Target cells are medically called codocytes. They are red blood cells that look like a shooting target with a bulls-eye. If you are going to look at it using an optical microscope, you will notice that the target cells have a dark center (filled with hemoglobin) and surrounded by a white ring with a dark outer second ring (contains a band of hemoglobin).
On the other hand, if you are going to examine the target cells using an electron microscope, you will notice that they look very thin and look like the shape of a bell. That is why target cells are called codocytes as codo means bell.
Other names for target cells are leptocytes because they are extremely thin and sombrero cells or Mexican hat cells because of their wavy bowl shape. (1, 2, 3, and 4)
Image 1: The slide contains a microscopic view of target cells/codocytes.
Picture Source: medical-labs.net
Target cell’s mechanism of formation
Codocytes or popularly known as target cells are formed if the red blood cell’s surface is increased disproportionately to its volume. Target cells are actually red blood cells, which are extremely thin and have an excessive cell membrane.
Hence, the red blood cells assume the shape of a bell while in circulation. If you flatten out the red blood cells in a smear, the top part of the bell is pushed to the center leading to the formation of a central target/bulls-eye.
In a film of blood, target cells look thinner than usual because of their pale appearance. In target cells, there is a disproportional increase in the ratio of surface membrane area to volume. As a result, there is a noticeable decreased osmotic fragility enabling the cells to take up more water for particular osmotic stress.
The formation of target cells leads to a significant decrease in the amount of oxygen circulating through the blood and unable to deliver the oxygen to different parts of the body. Hence, if there is an elevation in target cells, it could indicate a shift in the equilibrium exchanges between the red blood cells and the cholesterol. In other words, target cells have an abnormally high surface area for their volume. (2, 3, 4, 5, and 6)
Image 2: A standard hematologic investigation revealed target cells that look like a bulls-eye target.
Picture Source: hematologylearning.weebly.com
Image 3: A closer look at the target cells using an electron microscope.
Picture Source: acuclinic.com.au
Physical characteristics of target cells
- Target cells differ from the usual red blood cells because they are more resistant to hypertonic solutions than normal red blood cells.
- Hemolysis may less likely to take place until the saline concentration reaches 1.5% to 2%.
- Their envelope is too large for hemoglobin content.
- They have a lower oxygen concentration than the normal red blood cells. (2, 5, 7, and 8)
Image 4: In this view of target cells, the image resembles the shape of a bell.
Picture Source: yumpu.com
Image 6: A hematologic examination showing a closer image of target cells.
Picture Source: medical-labs.net
There are various diseases associated with the formation of codocytes/target cells. The formation of diseases can be linked to two reasons – direct increase in the surface area by having a direct impact on the concentration of lipids in the red blood cell membrane and decreased concentration of hemoglobin within the red blood cells.
These diseases are the following:
- Liver-related diseases such as obstructive liver disease. It is caused by a deficiency in LCAT (lechitin-cholesterol acyltransferase), an enzyme responsible for converting free cholesterol into cholesteryl ester. The absence of this enzyme leads to the increase in cholesterol phospholipid ratio leading to the build-up of cholesterol in the red blood cells and an increase in the size of the red blood cell membrane.
- Hemoglobin-related diseases such as hemoglobin C (hemoglobiopathies) and hemoglobin S (sickle cell anemia) disease. What happens is that one of the hemoglobin chains is genetically malformed causing the hemoglobin to be reduced significantly. In some cases, the hemoglobin isn’t functional at all causing a shortened lifespan of red blood cells.
- Autosplenectomy secondary to sickle cell anemia or hyposplenism in coeliac disease.
- Post-splenectomy. One of the functions of the spleen is to get rid of excess membrane from the red blood cells known as splenic conditioning. Target cells usually appear in the first few weeks post splenectomy. The level of target cells could increase by up to 10%. It causes a significant increase in membrane lipids, reduced osmotic fragility, and increased mean surface area to volume ratio. (2, 5, 8, 9, and 10)
- The other names for target cells are codocytes, Mexican hat cells (wavy bowl shape), and leptocytes (thinness).
- Target cells resemble the appearance of a shooting target with a bulls-eye but in electron microscope, target cells resemble the shape of a bell.
- There is a distinction between codocytes and leptocytes. In leptocytes, the central spot is not fully detached from the peripheral ring. On the other hand, codocytes assume a target configuration in the process of obtaining a blood film. Target cells look thinner than the usual secondary to paleness.
- The formation of target cells resulted from increased red blood cell membrane cholesterol to phospholipid ratio.
- Target cells are a common and usually a non-significant finding. However, the presence of target cells, especially in a large quantity can be associated with liver-related diseases.
- If target cells are increased, one of the approaches of doctors is to check for a liver enzyme panel.
- Target cells are increased significantly in patients with hemoglobin E and beta thalassemia traits.
- Macrocytic target cells are observed in people with liver-related diseases.
- Microcytic target cells are observed in people with thalassemia. (1, 4, 6, 9, and 10)