Hematopoiesis, Hematopoietic Precursors, and the Bone Marrow Flashcards
- Describe where hematopoiesis occurs before birth, in childhood, and in adulthood.
Hematopoiesis = production of all blood cells (granulocytes, lymphocytes, monocytes/macrophages, RBCs, platelets, and dendritic cells). Ongoing because you’re also losing cells all the time.
RBCs live 120 days, neutrophils live 7 hours, and platelets live 8.5 days.
In embryonic stage, hematopoiesis happens in the yolk sac until 3 mos. Gestational age.
In fetal development, hematopoiesis happens in the liver/spleen.
Bone marrow is the official site of hematopoiesis by time of birth.
-in children, occurs in most of the marrow cavity throughout skeletal system
-in adults, 90% hematopoiesis occurs in vertebrae, pelvis, sternum, ribs, and calvarium (skull)
-marrow encased by cortical bone (interspersed by trabecular bone with osteo cells). Between trabeculae are vascular sinusoids with leaky endothelial walls. Stromal cells bound on non-luminal side (produce protein of marrow, like type 4 collagen, and regulatory factors).
- Discuss hematopoietic stem cells, progenitor cells, and precursor cells
HSC’s themselves are very rare in marrow and not morphologically recognizable. Participate in asymmetric cell division 1 daughter cell is a stem cell and 1 daughter is a multipotent progenitor cell. So HSC’s can self renew. Gives rise to lymphoid and myeloid cells. Whole process works on amplification because you only have a few stem cells that give rise to ALL the other cells.
-pluripotent stem cells = CFU-GEMM and CFU-L (granulocytes/erythroid/monocyte/megakaryocyte and lymphoid)
Progenitor cells can be multipotent (can differentiate into all lymphoid and myeloid lineages), oligopotent (common progenitor cells, and lineage-restricted progenitor cells.
-ability to self renew is limited; generally irreversibly committed
-once a cell becomes lineage-restricted, it’s committed and can produce tons of cells
Precursor cells are recognizable, maturing cells; recognizable in a smear. Cannot self renew; give rise to mature, functional cells.
- Describe the major hematopoietic growth factors (HGFs).
HGF’s are complex and can work on more than cell lineage.
EPO – made by kidney in response to hypoxia, promotes erythropoiesis.
Thrombopoietin (TPO) – promotes megakaryopoiesis.
GM-CSF (granulocyte-monocyte colony stimulating factor) – promotes granulopoiesis and monopoiesis
G-CSF (granulocyte colony stimulating factor) – promotes granulopoiesis
M-CSF (monocyte colony stimulating factor) – promotes monopoiesis
IL-5 – promotes production of eosinophils.
IL-3 – promotes production of basophils.
- Discuss the names, characteristics, and order of progression of the different precursor and mature cell types in the erythroid, granulocytic, monocytic, and megakaryocytic lineages.
1) Erythrocyte maturation gradual condensation of chromatin, degeneration of nucleus, loss of organelles, can’t replicate.
-pronormoblast = large nucleus; cytoplasm has lots of RNA so stains blue. N:C = 8:1.
-basophilic normoblast = slightly smaller, cytoplasm is very basophilic/blue; relatively large oval nucleus., N:C = 6:1
-polychromatophilic normoblast = smaller, blue-gray/pink-gray cytoplasm, round, smaller nucleus (stained blue). N:C = 3:1.
-orthochromic normoblast = even smaller, increased pink cytoplasm; small pyknotic nucleus. N:C = 1:2
-reticulocyte = smaller, pink-grey cytoplasm, NO nucleus. Use supravital methylene blue staining to highlight RNA
-mature RBC = slightly smaller, pink cytoplasm, round and biconcave. No nucleus.
rate depends on Hgb level, EPO (stimulated by hypoxia) activates HSCs to become pronormoblasts, increases mitosis and maturation rate, increases Hgb production, and increases reticulocyte release.
2) Granulopoiesis. Neutrophils stain pink-rose, eosinophils red-orange, and basophils dark purple. Lose ability to replicate at metamyelocyte stage.
-myeloblast = large, small amount of blue (because of RNA), non-granular cytoplasm; round nucleus is 80% of cell. Auer rods only appear in myeloblasts, but only seen in disease.
