MH 034a-034bhr Bone Marrow Smear

Megakaryocytes

Platelets develop from the multipotential myeloid stem cell (CFU-GEMM) which differentiates into the megakaryocyte progenitor cell (CFU-Meg).

Megakaryoblasts are produced directly from megakaryocyte progenitor cell (CFU-Meg). They are large cells (~30 µm diameter) with a round nucleus.

Megakaryocytes are produced from megakaryoblasts under the influence of thrombopoietin. They grow in size by replicating DNA without going through cytokinesis (endomitosis).

Megakaryocytes are the only recognizable precursor of platelets.

• Large cells (50 to 70 µm diameter)
• Large multilobed, polyploid nucleus (up to 32 copies of the normal complement of DNA)
• Cytoplasm is blue with many small azure granules
• No longer capable of mitosis

Examples:

• Megakaryocyte 1(40x)[ + ]

• Megakaryocyte 2(40x)[ + ]

• Megakaryocyte 3(40x)[ + ]

Platelets

Platelets are small cell fragments produced by budding of from megakaryocytes under the influence of thrombopoietin. Each megakaryocyte produces between 5,000 and 10,000 platelets.

Platelets are much smaller than red blood cells.

• Small discs (2 to 3 µm diameter)
• No nucleus
• Contain basophilic structures

Platelets are typically found as clusters:

• Platelets 1(60x)[ + ] - small cluster of four platelets

• Platelets 2(60x)[ + ] - cluster with several cross-sections of platelets

• Platelets 3(60x)[ + ] - large cluster

Platelets are released from the bone marrow into the peripheral circulation. Too few platelets can cause excessive bleeding, while too many platelets can cause blood clot formation. Their life span is between 7 to 10 days.

A cross-section of a platelet is shown in EM 159 Platelet by transmission electron microscopy.