Haematopoiesis

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Haematopoiesis

• Dr. Khadija
• 22-03-10

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• PHASES OF HAEMOPOIESIS
• SITES OF HAEMOPOIESIS
• BONE MARROW

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Haemopoiesis

• A biological process in which new blood cells are
  formed, and is usually taking place in the bone
  marrow

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Types of Cellular Elements

• Red blood cells Erythrocytes
• White blood cells Leukocytes
• Neutrophils
• Eosinophils
• Basophils
• Lymphocytes
• Monocytes
• Platelets Thrombocytes (not true cells)

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GRANULOCYTES AGRANULOCYTES
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Sites of Hematopoiesis

• In 2 week old embryo, hematopoiesis begins in
  yolk sac. Production of erythrocytes first cells

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

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

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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

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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

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

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

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

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

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

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

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• 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.

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

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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

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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..

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Marrow types

• red marrow (consisting mainly of hematopoietic
  tissue) Red blood cells, platelets and most white
  blood cells arise in red marrow
• yellow marrow (consisting mainly of fat cells).

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• 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.

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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,
• in the cancellous ("spongy") material at the
  epiphyseal ends of the long bones such as the
  femur and humerus.

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Cells in bone marrow

• fibroblasts
• macrophages
• adipocytes
• osteoblasts
• osteoclasts
• endothelial cells

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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.

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