-promyelocyte = larger, pale blue cytoplasm with non-specific, primary granules. Oval nucleus occupies 50% of cell.
-myelocyte = smaller, start to see specific/secondary granules appear in cytoplasm. Nucleus is round. Last precursor cell with ability to divide.
-metamyelocyte = smaller; abundant cytoplasm with lots of secondary granules. Smaller, kidney-shaped/indented nucleus.
-band = smaller. Nucleus begins to look rod/band shaped.
-segmented granulocyte = smaller, abundant cytoplasm with specific granules. Neutrophils have 2-5 nuclear lobules (usually 3).
myeloblasts, promyelocytes, and myelocytes undergo cell division = mitotic pool. Metamyelocytes, bands, and segs don’t (maturation and storage pools). Main cytokine = G-CSF. Granules contain destructive enzymes (mainly myeloperoxidase). Neutrophils are phagocytes (AKA: PMN’s, segs, and polys). Eosinophils have enzymes used to fight bigger organisms (fungi, parasites) and can modulate mast cell activity in response to hypersensitivity/allergy; IL-5. Basophils/mast cells also involved in hypersensitivity; IL-3 and Stem Cell Factor SCF.
3) Monopoiesis. Started by M-CSF; monocytes in peripheral blood 20 days before entering tissues. Nucleus goes from round to irregularly shaped. Cytoplasm gets granules/vacuoles.
- monoblast = ovoid/round nucleus with blue-purple chromatin; abundant non-granular cytoplasm (blue-gray).
- promonocyte = larger; irregularly shaped, indented nucleus; abundant blue- gray cytoplasm with some granules.
- monocyte = about same size; abundant, blue-gray cytoplasm with many granules and vacuoles. Nucleus is variably shaped.
4) Megakaryopoiesis. Initiated by TPO on megakaryocyte/erythroid progenitor.
- megakaryoblast has indented nucleus and thin rim of basophilic cytoplasm (RNA)
- promegakaryocyte = more lobulated nucleus; granules appear.
- megakaryocyte = LOTS of cytoplasm, more lobulated nucleus, cytoplasm is granular and purple.
- platelet = get injected into marrow sinuses by megakaryocyte. No nuclei, appear granular and purplish with no defined borders.
- Discuss the general time frame of granulopoiesis and erythropoiesis.
Erythropoiesis requires 2-7 days to go from pronormoblast orthochromic normoblast, 1 day to extrude nucleus, 2-3 days for reticulocyte to mature. ~11 days total.
In granulopoiesis, 3-6 days spent in mitotic pool, and 5-7 days in maturation/storage pools.
- Describe the length of time mature blood cells exist, on average, in the peripheral blood.
120 days.
- Describe the process of bone marrow biopsy and aspirate.
Do these to determine marrow cellularity, detect focal lesions, evaluate lineages, determine diagnoses/disease states.
To get the biopsy, you pull out a core bit of bone + marrow. To get the aspirate, you only take marrow. Aspirate is usually fluid? Commonly done at the posterior superior iliac crest.
- Discuss some of the main features a pathologist evaluates when reviewing a bone marrow core biopsy and bone marrow aspirate smears.
On the aspirate smears, look for the marrow differential (% of different cell types [if they’re abnormal]), morphology (do cells look normal), and iron content (is marrow iron high/low/normal).
On the core biopsy, look for cellularity, the myeloid: erythroid ratio (generally 2:1, 4:1), megakaryocyte frequency (are there more or less?), and focal findings/lesions (any lesions indicative of cancer?).
Also check if precursors seem to be maturing properly.
- Explain the concept of marrow cellularity.
Cellularity = portion of the marrow that is hematopoietically active; non-active marrow is occupied by stromal elements (fat). If half marrow is active, it has 50% cellularity. Rule of thumb cellularity = 100-age. Cellularity decreases with age.
Hypercellular marrow = hyperplastic and increased proliferation, or neoplastic due to neoplasm of hematopoietic cells.
Hypocellular marrow = cellularity is less, but has some cells present or aplastic (basically marrow cells absent). Can be due to autoimmune attack or viral attack on marrow cells, neoplasms, or malnourishment.
