Blood Composition & Hematopoiesis

Blood is a specialized connective tissue: a liquid extracellular matrix (plasma) suspending living cells (formed elements). By volume, an adult has about 5 liters of blood, roughly 7-8% of body mass. Centrifuging a blood sample separates it into three visible layers: a straw-colored plasma on top (~55%), a thin white-tan "buffy coat" of leukocytes and platelets in the middle (<1%), and a dense red column of erythrocytes at the bottom (~40-45%). This red cell fraction, called the hematocrit (or packed cell volume), is one of the most frequently measured numbers in clinical medicine. Plasma is about 90% water, with the remaining 10% consisting of dissolved plasma proteins (albumin, globulins, fibrinogen), electrolytes, nutrients, hormones, gases, and metabolic wastes. Albumin, made by the liver, provides most of plasma's colloid osmotic pressure and holds water inside vessels — a concept that connects directly to capillary fluid exchange in circulatory physiology. Globulins include antibodies (immunoglobulins) and transport proteins; fibrinogen is the soluble precursor of the fibrin clot. Remove the clotting factors from plasma (by letting blood clot and collecting the fluid) and what remains is serum. The formed elements are three functionally distinct cell families. Erythrocytes (red blood cells) are biconcave, anucleate discs about 7-8 micrometers in diameter, packed with hemoglobin for oxygen and carbon dioxide transport — the subject taken up in depth in Hemoglobin & Oxygen Transport. Leukocytes (white blood cells) are a heterogeneous set of nucleated immune cells — neutrophils, eosinophils, basophils, lymphocytes, and monocytes — that patrol tissues and blood for pathogens and abnormal cells. Platelets (thrombocytes) are not true cells but small anucleate cell fragments budded from megakaryocytes, essential for clot formation. All three lineages are continuously replaced from a common origin: the hematopoietic stem cell.

Step 1 — The stem cell niche. In adults, active blood formation (hematopoiesis) occurs in red bone marrow, concentrated in flat bones (sternum, pelvis, vertebrae, ribs, skull) and the proximal ends of long bones. Within the marrow, a small, specialized microenvironment called the stem cell niche — built from osteoblasts lining bone surfaces and marrow endothelial and stromal cells — houses hematopoietic stem cells (HSCs). HSCs are exceedingly rare, roughly 1 in 10,000 to 1 in 100,000 nucleated marrow cells, yet they sustain lifelong blood production because they can both self-renew (dividing to produce another HSC) and differentiate (dividing to produce more committed progeny).

Step 2 — The first branch point. An activated HSC divides asymmetrically or gives rise to a population that splits into two multipotent progenitor lines:

• The common myeloid progenitor (CMP), which generates erythrocytes, platelets (via megakaryocytes), granulocytes (neutrophils, eosinophils, basophils), and monocytes.
• The common lymphoid progenitor (CLP), which generates B lymphocytes, T lymphocytes, and natural killer cells.

Step 3 — Progressive commitment. Each progenitor undergoes further restriction. The CMP branches into a megakaryocyte-erythroid progenitor (MEP) and a granulocyte-monocyte progenitor (GMP). The MEP splits toward either proerythroblasts (the erythroid line) or megakaryoblasts (the platelet line). At each branch point, the cell's fate is progressively narrowed and becomes largely irreversible, driven by combinations of transcription factors (e.g., GATA1 favors erythroid/megakaryocyte fate; PU.1 favors myeloid/lymphoid fate) that cross-antagonize each other. This is a bistable switch: high GATA1 suppresses PU.1 and vice versa, locking the cell into one lineage.

Step 4 — Lineage-specific maturation.

• Erythropoiesis: proerythroblast → basophilic erythroblast → polychromatic erythroblast → orthochromatic erythroblast → reticulocyte → mature erythrocyte. During this sequence the cell shrinks, accumulates hemoglobin, and ultimately extrudes its nucleus (a feature unique among human blood cells), becoming a reticulocyte that still contains residual ribosomal RNA. Reticulocytes circulate 1-2 days before losing that RNA and becoming fully mature erythrocytes, which then live about 120 days before removal by macrophages in the spleen and liver.
• Granulopoiesis/monopoiesis: myeloblast → promyelocyte → myelocyte → metamyelocyte → band cell → mature neutrophil/eosinophil/basophil, or monoblast → promonocyte → monocyte (which later becomes a tissue macrophage).
• Thrombopoiesis: megakaryoblast → promegakaryocyte → megakaryocyte, an enormous polyploid cell (up to 64N DNA content) that extends long cytoplasmic processes (proplatelets) into marrow sinusoids; shear stress in the blood shears these into thousands of individual platelets.
• Lymphopoiesis: CLP-derived progenitors give rise to B cells (maturing in marrow) and T cells (which migrate to the thymus to complete maturation), plus NK cells.

