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Hematopoiese
O processo de formação de células sanguíneas é denominado hematopoiese. A hematopoiese começa cedo durante o desenvolvimento, no sétimo dia da embriog…
A hematopoiese é o processo pelo qual diferentes tipos de células sanguíneas são produzidos a partir de um único tipo de células chamadas células-tronco hematopoiéticas ou HSCs.
Em adultos, a hematopoiese ocorre na medula óssea. A medula óssea é cercada por células reticulares, células estromais e uma intrincada rede de vasos sanguíneos. A hematopoiese ocorre nos espaços entre esses vasos sanguíneos e os componentes celulares.
Os HSCs se multiplicam e se auto-renovam continuamente passando por uma divisão assimétrica. Eles produzem duas células-filhas, uma das quais pode se auto-renovar como a célula-mãe e manter o pool de HSC na medula óssea.
Em contraste, fatores moleculares específicos liberados em resposta à lesão tecidual, surto de crescimento ou infecção ajudam a diferenciar a segunda célula-filha em células mais especializadas, incluindo as células progenitoras mieloides e as células progenitoras linfoides.
Os progenitores do HSC podem produzir todos os tipos de células sanguíneas, incluindo glóbulos vermelhos, linfócitos, macrófagos e células natural killer.
Uma vez comprometidas com seus papéis específicos, essas células sanguíneas deixam a medula óssea e entram nos vasos sanguíneos para migrar para seu local de ação.
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Q1: What is hematopoiesis and where does it occur in adults?
Hematopoiesis is the process by which different types of blood cells are produced from hematopoietic stem cells (HSCs). In adults, hematopoiesis occurs in the bone marrow, which is surrounded by reticular cells, stromal cells, and blood vessels. The process takes place in the spaces between these components, where HSCs continuously multiply and self-renew.
Q2: How do hematopoietic stem cells maintain themselves through asymmetric division?
HSCs undergo asymmetric division to produce two daughter cells with different fates. One daughter cell self-renews like the parent cell, maintaining the HSC pool in the bone marrow. The other daughter cell differentiates into specialized cells such as myeloid progenitor cells and lymphoid progenitor cells in response to molecular factors released during tissue injury, growth spurts, or infection.
Q3: What types of blood cells can hematopoietic stem cells produce?
HSC progenitors can produce all types of blood cells due to their multipotent nature, including red blood cells, lymphocytes, macrophages, and natural killer cells. Once committed to their specific roles, these differentiated blood cells leave the bone marrow and enter blood vessels to migrate to their sites of action throughout the body.
Q4: What are the differences between dormant and active hematopoietic stem cells in bone marrow?
Bone marrow contains two HSC populations with distinct characteristics. Dormant or quiescent HSCs are non-proliferative and remain attached to the inner bone surface. Active or primed HSCs rapidly proliferate in response to vascular endothelial growth factors and are located in the central marrow region, allowing them to respond quickly to physiological demands.
Q5: How do hematopoietic stem cells migrate and home to target tissues?
HSCs migrate from bone marrow into circulating blood by losing cell adherence molecules and chemokine receptor CXCR4. During HSC homing, they regain these molecules and receptors, allowing them to bind and anchor to target tissues following chemokine gradients. Homing to bone marrow is essential for regulating HSC homeostasis and maintaining the HSC pool through proliferation.
Q6: When does hematopoiesis begin during embryonic development?
Hematopoiesis starts early during development on the seventh day of embryogenesis in a phase called the primitive wave. During this phase, the extraembryonic yolk sac produces erythroid cells and endothelial cells from a common precursor called hemangioblast. These erythroid cells provide oxygen to support the rapidly dividing embryo's growth.
Q7: Where do hematopoietic stem cells migrate during fetal development?
During fetal development, hemangioblasts develop into hematopoietic stem cells that migrate to the liver. This represents a shift from the yolk sac as the primary hematopoietic site during the primitive wave to the liver during fetal stages, before hematopoiesis eventually becomes established in the bone marrow of adults.