Monocytes Under The Microscope

Observation and Discussion

Monocytes are a type of white blood cell (leukocyte) that are produced from the pluripotent stem cells found in the bone marrow.

Once they are produced in the bone marrow, they remain there for a very short period of time (less than 24 hours) before moving to the circulatory system and the marginating pool. They also remain in these environments for a few days (2 to 3 days) and ultimately migrate to various tissues where they change into macrophages.

While monocytes are the largest white cells, they only make up about 5 percent of the total leukocytes in circulation. However, their numbers will increase during an infection (when bacteria or protozoa invade body tissues and cause inflammation).


Unstained, the different types of leukocytes are almost impossible to identify. However, staining a blood smear with such stains as Wright-Giemsa stain can help increase contrast and thus be in a position to differentiate the different types of white blood cells. Here, this technique will be used to identify monocytes.



Procedure (Blood Smear)

  • Using a cotton swab with alcohol, clean the tip of the middle or ring finger (this is done to prevent an infection)
  • Prick the tip of the finger using a clean and unused laboratory pricking needle and wipe off the first drop of blood using another cotton swab with alcohol 
  • Gently press the finger and make a round smear on one glass slide (this is the thick film)
  • Press the finger tip again (gently) and place a drop or two of blood on to the center of another clean slide (for a thin film)
  • Press a clean swab with a little alcohol on the pricked finger (to prevent bleeding or infection)
  • Using another clean glass slide at an angle to the glass slide with 1 or 2 drops of blood, create a thin blood film 
  • Place the two glass slides (with a thick and thin blood film) on the drying rack and allow them to dry completely 


* Fix the thin slide with acetone-free methanol before staining and allow it to dry completely.


  • Stain the slide using modified Wright-Giemsa stain that has been buffered using methanol (pH 6.8). Basically, this will involve immersing the slides in a jar/beaker containing the stain for a period of about 30 minutes.
  • Remove the slide from the jar and stand it on a rack and allow it to dry properly
  • Place the slide on the microscope stage and view starting with low power
  • At 1000x, add a drop of immersion oil to view the slide


When viewed under the microscope at high power, the slide shows numerous red blood cells that appear to have a space in the middle. However, students will also be able to see several monocytes that will appear spherical or bean shaped with a dark material inside.

Compared to other leukocytes, monocytes will appear larger therefore setting them apart.

Monocytes Discussion

As mentioned, monocytes are spherical in shape or may appear amoeboid. They are large in size and have a large nucleus that is usually bean shaped. This characteristic differentiates monocytes from neutrophils, which have several lobes with a divided nucleus. In tissues, monocytes transform to macrophages.

As macrophages, monocytes play an important role of eliminating foreign material, tumorous cells and microorganisms through a process known as phagocytosis.


* Essentially, phagocytosis involves the cell engulfing foreign material or microorganisms in to the cell vacuole where they are ingested and destroyed by enzymes.


Depending on the tissue in which they reside, macrophages will have different names.

These include:


  • Intraglomerular Mesangial Cells - These are macrophages that reside in the kidney
  • Kupfer Cells - Macrophages residing in the liver
  • Sinus Histiocytes - Macrophages found in the lymph nodes
  • Langerhans - Macrophages found in the skin (Dendritic cells of the skin)

See Also:  Basophils, Lymphocytes, Neutrophils, Eosinophils, Macrophages

More on Leukocytes here

Return to White Blood Cells Main Page

Return to Blood Smear Guide

Return from Monocytes to Cell Biology Main Page

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