MLAB Hematology Keri Brophy-Martinez Chapter 4 : Hematopoietic Organs

Hematopoiesis: blood formation Intramedullary Origin of blood cells and sequential sites of normal blood production within the bone marrow Origin of blood cells and sequential sites of normal blood production within the bone marrowFetus Yolk sac Yolk sac Liver and spleen Liver and spleen Bone marrow (all bones) Bone marrow (all bones)Adult Child up to teen years - all bones Child up to teen years - all bones 18 years and up - flat bones (sternum, ribs, pelvis, vertebra, skull) 18 years and up - flat bones (sternum, ribs, pelvis, vertebra, skull) Adults- bone marrow Adults- bone marrow

Ontogeny (origin) of Hematopoiesis Begins in mesoderm of yolk sac with mesenchymal cells forming primitive nucleated erythoid cells. Production continues for about six weeks before declining and ends in about two months. Fetal liver now assumes responsibility for hematopoiesis during second month. Yolk sac erythoid cells move to liver and remain there until the seventh month remain there until the seventh month From third to sixth month, hematopoiesis occurs in hematopoiesis occurs in kidney, thymus, spleen and lymph kidney, thymus, spleen and lymph nodes nodes

Ontogeny (origin) of Hematopoiesis Around seventh month, hematopoiesis shifts to bone marrow which remains major site of blood cell production. Fetal marrow fills with RBCs. Bones of toes, fingers, vertebrae, ribs, pelvis, long bones and cranium also fill with RBCs. About three months of life, see platelets; five months of life, see leukocytes.

Ontogeny (origin) of Hematopoiesis At birth, liver and spleen stop hematopoietic activity. Sites of hematopoiesis are in bone marrow. Cell production occurs in extravascular part of red marrow. When cells almost mature, migrating cells leave marrow and emerge into venous sinuses.

Ontogeny (origin) of Hematopoiesis Ontogeny (origin) of Hematopoiesis During infancy and early childhood, hematopoiesis occurs entirely in medullary space. Hematopoiesis gradually decreases in shafts of long bones. At years of age, hematopoiesis occurs only in sternum, ribs, pelvis, vertebrae, and skull. After age 40, less area in these sites available for hematopoiesis.

Ontogeny (origin) of Hematopoiesis Ontogeny (origin) of Hematopoiesis If have increased demand, liver, spleen, and other tissues may act as compensatory mechanism called extramedullary hematopoiesis.

Ontogeny (origin) of Hematopoiesis Bone marrow hematopoietic activity divided into two pools: stem cell pool (containing undifferentiated stem cells) and bone marrow pool (containing cells that are proliferating and maturing) with eventual release of mature cells into peripheral blood.

Storage Pools Granulocytes and platelets also exist in two pools: functional cells in the circulatory system, and those that exist in storage form( marginal).

Bone Derived Cells Non-hematopoietic cells involved with bone turnover closely linked with hematopoiesis Osteoblasts Osteoblasts Responsible for the formation, calcification and maintenance of the bone structure Large irregularly shaped cells in the bone marrow that may be confused with plasmacytes or malignant cells Osteoclasts Osteoclasts Responsible for absorption of bone Giant multinucleated, irregularly shaped phagocytic cells in the bone marrow that may be confused with megakaryocytes