Target cells are red blood cells, whose abnormally high surface to volume ratio gives them a “porthole” -like appearance. They are characterized by darker regions in the center and on the edge of the cell, with a paler region in between. These target-like bands are caused by a large cell membrane surface 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. Under the microscope they look like a “bullseye” with a dark center and a dark outer ring and a lighter area in between.
This banded appearance is caused by an uneven distribution of hemoglobin that is concentrated in the center and edge of the cell.
Structure of the target cells
The “bullseye” -like appearance of the target cells is due to an uneven distribution of hemoglobin within the cell, which is caused by an increase in the ratio of cell membrane to volume. They essentially have an excess of lipid membrane, which causes them to take on a bell shape as it circulates in the bloodstream. Hemoglobin is concentrated in the center and on the outer edge of these bell-shaped cells. When viewed from above on a slide, the hemoglobin bands are clearly visible, making the cells look like targets.
The increase in the surface to volume ratio of the target cells makes them more resistant to osmotic stress. They can hold a larger volume of water than normal 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 that can be caused by one of two things; high cholesterol or a decrease in cell volume.
Cholesterol is an important part of the lipid bilayer, so higher cholesterol levels lead to an increase in cell surface. Target cells have more cell membranes in relation to their volume and assume a bell-shaped shape, with hemoglobin distributed in the center and on the outer edge of the cell. High cholesterol can be caused by severe liver disease, especially obstructive liver disease or a lack of lecithin cholesterol acetyltransferase (LCAT).
Target cells can also develop as a result of decreased hemoglobin production and thus decreased cell volume. This can be caused by iron deficiency anemia and certain hemoglobinopathies such as thalassemia, hemoglobin C disease, and sickle cell anemia.
Causes of target cells
Target cells have been linked to a variety of diseases, including iron deficiency anemia, certain hemoglobinopathies, and severe liver disease. They also occur in people who have recently had a splenectomy.
Obstructive liver disease can lead to a deficiency in 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 level of cholesterol in the lipid bilayer of red blood cells. This causes an expansion of the cell membrane surface 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 decrease in hemoglobin production. This leads to a decrease in blood cell volume and therefore an increase in the surface to volume ratio of the red blood cells.
Target cells can be caused by a variety of genetic diseases called hemoglobinopathies, which affect the hemoglobin component of red blood cells. Hemoglobinopathies such as thalassemia, hemoglobin C disease, and sickle cell anemia can all cause an uneven distribution of hemoglobin in red blood cells and lead to the formation of target cells.
A primary function of the spleen is to identify old, damaged, or otherwise defective red blood cells and to remove them from the circulation. When the spleen is removed, abnormal blood cells are not removed from the bloodstream as effectively. As a result, the patient can develop an increased number of target cells.