Clinical Heme Lecture 2

Hematopoiesis

The continuous process of blood cell production, including cell renewal, proliferation, differentiation, and maturation.

Three Phases of Fetal Hematopoiesis

1. Mesoblastic Phase 2. Hepatic Phase 3. Myeloid (Medullary) Phase

Mesoblastic Phase

Begins around day 19 of embryonic development; cells from mesoderm migrate to yolk sac to produce primitive blood cells.

Hepatic Phase

Begins at 5-7 gestational weeks; liver is main site (erythroblasts, granulocytes, monocytes), spleen produces B cells. Fetal Hb (HbF) dominates.

Myeloid (Medullary) Phase

Begins around 18-20 weeks gestation in bone marrow; by 24 weeks, bone marrow becomes primary hematopoietic site.

Retrogression

Process where active red marrow is gradually replaced by inactive yellow (fatty) marrow as age increases.

Red Marrow vs. Yellow Marrow

Red marrow = Hematopoietically active. Yellow marrow = Inactive (fat). Normal adult ratio is ~50% red / 50% yellow in active sites.

Adult Active Marrow Sites

Sternum, vertebrae, ribs, pelvis (iliac crest), skull, and proximal ends of long bones (femur/humerus).

Bone Marrow Microenvironment (Stromal Cells)

Includes endothelial cells, adipocytes, macrophages, osteoblasts, osteoclasts, and adventitial cells. They secrete matrix and cytokines to nurture HSCs.

Erythroblastic Islet

Specialized BM niche where developing erythroid precursors surround a central iron-laden macrophage that provides iron and growth signals.

Path of Blood Cells into Circulation

Adventitial cells -> Basement membrane -> Endothelial cells -> Vascular sinus -> Peripheral blood.

Kupffer Cells

Specialized macrophages in liver sinusoids that phagocytize aged RBCs, cellular debris, and foreign material.

Extramedullary Hematopoiesis

Blood cell production outside the bone marrow (mainly in liver and spleen) occurring when bone marrow fails or cannot meet demand.

Splenic Functions: Culling vs. Pitting

Culling = Phagocytosis and total destruction of aged/damaged RBCs. Pitting = Macrophages strip inclusions (e.g., Heinz bodies) from RBCs without destroying them.

Splenomegaly vs. Autosplenectomy

Splenomegaly = Enlarged spleen (leukemias/MPNs). Autosplenectomy = Spleen shrinks and becomes non-functional due to repeated sickling/infarctions (Sickle Cell Disease).

Hematopoietic Stem Cell (HSC) Characteristics

Multipotent cells capable of self-renewal and giving rise to all differentiated blood lineages.

Three Fates of HSCs

1. Self-renewal 2. Differentiation 3. Apoptosis

Stochastic vs. Instructive Model

Stochastic = HSC commitment decision is random. Instructive = HSC commitment is directed by microenvironmental signals (cytokines). Both models contribute.

HSC Surface Markers (Immunophenotype)

CD34+, CD38-, HLA-DR low, Thy-1 low, Lin- (Lineage negative).

CD34 vs. CD38

CD34 = Surface glycoprotein marker on HSCs and early progenitors. CD38 = Early myeloid differentiation marker (absent/low on true HSCs).

Colony-Forming Units (CFUs)

Committed progenitor cell populations (e.g., CFU-GEMM = Granulocyte, Erythrocyte, Monocyte, Megakaryocyte; CFU-E = Erythrocyte; CFU-GM = Granulocyte/Monocyte).

BFU vs. CFU

BFU (Burst-Forming Unit) represents a more primitive progenitor stage than CFU (Colony-Forming Unit) in lineage differentiation (e.g., BFU-E -> CFU-E).

Hematopoietic Cytokines / Growth Factors

Glycoproteins that prevent apoptosis, stimulate division, and direct differentiation of hematopoietic precursor cells.

Erythropoietin (EPO)

Produced in kidneys (peritubular cells); acts on BFU-E & CFU-E to stimulate erythropoiesis. Used for renal disease and chemo-induced anemia.

Granulocyte Colony-Stimulating Factor (G-CSF)

Stimulates neutrophil differentiation and precursor maturation; used for chemotherapy-induced neutropenia and HSC mobilization.

RBC Maturation Size Trend

Cell size gradually decreases from 19 µm down to 6-8 µm.

RBC Maturation N:C Ratio Trend

Decreases significantly from 8:1 in pronormoblasts to 1:2 before nuclear extrusion.

RBC Cytoplasm Color Evolution

Shifts from deep royal blue to grey-blue and finally salmon pink.

BFU-E Characteristics

Earliest committed erythroid progenitor; highly proliferative; takes ~1 week to mature into CFU-E.

CFU-E Characteristics

Derived from BFU-E; high concentration of EPO receptors; highly sensitive to EPO to prevent apoptosis.

Pronormoblast (Stage 1)

Size 12-19 µm, N:C 8:1, 1-2 distinct nucleoli, and dark blue cytoplasm.

Basophilic Normoblast (Stage 2)

Size 12-17 µm, N:C 6:1, coarser chromatin, and intensely dark royal blue cytoplasm.

Polychromatophilic Normoblast (Stage 3) Milestone

The LAST stage capable of cell division (mitosis).

Orthochromic Normoblast (Stage 4) Key Event

Features a pyknotic nucleus that is extruded at the end of the stage.

Reticulocyte Supravital Stain

New Methylene Blue precipitates RNA into a visible dark blue fine mesh network.

Mature Erythrocyte Lifespan

Circulates for 120 days delivering oxygen and assisting CO2 transport.

Primary EPO Stimulator & Sites

Stimulated by hypoxia; produced by renal peritubular interstitial cells (~90%) and liver (~10%).

Primary Regulatory Action of EPO

Inhibits apoptosis by rescuing CFU-E and early normoblasts from programmed cell death.

Extravascular Hemolysis Primary Sites

Spleen (red pulp), liver (Kupffer cells), and bone marrow macrophages.

Extravascular Protoporphyrin Breakdown

Cleaved into carbon monoxide and biliverdin, then converted to unconjugated bilirubin.

Intravascular Hemolysis Definition

Rupture of RBCs directly within circulating blood vessels.

Haptoglobin Function

Binds free hemoglobin alpha-beta dimers and is rapidly depleted during intravascular hemolysis.