Formation and Significance of Keratocytes in Peripheral Blood Smears

Eve M. Boggs, DVM; Kenneth S. Latimer, DVM, PhD; Heather L. Tarpley, DVM; Bruce E. LeRoy, DVM, PhD

Class of 2003 (Boggs) and Department of Pathology (Latimer, Tarpley, LeRoy), College of Veterinary Medicine, The University of Georgia, Athens, GA 30602-7388

Introduction

Healthy, mature erythrocytes of humans and most domestic animals normally circulate as biconcave disks. This special shape allows for gas exchange as well as deformation and return to a preexisting shape without permanent damage as these cells pass through microvasculature.⁹

Microscopic examination of Romanowsky-stained blood smears is performed routinely as a portion of the complete blood cell count. In sick patients, erythrocyte abnormalities may indicate the presence of disease. Abnormally shaped red blood cells generally can be classified as poikilocytes (misshapen erythrocytes), some of which are spiculated or have surface projections. These latter cells may be classified more specifically as keratocytes, echinocytes, schistocytes, or acanthocytes.¹ Keratocytes are abnormally shaped erythrocytes that appear to have horns (kerat in Latin means horn; Fig. 1).² Another synonym for keratocyte is "helmet cell" because the cell may resemble a helmet with chinstraps.

Figure 1. Keratocytes (arrows) are present in a Wright stained blood smear of a dog.

Mechanism of Formation and Diseases Associated with Keratocytes

Keratocyte formation may be associated with trauma, especially cellular damage from contact with fibrin strands within the microvasculature. Other processes that can contribute to microvascular injury include endotoxemia and antigen-antibody reactions. These forms of microangiopathy subsequently may lead to platelet aggregation, fibrin formation, and, ultimately, intravascular coagulation.¹⁰ As normal erythrocytes encounter a mesh of fibrin strands, they can become entrapped. Sometimes these cells are impaled on fibrin strands. Blood flow then pushes the cell against the strand of fibrin and the cell may bisect. Alternatively, the opposing sides of the cell may adhere to one another around the fibrin strand. When blood flow frees the cell, the opposing sides rejoin forming a pseudovacuole. Cells with pseudovacuoles are called "blister" cells or pre-keratocytes (Fig. 2). When the vacuole ruptures (usually within minutes), the remaining cell resembles a helmet with straps or a horned cell that is designated a keratocyte (Figs. 2, 3, and 4).² Prekeratocytes also can form via fusion of injured cell membranes (e.g., administration of doxorubicin⁶ and some iron deficiency anemias^7,5). These new cells are more fragile than the original parent cell and may rupture, forming a keratocyte.²

Figure 2. Prekeratocytes or blister cells in a Wright stained blood smear of a dog.

Figure 3. Phase contrast photomicrographs demonstrating successive stages of formation of keratocytes in the blood (from Bessis M: Blood Smears Reinterpreted, Springer-Verlag, 1977, p. 85).

Figure 4. Scanning electron micrographs of keratocytes showing the formation of irregular spicules that resemble horns (modified from Bessis M: Blood Smears Reinterpreted, Springer-Verlag, 1977, p. 85).

As previously mentioned, many seemingly unrelated disease processes that can result in keratocyte formation. Various diseases and mechanisms of keratocyte formation are listed in Table 1.

Table 1. Diseases associated with and mechanisms of formation of keratocytes.

Disease Proposed Mechanism of Keratocyte Formation

This list is not all inclusive; the disease processes listed are excerpted from the published literature. Understanding the mechanism of cellular injury in the formation of keratocytes is important in interpreting the significance of these cells in the stained blood smear.

Other Facts Concerning Keratocytes

Formation of keratocytes is not commonly associated with a decreased hematocrit or packed cell volume. However, if keratocytes are formed PCV, these abnormal cells will be removed from the circulation by the spleen. The feline spleen is less efficient in removing abnormal cells from circulation, so it is not uncommon to see more keratocytes in peripheral blood smears of cats.⁶

Keratocytes usually have a normal concentration of hemoglobin; therefore, they may retain an area of central pallor.² Keratocytes, blister cells, and schistocytes may be observed commonly in the same blood smear because they may be formed by a common mechanism (e.g., fragmentation in circulation).

It has been reported that blood collected with EDTA had a greater potential for keratocyte formation.⁴ Furthermore, morphological artifacts of erythrocytes may be associated with cellular damage when the blood specimen is obtained or the blood smear is prepared. When possible apply the blood immediately to a clean, debris free slide and use proper technique when preparing the smear.

References

1. Andreasen CB, Brockus CW: Erythrocytes. In: Latimer KS, Mahaffey EA, Prasse KW (eds): Duncan and Prasse's Veterinary Laboratory Medicine: Clinical Pathology, 4^th ed. Iowa State Press, Ames, IA, 2003.

2. Bessis M: Blood Smears Reinterpreted (translated by G. Brecker). Springer International, New York, NY, 1977.

3. Christopher MM, Lee SE: Red cell morphologic alterations in cats with hepatic disease. Vet Clin Pathol 23:7-12 1994.

4. Harvey JW: Erythrocytes. In: Atlas of Veterinary Hematology: Blood and Bone Marrow of Domestic Animals. W.B. Saunders Company, Philadelphia, PA, 2001, pp. 21-44.

5. Harvey JW: Microcytic Anemias. In: Feldman BF, Zinkl JG, Jain NC (eds): Schalm’s Veterinary Hematology, 5^th ed. Lippincott Williams & Wilkins, Philadelphia, PA, 2000, pp 200-204.

6. O’Keefe DA, Schaeffer DJ: Hematologic toxicosis associated with doxorubicin adminis-tration in cats. J Vet Intern Med 6:276-283, 1992.

7. Weiser G: Erythrocyte responses and disorders. In: Ettinger SW, Feldman EC (eds): Textbook of Veterinary Internal Medicine Diseases of the Dog and Cat, vol. 2, 4^th ed. WB Saunders, Philadelphia, PA, 1989, pp 1864-1891.

8. Weiss DJ, Kristensen A, Papenfuss N: Quantitatitve evaluation of irregularly spiculated red blood cells in the dog. Vet Clin Pathol 22:117-121,1993.

9. Weiss DJ, Kristensen A, Papenfuss N, McClay CB: Quantitative evaluation of echinocytes in the dog. Vet Clin Pathol 19:114-118,1990.

10. Rebar AH, Lewis HB, DeNicolo DB, et al: Red cell fragmentation in the dog: An editorial review. Vet Pathol 18:415-426, 1981.

Acknowledgement

Photograph of the Holy Cult Horns, Palace of King Minos, Knossos, Crete from the page Impressions of Crete on the web site Sights and Cultural Features Around the World is used with permission.

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