University:
Mt. San Antonio College
Course:
MEDI 90 | Medical Terminology
Academic year:

2024
Views:

48

Pages:

1
Author:

Larbi M.

Hematopoiesis Hematopoiesis - whose name means "production of blood" - is the function by which the body produces and renews the formed elements of the blood (red blood cells, leukocytes and platelets). This production, very finely regulated, comes from hematopcietic stem cells, capable of self-renewal, which allows the maintenance of a constant number of stem cells, and of differentiating to ensure the renewal of cells which die physiologically (and even ensure even faster renewal in the event of increased needs). Within hematopoiesis, we distinguish between myelopoiesis, allowing the production of myeloid cells (red blood cells, polymorphonuclear cells, monocytes, platelets), and lymphopolesis allowing the production of lymphocytes. The regulation of hematopoiesis is under the control of numerous growth factors. Localization of hematopolesis During the embryonic period, three distinct locations of hematopolesis have been characterized, each corresponding approximately to a trimester. The first phase takes place in a poorly defined region called AGM (Aorta-Gonad-Mesonephros) located at the level of the mesoderm, exclusively erythroblastic. The second phase takes place mainly in the liver and spleen. From the sixth month, hematopolesis becomes medullary. At birth, hematopolesis is almost exclusively medullary. It occurs in all bones, including the skull and phalanges. The marrow is, at this age, almost entirely hematopoietic, with very few fat cells. During aging, hematopoiesis undergoes "centripetal" regression which, in adults, is limited to the flat bones: sternum, ribs, vertebrae and pelvis. In the elderly, bone marrow density gradually decreases, especially after age 70, but without a notable reduction in the number of circulating blood cells. There is no physiological aplasia in the elderly. Hematopoietic stem cells These are rare cells (representing only a very small percentage of marrow cells), characterized by self- renewal capacities and commitment to differentiation (totipotent, i.e. only a single stem cell will be able to give rise to red blood cells, polynuclear cells, monocytes, megakaryocytes and lymphocytes).