Step 5 — Hormonal control (feedback regulation). Production of each lineage is tuned independently by circulating growth factors, mostly made outside the marrow itself, closing negative feedback loops:

The erythropoietin loop is the clearest example of physiological feedback. When blood oxygen delivery falls — from altitude, blood loss, or lung disease — kidney fibroblast-like cells sense hypoxia via the transcription factor HIF (hypoxia-inducible factor), which is normally degraded in the presence of oxygen but stabilizes and accumulates in hypoxia. Stabilized HIF drives EPO gene transcription, EPO is released into blood, travels to marrow, binds EPO receptors on erythroid progenitors, and rescues them from programmed cell death (apoptosis), boosting red cell output within days. As oxygen delivery is restored, HIF is degraded again and EPO output falls — a classic negative feedback loop.

Till and McCulloch — proving stem cells exist (1961, University of Toronto). Before 1961, it was widely suspected that blood cells arose from some kind of precursor, but no one had rigorously shown that a single cell could both self-renew and generate multiple blood lineages. James Till (a biophysicist) and Ernest McCulloch (a hematologist) were studying radiation sensitivity of marrow cells. They injected known numbers of mouse bone marrow cells into lethally irradiated mice (whose own marrow was destroyed) and examined the spleens 8-14 days later. They observed discrete nodules — colonies — on the spleen surface, and crucially, the number of colonies was directly proportional to the number of marrow cells injected, following Poisson statistics consistent with each colony arising from a single cell (a colony-forming unit, CFU-S). Microscopic examination showed each colony contained a mixture of erythroid, granulocytic, and megakaryocytic cells — proof that one progenitor cell could give rise to multiple blood cell types. In a critical follow-up, Lou Siminovitch, Till, and McCulloch (1963) showed that cells taken from one spleen colony could be re-injected into a second irradiated mouse and form new colonies again, some of which still contained the full range of lineages — direct proof of self-renewal, the second defining property of a stem cell. This spleen colony assay (CFU-S assay) is regarded as the founding experiment of stem cell biology, predating even the term "stem cell" in its modern usage.

Becker, McCulloch, and Till (1963) — clonality proof using chromosome markers. A remaining worry was that each spleen colony might arise from several cells clumped together rather than truly one cell. The team exposed marrow cells to low-dose radiation before injection, which produced rare, cell-specific chromosomal breakage patterns (visible under the microscope as unique karyotypic markers) — essentially a natural barcode. When they examined cells within a single spleen colony, every cell in that colony carried the same unique chromosomal marker, proving unambiguously that the colony had descended from one single marked cell, not a mixture. This chromosome-marking experiment is a model of rigorous clonal-lineage proof, analogous in spirit to using genetic markers to trace clonal descent in other tissues.

Locating the niche and the marrow origin. Complementary classic work (parabiosis experiments and later cross-transplantation of genetically marked marrow, e.g., using the Y chromosome or later GFP markers, into irradiated recipients) confirmed that transplanted marrow cells could fully reconstitute all blood lineages in an animal whose own marrow had been ablated — the conceptual basis of modern bone marrow and hematopoietic stem cell transplantation, now standard therapy for leukemias, lymphomas, and some genetic blood disorders. Together, these experiments established the two non-negotiable defining properties of any stem cell: multipotency (one cell, many descendant cell types) and self-renewal (one cell, more stem cells), both demonstrated directly and quantitatively rather than inferred.

Example 1 — Interpreting a complete blood count (CBC). A patient presents with fatigue. Labs show: hematocrit 28% (normal male range 40-52%), hemoglobin 9.2 g/dL (normal 13.5-17.5), reticulocyte count 0.3% (normal 0.5-2.5%), platelet count 210,000/µL (normal 150,000-450,000), white cell count 6,200/µL (normal 4,500-11,000). Reasoning: hematocrit and hemoglobin are both low → anemia is present. Platelets and white cells are normal, so this is an isolated erythroid problem, not a global marrow failure (which would depress all three lineages, so-called pancytopenia). The reticulocyte count is the key diagnostic clue: reticulocytes are the immature red cells released 1-2 days before full maturation, so a low reticulocyte count in the face of anemia means the marrow is not compensating — production is inadequate, pointing toward causes like iron deficiency, chronic disease, or marrow suppression, rather than active blood loss or hemolysis (destruction), which would instead trigger a high reticulocyte count as the marrow ramps up EPO-driven output to compensate. This single differential — low reticulocyte production anemia vs. high reticulocyte compensatory anemia — is a direct clinical application of the erythropoietin feedback loop described above.

