Codocyte

Codocytes, also known as target cells or Mexican hat cells, are red blood cells that have the appearance of a shooting target with a bullseye. In optical microscopy these cells appear to have a dark center (a central, hemoglobinized area) surrounded by a white ring (an area of relative pallor), followed by dark outer (peripheral) second ring containing a band of hemoglobin. However in electron microscopy they appear very thin and bell shaped. Because of their thinness they are referred to as leptocytes and because of the wavy bowl shape they are referred to as Mexican hat cells. On routine smear morphology, some people like to make a distinction between leptocyes and codocytes- suggesting that in leptocytes the central spot is not completely detached from the peripheral ring, i.e. the pallor is in a C shape rather than a full ring.

These cells are characterized by a disproportional increase in the ratio of surface membrane area to volume. This is also described as a "relative membrane excess." It is due to either increased red cell surface area (increased beyond normal), or else a decreased intracellular hemoglobin content (which may cause an abnormal decrease in cell volume without affecting the amount of membrane area). The increase in the surface area to volume ratio also gives the cell decreased osmotic fragility, as it allows it to take up more water for a given amount of osmotic stress.