Learning Objective

By the end of this article, you should be able to define haematopoiesis and erythropoiesis, describe the changing anatomical sites of erythropoiesis from fetal development to adulthood, outline the stages involved in red blood cell maturation, explain the regulatory role of erythropoietin (EPO) in maintaining red cell mass, and recognise the key clinical conditions associated with both underproduction and overproduction of red blood cells.

Overview

Haematopoiesis is the process by which all blood cells are produced. A key component of this process is erythropoiesis, the production of red blood cells (erythrocytes).

The human body produces approximately 2.5 billion red blood cells per kilogram per day, reflecting the remarkable efficiency and tight regulation of this system. Red cell production must be sufficient to maintain oxygen delivery while avoiding excessive blood viscosity.

This article reviews the sites, stages, regulation, and clinical relevance of erythropoiesis.

Sites of Erythropoiesis

The anatomical site of erythropoiesis changes during development:

Early fetal life: Yolk sac
2–5 months gestation: Liver and spleen
From ~5 months of gestation onward: Bone marrow

In children, erythropoiesis occurs in the bone marrow of most bones.
In adults, it is restricted primarily to the axial skeleton:

Vertebrae
Ribs
Sternum
Sacrum
Pelvis
Proximal femur

When bone marrow production is inadequate, extramedullary haematopoiesis (blood cell production outside the marrow) may occur, particularly in the liver and spleen. This is commonly seen in conditions such as:

Thalassemia
Myelofibrosis

Stages of Erythropoiesis

All blood cells originate from a haematopoietic stem cell (haemocytoblast) in the bone marrow. These multipotent cells have extensive self-renewal capacity.

Differentiation Pathway

Haematopoietic stem cell (HSC)
Common myeloid progenitor
Erythroblast (normoblast) – nucleated, found in bone marrow
Reticulocyte – immature red blood cell, nucleus extruded
Erythrocyte – mature red blood cell

Key Maturational Changes

Progressive reduction in cell size
Increasing haemoglobin concentration
Nuclear condensation and extrusion
Loss of remaining organelles

Reticulocytes may enter the peripheral blood before completing maturation.
Mature erythrocytes have no nucleus, maximising space for haemoglobin and enabling flexibility.

The average lifespan of a red blood cell is ~120 days. The presence of nucleated red blood cells in peripheral blood suggests pathology, such as:

Severe anaemia
Haematological malignancy
Thalassemia

Activity

Regulation of Erythropoiesis

Erythropoiesis is primarily regulated by erythropoietin (EPO), a glycoprotein hormone produced by renal peritubular interstitial cells.

Mechanism of Regulation

Reduced haemoglobin → decreased oxygen delivery
Reduced renal pO₂ detected
Increased EPO production
Stimulation of bone marrow erythropoiesis
Increased haemoglobin → restoration of oxygen levels
Reduced EPO production (negative feedback)

This tightly controlled feedback loop maintains appropriate red cell mass.

Activity

Clinical Relevance

Chronic Kidney Disease

In Chronic kidney disease, damaged kidneys produce insufficient EPO. Consequences:

Reduced red cell production
Normocytic anaemia

Management may include exogenous EPO therapy.

Drug Doping

Exogenous EPO has been misused in competitive sports to increase haemoglobin concentration and improve oxygen delivery to muscles. Notable cases have occurred in professional cycling, including during the Tour de France.