Page 343 - Textbook of Pathology, 6th Edition

Disorders of Platelets, 327

Chapter 13
Chapter 13 Bleeding Disorders and

Basic Transfusion Medicine

THROMBOPOIESIS granules. Platelets are formed from pseudopods of
megakaryocyte cytoplasm which get detached into the blood
As outlined in previous chapter and illustrated in Fig. 12.3, stream. Each megakaryocyte may form up to 4000 platelets.
the trilineage myeloid stem cells in the bone marrow The formation of platelets from the stem cell takes about
differentiate into erythroid progenitor, granulocyte- 10 days.
monocyte progenitor, and megakaryocyte progenitor cells. PLATELETS. Platelets are small (1-4 μm in diameter),
Platelets are formed in the bone marrow by a process of discoid, non-nucleate structures containing red-purple CHAPTER 13
fragmentation of the cytoplasm of megakaryocytes. Platelet granules. The normal platelet count ranges from 150,000-
production is under the control of thrombopoietin, the nature 400,000/μl and the lifespan of platelets is 7-10 days. About
and origin of which are not yet established. The stages in 70% of platelets are in circulation while remaining 30% lie
platelet production are: megakaryoblast, promegakaryocyte, sequestered in the spleen. Newly-formed platelets spend
megakaryocyte, and discoid platelets (Fig. 13.1).
24-36 hours in the spleen before being released into
MEGAKARYOBLAST. The earliest precursor of platelets circulation but splenic stasis does not cause any injury to the
in the bone marrow is megakaryoblast. It arises from platelets normally. Factors such as stress, epinephrine and
haematopoietic stem cell by a process of differentiation. exercise stimulate platelet production.
The main functions of platelets is in haemostasis which
PROMEGAKARYOCYTE. A megakaryoblast undergoes includes two closely linked processes:
endo-reduplication of nuclear chromatin i.e. nuclear
chromatin replicates repeatedly in multiples of two without 1. Primary haemostasis. This term is used for platelet plug
division of the cell. Ultimately, a large cell containing up to formation at the site of injury. It is an immediate phenomenon
32 times the normal diploid content of nuclear DNA appearing within seconds of injury and is responsible for
(polyploidy) is formed when further nuclear replication cessation of bleeding from microvasculature. Primary
ceases and cytoplasm becomes granular. haemostasis involves three steps: platelet adhesion, platelet
granule release and platelet aggregation which are regulated
MEGAKARYOCYTE. A mature megakaryocyte is a large by changes in membrane phospholipids, and calcium
cell, 30-90 μm in diameter, and contains 4-16 nuclear lobes (Fig. 13.2). At molecular level, these important events are
having coarsely clumped chromatin. The cytoplasm is depicted diagrammatically in Fig. 13.3 and briefly outlined
abundant, light blue in colour and contains red-purple below: Disorders of Platelets, Bleeding Disorders and Basic Transfusion Medicine

Figure 13.1 Thrombopoiesis.