A peripheral blood smear demonstrating sickle cells (indicated by arrows). From MLS Collection, University of Alberta.
Image 1: 100x oil immersion. https://doi.org/10.7939/R3TQ5RV8M
Image 2: 100x oil immersion. https://doi.org/10.7939/R3X05XT8X
Red blood cells that lack an area of central pallor, are thin, and appear curved or S-shaped (cells resemble a sickle or crescent). The ends of the cell are pointed.1
A genetic mutation in the β globin chain results in the production of abnormal hemoglobin S. The mutation results in an amino acid substitution in the 6th position from glutamine to valine. Red blood cells have normal morphology under normal conditions but under hypoxic conditions (decreased oxygen tension), hemoglobin S polymerizes and causes the red blood cell to assume the characteristic sickle shape.2 Sickle cell formation causes the red blood cell to become rigid and inflexible.3
Factors that contribute to hemoglobin S polymerization and RBC sickling include:
- Low oxygen saturation
- Decreased pH
- Increased 2,3-BPG
These factors affect either the oxygenation or concentration of hemoglobin S inside the red blood cell, leading to polymerization and sickling.
Note: Formation of sickle cells can be reversible when hypoxic conditions are corrected however not all sickle cells have the ability to revert back to a normal morphology.2,3
Associated Disease/Clinical States:1
Sickle Cell Disease (Homozygous Hemoglobin S disease)
Hemoglobin SC Disease
Note: Sickle cells not usually seen in heterozygous hemoglobin S (Hemoglobin AS or Sickle Cell Trait).1
1. Rodak BF, Carr JH. Variations in shape and distribution of erythrocytes. In: Clinical hematology atlas. 5th ed. St. Louis, Missouri: Elsevier Inc.; 2017. p. 93-106.
2. Landis-Piwowar K, Landis J, Keila P. The complete blood count and peripheral blood smear evaluation. In: Clinical laboratory hematology. 3rd ed. New Jersey: Pearson; 2015. p. 154-77.
3. Jones KW. Evaluation of cell morphology and introduction to platelet and white blood cell morphology. In: Clinical hematology and fundamentals of hemostasis. 5th ed. Philadelphia: F.A. Davis Company; 2009. p. 93-116.