Page 2095 - Hematology_ Basic Principles and Practice ( PDFDrive )

1858 Part XII Hemostasis and Thrombosis

A B

Fig. 124.1 POLYPLOID MEGAKARYOCYTES IN THE BONE MARROW. Large polyploid megakaryo-
cytes are seen in the bone marrow on a typical hematoxylin- and eosin-stained slide and are recognized by
their abundant pink cytoplasm and large nuclei (A). The degree of polyploidization is difficult to determine.
Rarely, megakaryocytes can be seen in mitosis (B), and when chromosomes are aligned in metaphase plates,
the high ploidy level become quite apparent. In the mitotic figure illustrated, the megakaryocyte is a 16N
form with eight 2N metaphase plates.

Production of
platelet-specific granules
NF-E2
GATA1
Tpo FOG

α-granules
Cortical
microtubule arrays
Dense granules
Production of
proplatelets—microtubule
dependent
Platelets release

Branching of
Apoptosis of proplatelets—actin
residual cell dependent
body
Production
site of platelets
Fig. 124.2 SUMMARY OF THE MAJOR EVENTS THAT LEAD TO PLATELET FORMATION AND
RELEASE FROM MEGAKARYOCYTES. Hematopoietic stem cells are converted into megakaryocytes by
exposure to the specific growth factor TPO. TPO initiates a maturation program that amplifies the mega-
karyocyte DNA and leads to synthesis of platelet-specific proteins. In particular, cytoskeletal elements,
membrane systems, and receptor proteins are made in bulk, and the megakaryocyte becomes filled with
platelet-specific granules. Platelet production begins when microtubules aggregate in the cell cortex, and one
pole of the megakaryocyte spontaneously elaborates pseudopodia. These begin as large blunt pseudopodia,
which subsequently thin and branch into proplatelets. The branching reaction is dependent on a localized
assembly of actin and is inhibited by drugs that disrupt actin filaments. Platelets are assembled primarily at
the ends of the proplatelets. Intracellular organelles are delivered to the platelet buds along microtubule tracks
in the shafts. Platelets are released from the ends of proplatelets. FOG, Friend of GATA1; GATA1, GATA
binding protein 1; TPO, thrombopoietin.