselves around the cavities in the blood island to form the endothelium.

 These endothelium-lined cavities soon fuse to form networks of endothelial channels (vasculogenesis).

 Vessels sprout into adjacent areas by endothelial budding and fuse with other vessels

 Hemangioblasts in the center of blood islands form hematopoietic stem cells, the precursors of all blood cells,  whereas peripheral hemangioblasts differentiate into angioblasts, the precursors to blood vessels.

 Once the process of vasculogenesis establishes a primary vascular bed, additional vasculature is added by angiogenesis, the sprouting of new vessels

aorta-gonad-mesonephros region (AGM)  The definitive hematopoietic stem cells arise from mesoderm surrounding the aorta in a site called the aorta-gonad- mesonephros region (AGM). These cells will colonize the liver, which becomes the major hematopoietic organ of the fetus. Later, stem cells from the liver will colonize the bone marrow, the definitive blood-forming tissue

Bone marrow  Bone marrow is the flexible tissue found in the hollow interior of bones. In adults, marrow in large bones produces new blood cells.. tissuebones blood cellstissuebones blood cells

Marrow types  red marrow (consisting mainly of hematopoietic tissue) Red blood cells, platelets and most white blood cells arise in red marrow hematopoietic tissueRed blood cells plateletswhite blood cells hematopoietic tissueRed blood cells plateletswhite blood cells  yellow marrow (consisting mainly of fat cells). fat cellsfat cells

 At birth, all bone marrow is red. With age, more and more of it is converted to the yellow type. About half of adult bone marrow is red. 

SITES OF RED MARROW Red marrow is found mainly in the  flat bones, such as the hip bone, breast bone, skull, ribs, vertebrae and shoulder blades, flat boneship bonebreast boneskullribsvertebraeshoulder blades flat boneship bonebreast boneskullribsvertebraeshoulder blades  in the cancellous ("spongy") material at the epiphyseal ends of the long bones such as the femur and humerus. cancellousepiphyseallong bonesfemurhumeruscancellousepiphyseallong bonesfemurhumerus

Cells in bone marrow  fibroblasts fibroblasts  macrophages macrophages  adipocytes adipocytes  osteoblasts osteoblasts  osteoclasts osteoclasts  endothelial cells endothelial cells endothelial cells

Compartmentalization  There is biologic compartmentalization in the bone marrow, in that certain cell types tend to aggregate in specific areas. For instance, erythrocytes, macrophages and their precursors tend to gather around blood vessels, while granulocytes gather at the borders of the bone marrow. biologic compartmentalizationcell typeserythrocytesmacrophagesblood vesselsgranulocytesbiologic compartmentalizationcell typeserythrocytesmacrophagesblood vesselsgranulocytes

Hematopoietic precursor cells: promyelocyte in the center, two metamyelocytes next to it and band cells from a bone marrow aspirate promyelocytemetamyelocytes band cellspromyelocytemetamyelocytes band cells