Hematopoiesis and Bone Marrow

What are the potential fates of hematopoietic stem cells?

They can be quiescent, die, self-renew, or differentiate into multipotent progenitor cells (MPP).

What are the main sites of hematopoiesis during embryo, fetus, and adult stages?

Embryo: yolk sac; Fetus: liver and spleen; Adult: bone marrow (spleen can resume function in crisis).

What nutrients are required for hematopoiesis?

Iron, vitamin B6, vitamin B12, and calories.

What are the three major pathways hematopoietic stem cells can differentiate into?

Leukopoiesis (WBCs), erythropoiesis (RBCs), megakaryopoiesis (platelets).

What is megakaryopoiesis?

The development of megakaryocytes—large cells with multilobular nuclei and pale cytoplasm.

What is thrombopoiesis?

Platelet production via cytoplasmic fragments from megakaryocytes.

How do megakaryocytes release platelets?

They form long cytoplasmic arms that extend into sinusoids and fragment into platelets.

What is thrombopoietin (Tpo), its source, and function?

A cytokine produced by hepatocytes in the liver; stimulates maturation of megakaryocytes and platelet production.

Describe the cellular changes during erythropoiesis.

Cells decrease in size, lose RNA and nucleus, and increase in hemoglobin content.

What is erythropoietin (Epo), its source, and stimulus?

A cytokine produced by the kidneys in response to renal hypoxia; stimulates proliferation, maturation, and release of RBCs.

What are the two erythroid pools in bone marrow?

Proliferation pool (undergoes mitosis) and maturation pool (no longer divides).

What is the relationship between macrophages and erythroid precursors?

Erythroid precursors form "islands" around macrophages (nurse cells) for iron supply; macrophages phagocytize nuclei from maturing RBCs.

Describe the cellular transitions in granulopoiesis.

Cells go from large with abundant RNA and open chromatin to smaller cells with less RNA, condensed chromatin, and granules.

What cytokines promote neutrophil and monocyte production?

G-CSF (primary stimulator of neutrophils) and GM-CSF (secondary stimulator of neutrophils/monocytes).

What are the neutrophil pools in bone marrow and blood?

Bone marrow: proliferation, maturation, storage pools. Blood: circulating, marginated, and extravascular pools.

What other cell types are found in low numbers in healthy bone marrow?

Megakaryocytes, eosinophils, basophils, lymphocytes, macrophages.

Define hematopoietic cellularity and expected percentages by age.

% of marrow space occupied by hematopoietic cells. Juveniles: >50%; Adults: ~50%; Seniors:

What is the normal myeloid-to-erythroid (M:E) ratio?

1:1 (myeloid cells to nucleated erythroid cells).

What does increased hematopoietic cellularity with an increased M:E ratio indicate?

Increased myeloid production.

What does decreased M:E ratio indicate?

Increased erythroid production.

What are the main sites for bone marrow collection in small and large animals?

Dogs & cats: ilium, humerus, femur. Horses & cattle: sternum, ribs, ilium.

What is the difference between red and yellow marrow?

Red marrow: hematopoietically active; Yellow marrow: mostly adipocytes, inactive; relevant for choosing collection sites.

What is the method and sample type for bone marrow cytology?

Aspirate using negative pressure → cellular sample.

What is the method and sample type for bone marrow histology?

Core biopsy → intact tissue structure.

What are advantages and disadvantages of cytology vs histology?

Cytology: better for cell detail but may miss architecture. Histology: shows architecture but less cellular detail.

What defines high vs low diagnostic quality for marrow aspirates and cores?

High quality: adequate cellularity, well-preserved cells/tissue. Low quality: diluted, poorly preserved, insufficient material.