Target cells are red blood cells whose abnormally high surface-area-to-volume ratio gives them a ‘bullseye’-like appearance. They are characterized by darker regions at the center and periphery of the cell, with a paler region between the two. These target-like bands are caused by a large cell membrane surface area and an uneven distribution of hemoglobin within the cell. Target cells are associated with a variety of medical conditions including hemoglobinopathies, liver disease, iron deficiency anemia, and splenectomy.
What Are Target Cells?
Target cells (AKA codocytes) are red blood cells that have a target-like appearance. When viewed under a microscope, they look like a ‘bullseye’ with a dark center and dark outer ring, and a paler region between the two.
This banded appearance is caused by an uneven distribution of hemoglobin, which is concentrated in the middle and at the periphery of the cell.
Structure of Target Cells
The ‘bullseye’-like appearance of target cells is due to an uneven distribution of hemoglobin within the cell, which is caused by an increase in cell membrane-to-volume ratio. They essentially have an excess of lipid membrane, which causes them to take on a bell shape when circulating in the bloodstream. Hemoglobin is concentrated in the center and the outer edge of these bell-shaped cells. When viewed from above on a slide, the bands of hemoglobin are clearly visible and cause the cells to look like targets.
The increase in surface area to volume ratio of target cells makes them more resistant to osmotic stress. They can take up a larger volume of water than regular red blood cells, and are less likely to burst when placed in a hypertonic solution.
Formation of Target Cells
Target cells are red blood cells with an increased cell membrane-to-volume ratio, which can be caused by one of two things; high cholesterol, or a decrease in cell volume.
Cholesterol is a key component of the lipid bilayer, so higher levels of cholesterol lead to an increase in the cell surface area. Target cells have more cell membrane relative to their volume and take on a bell-like shape with hemoglobin distributed in the middle and outer edge of the cell. High cholesterol levels can be caused by severe liver disease, especially in cases of obstructive liver disease or lecithin-cholesterol acetyltransferase (LCAT) deficiency.
Target cells can also develop as a result of reduced hemoglobin production and, therefore, decreased cell volume. This can be caused by iron deficiency anemia and certain hemoglobinopathies such as thalassemia, hemoglobin C disease, and sickle cell disease.
Causes of Target Cells
Target cells are linked to a variety of diseases including iron deficiency anemia, certain hemoglobinopathies, and severe liver disease. They also appear in people who have undergone a recent splenectomy.
Obstructive liver disease can lead to a deficiency of the enzyme lecithin-cholesterol acetyltransferase (LCAT). LCAT plays a key role in removing cholesterol from the blood, so a decrease in the activity of this enzyme increases the proportion of cholesterol in the lipid bilayer of red blood cells. This causes expansion of the cell membrane surface area and the consequent formation of target cells.
Iron Deficiency Anemia
Iron is a vital component of hemoglobin, so iron deficiency anemia can lead to a reduction in hemoglobin production. This causes a decrease in blood cell volume and, therefore, an increase in the surface area-to-volume ratio of red blood cells.
Target cells can be caused by various genetic conditions, called haemoglobinopathies, that affect the hemoglobin component of red blood cells. Haemoglobinopathies such as thalassemia, hemoglobin C disease, and sickle cell disease, can all cause an uneven distribution of hemoglobin in red blood cells and lead to the formation of target cells.
One major function of the spleen is to recognize and remove old, damaged, or otherwise faulty red blood cells from circulation. If the spleen is removed, abnormal blood cells will not be removed from the bloodstream as effectively. As a result, the patient may develop increased numbers of target cells.