Page 300 - Textbook of Pathology, 6th Edition

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Section II HAEMATOLOGY AND

LYMPHORETICULAR TISSUES

Introduction to Haematopoietic

Chapter 12
Chapter 12 System and Disorders of

Erythroid Series

The section on disorders of the haematopoietic system is of phagocytic macrophages, both of which together constitute
concerned with diseases of the blood and the bone marrow. mononuclear-phagocyte system (Chapter 4). Lymphopoietic
SECTION II
Conventionally, it includes study of constituents of cells in the marrow undergo differentiation to form B, T and
circulating blood and thus comprises of discussion on natural killer (NK) cells of the immune system.
diseases of red blood cells, white blood cells, platelets and Circulating blood normally contains 3 main types of
bleeding disorders, and an account of blood groups and mature cells—the red cells (erythrocytes), the white cells
blood transfusion. However, with wide acceptance of the (leucocytes) and the platelets (thrombocytes). These blood
newer WHO classification of malignancies of haematopoietic cells perform their respective physiologic functions:
system and lymphoreticular tissues over previous FAB erythrocytes are largely concerned with oxygen transport,
(French-American-British) Cooperative Group classification, leucocytes play various roles in body defense against infection
this section needed rational redistribution. While the FAB and tissue injury, while thrombocytes are primarily involved
group had taken the criteria of location of leucocytes (in blood in maintaining integrity of blood vessels and in preventing
and in tissues) for classification, the WHO classification has blood loss. The lifespan of these cells in circulating blood is
taken into consideration both ‘circulating’ leucocytes and variable—neutrophils have a lifespan of 6-8 hours, followed
‘fixed’ leucocytes present anywhere in the tissues together by platelets with a lifepan of 10 days, while the RBCs have
since these diseases tend to disseminate from one pool to the longest lifespan of 90-120 days. The rates of production
the other. of these blood cells are normally regulated in healthy
Thus, in this edition, the present chapter encompasses individuals in such a way so as to match the rate at which
basic introduction to the haematopoietic system and the bone they are lost from circulation. Their concentration is normally
marrow, followed by discussion on erythroid series of maintained within well-defined limits unless the balance is
disorders. The disorders of leucocytes (myeloid and disturbed due to some pathologic processes.
lymphoid series) and diseases of lymphoreticular tissues
have been put together in Chapter 14. A separate chapter on HAEMATOPOIESIS
disorders of platelets and bleeding along with transfusion
Haematology and Lymphoreticular Tissues
medicine (Chapter 13) has been placed interposed between In the human embryo, the yolk sac is the main site of
these two chapters. haematopoiesis in the first few weeks of gestation. By about
Since study of haematopoietic system involves two 3rd month, however, the liver and spleen are the main sites of
phases—laboratory haematology and clinical haematology, blood cell formation and continue to do so until about 2 weeks
understanding of broad principles of management of after birth. Haematopoiesis commences in the bone marrow
common haematological diseases has been considered by 4th and 5th month and becomes fully active by 7th and
desirable part of the discussion on these topics. 8th month so that at birth practically all the bones contain
active marrow. During normal childhood and adult life,
BONE MARROW therefore, the marrow is the only source of new blood cells.
However, during childhood, there is progressive fatty
The pluripotent stem cells in the bone marrow give rise to replacement throughout the long bones so that by adult life
two types of multipotent stem cells: non-lymphoid stem cells the haematopoietic marrow is confined to the central skele-
which differentiate in the bone marrow, and lymphoid stem ton (vertebrae, sternum, ribs, skull, sacrum and pelvis) and
cells which differentiate in the bone marrow and then migrate proximal ends of femur, tibia and humerus (Fig. 12.1). Even
to the lymphoid tissues. The non-lymphoid stem cells form in these haematopoietic areas, about 50% of the marrow
the circulating erythrocytes, granulocytes, monocytes and consists of fat (Fig. 12.2). Non-haematopoietic marrow in the
platelets. Monocytes on entering the tissues form a variety adult is, however, capable of reverting to active haemato-