Example 2 — Quantifying erythropoiesis under hypoxic stress. Suppose a healthy adult normally produces red cells at a steady-state rate sufficient to replace 1% of the roughly 25 trillion (2.5 × 10^13) circulating erythrocytes each day (consistent with a ~120-day lifespan, since 1/120 ≈ 0.83% per day, close to 1%). This person ascends to high altitude (e.g., 4,300 m, comparable to La Paz, Bolivia), where arterial oxygen saturation falls from 97% to about 85%. The resulting tissue hypoxia stabilizes HIF in renal peritubular cells, driving a several-fold rise in circulating EPO within 24-48 hours, and marrow erythroid output can increase roughly 2- to 3-fold above baseline within one to two weeks (though the rise in mature circulating red cell mass lags behind, since maturation from proerythroblast to reticulocyte release takes about 5-7 days). If baseline daily production is: 0.0083 × 2.5 × 10^13 ≈ 2.1 × 10^11 red cells/day then a threefold acclimatization response yields approximately 3 × 2.1 × 10^11 ≈ 6.2 × 10^11 red cells/day Over several weeks this elevated output raises hematocrit from a sea-level baseline of ~45% toward 50-60% in well-acclimatized high-altitude residents (a phenomenon also exploited, illegally, in blood doping and altitude training for endurance athletes). This calculation illustrates quantitatively how the EPO feedback loop converts a sensed environmental variable (oxygen tension) into a measurable, delayed, and reversible change in marrow output.

McCulloch and Till were an unusual pairing: McCulloch the clinician who understood blood disease, Till the quantitative biophysicist who insisted on rigorous, countable, statistically testable experiments rather than qualitative description. Their 1961 paper analyzing spleen colonies formed after marrow transplantation into irradiated mice is now considered the founding document of experimental stem cell biology, though the word "stem cell" in its current technical sense only crystallized as their findings and terminology were absorbed by the field over the following decade. Neither had originally set out to discover stem cells; they were investigating a narrower question about radiation dosimetry and marrow cell survival, and the colony-forming assay they built to answer it turned out to reveal something far larger. McCulloch continued to work on human leukemia biology for the rest of his career, applying stem cell concepts to understand how leukemic cells subvert the normal hierarchy of blood formation — a link between fundamental stem cell biology and cancer that remains central to hematology and oncology today.

Bone marrow and stem cell transplantation. The entire clinical practice of hematopoietic stem cell transplantation — used to treat leukemias, lymphomas, aplastic anemia, and inherited immune and blood disorders — rests directly on the Till-McCulloch demonstration that transplanted marrow cells can fully reconstitute a destroyed hematopoietic system. Patients receive high-dose chemotherapy or radiation to ablate diseased marrow, then are "rescued" with donor (allogeneic) or their own previously harvested (autologous) HSCs.

Recombinant growth factors as drugs. Recombinant human erythropoietin (epoetin) is used to treat anemia of chronic kidney disease, since diseased kidneys cannot produce enough native EPO. Recombinant G-CSF (filgrastim) is given to cancer patients after chemotherapy to accelerate neutrophil recovery and reduce infection risk, and to mobilize HSCs from marrow into peripheral blood for easier, non-surgical stem cell harvesting.

Diagnosing anemia and marrow failure. The complete blood count and reticulocyte count, interpreted through the lineage and feedback framework described above, are the first-line tools for diagnosing anemia, infection, and marrow disorders in virtually every clinical setting worldwide — one of the most frequently ordered laboratory tests in medicine.

Leukemia biology and targeted therapy. Leukemias arise when mutations disrupt the normal balance between self-renewal and differentiation in the stem/progenitor hierarchy, producing a clonal population of abnormal cells stuck at an immature stage. Modern targeted therapies (e.g., tyrosine kinase inhibitors for chronic myeloid leukemia) work by correcting or exploiting specific molecular lesions within this hierarchy.

Altitude physiology, sports medicine, and blood doping detection. Understanding the EPO feedback loop underlies both legitimate altitude-training protocols for endurance athletes and the anti-doping tests designed to detect illicit recombinant EPO use or blood transfusion by analyzing reticulocyte percentages and hematocrit patterns inconsistent with natural physiology.

Public health and global anemia burden. Iron-deficiency anemia, caused by insufficient substrate for hemoglobin synthesis during erythropoiesis, remains one of the most prevalent nutritional deficiencies worldwide, disproportionately affecting women and children, making the mechanistic understanding of erythropoiesis directly relevant to global nutrition policy.

Ask the tutor: How does the structure of hemoglobin allow a single red blood cell to carry so much more oxygen than could dissolve directly in plasma?