Red blood cells – erythrocytes – are cells reduced to the bare essentials. Some argue that they should no longer be considered as cells. They carry no genetic information, they have no mitochondria and are incapable of protein synthesis. Stripped of organelles they are instead exquisitely optimised for delivering oxygen throughout the body. To perform this task, the cell is adapted to three main functions: transporting oxygen; protecting haemoglobin from oxidation; maintaining osmotic balance. The key to the first lies in its unique shape which allows optimal gas exchange and enables cells to squeeze through microcapillaries. The other features are primarily determined by its specialised metabolic systems which are actually directly linked to cell shape.
Anisocytes are cells with a significant change in size. Microcytes, a feature of microcytic anaemia and impaired haemoglobin production, are smaller, usually defined as less than 5um in diameter or, as a rule of thumb if you’re looking at a blood film, smaller than a lymphocyte nucleus.
Macrocytes refer to cells than are larger than usual and result from defects in cell division and differentiation so that the erythrocyte ends up having too large a volume and not enough haemoglobin. This impairs oxygen transport and passage through capillaries.
Variability in cell shape is called poikilocytosis and this is where it starts to get messy. The biconcave shape of the erythrocyte, sometimes referred to as a discocyte, is part of a spectrum of possible shapes. On one hand, extreme concavity produces stomatocytes, whereas the opposite convex shape is a spherocyte. The membrane can also deform unevenly to produce echinocytes covered in protrusions. Each of these morphologies can be broadly linked to certain blood disorders but as the result of different underlying mechanisms.
There are myriad other shapes that an erythrocyte can assume within these extremes and there is not unanimous agreement on how these are defined. A stomatocyte with two, or even more, invaginations is a knizocyte. I have seen knizocytes referred to as ovalocytes which in turn is sometimes used interchangeably with elliptocyte. An elliptocyte is an elongated cell, unless one end is more squashed in which case it is a dacrocyte. To make matters more confusing, you might see the same shape referred to by its classically derived name – ‘dacrocyte’ from ‘dacryon’, ancient Greek for ‘tear’ – or as a descriptive name ‘teardrop cell’. This is not helped by some cells having two classical names such as ‘codocyte’ from the Greek for ‘bell’ and ‘leptocyte’ meaning ‘thin’ both referring to the cell with the descriptive name ‘target cell’. Flattened on a blood film they look like a bullseye but in the circulation they are actually bell shaped, a bit like a stomatocyte. On top of this, authors will sometimes throw in their own anecdotal observations of ‘button cells’, ‘mushroom cells’ and ‘fish cells’.
We can all stare at a blood film, like a medical form of cloudwatching and decide what shapes are in there. The point is whether such cells occur at a clinically significant frequency – there are so many billions of cells in every sample that some will always look different even in the healthiest individual – and whether it says anything meaningful about the underlying pathology. I have found over two dozen different ‘-cytes’ in the literature referring to red blood cell shape and had a lot of fun drawing them. Trying to make them expressive whilst retaining the characteristic shape has been an interesting challenge. I found that less was more, it seemed to work better if I was strict about how much detail I would allow myself especially with the eyes.
These designs, including the spotters guide poster (you could have it printed on a shower curtain and be the envy of all the other haematologists in the bathroom!) are available on a range of cool products from my RedBubble store. You can see all the designs in the gallery below or use the search function. For cartoons and information about another major class of blood cell – platelets – follow this link.