Monocytes and platelets

Monocytes are agranulocytes that are produced in the bone marrow. These cells travel to organs of the body where they enter into the connective tissue and differentiate into phagocytic and antigen presenting cells, such as macrophages, dendritic cells, and osteoclasts. These cells have major roles in clearing apoptotic cells, removing debris, ingesting pathogens, and mediating immune responses.

• Examine the images below and the peripheral blood smear again. Identify monocytes, noting their size (12-15 microns in diameter), pale basophilic cytoplasm, and their large, indented or C shaped nuclei.
• To review the ultrastructural features of monocytes and platelets in TEM images, please see Figures 12-12 and 12-13 in Junqueira’s Histology 14e

Platelets (thrombocytes) are small, round to oval cellular fragments (2-4 microns) that normally range in number between 150,000-400,000 per microliter of blood. Platelets are essential for blood stasis and healing of damaged blood vessels. They contain cell surface proteins that allow them to adhere to collagen exposed by wounds in the vasculature, helping to plug the wound. They also possess a variety of granules with numerous substances involved in blood clotting, clot retraction, and clot removal. Platelets are produced in the bone marrow as cellular fragments that are released from megakaryocytes. Megakaryocytes are not present in peripheral blood smears. We will look at megakaryocytes when we examine a bone marrow smear in a moment.

• Identify platelets in the peripheral blood smear. Note their small size, lack of nuclei, distribution (singly or in clusters), and basophilia relative to the RBCs.

So, where does it all happen?