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Tissue slides of blood are difficult to confuse with other tissues.  Blood smears have a very characteristic appearance to them.

Consider the fact a single cubic milliliter of blood contains on average 6-7 million erythrocytes (red blood cells) and up to 7,000 leukocytes (white blood cells).  If these were stacked on top of one another, you could not determine key cell characteristics.    The purpose of a smear in blood histology is to spread cells across the surface of glass slides to separate them. The blood smear, if done correctly, results in a zone where a single layer of cells provides separation and therefore, potential for correct identification.

Blood slides are always stained with a differential stain.  As the name implies, this stain allows the differentiation or identification of cells.  In blood slides, the objective is to stain and differentiate leukocytes as variations in numbers can be very informative and in some cases, diagnostic of specific pathologies.

In this view of blood, the erythrocytes are visible as faint pink or red, oval shaped doughnut-like cells.  As biconcave discs, erythrocytes appear doughnut-like but they do not have holes.  Erythrocytes also lack nuclei!  It is believed erythrocytes lose nuclei during development to allow more room within cells for the oxygen carrying protein hemoglobin.

Leukocytes are not difficult to identify if certain criteria are kept in mind.  The differential stain, as you can see, causes these leukocytes to look quite different from one another.

I'd like to learn more about leukocyte identification!

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