Page 361 - Textbook of Pathology, 6th Edition

progression. Microscopic findings of node at this stage enter extravascular tissues where they perform their main 345
reveal follicular involution and lymphocyte depletion. At function of active phagocytosis. The extravascular lifespan
this stage, other stigmata of AIDS in the lymph node may of tissue macrophages which are the transformed form of
also appear e.g. lymphoma, mycobacterial infection, blood monocytes, may vary from a few months to a few years.
toxoplasmosis, systemic fungal infections etc.
Myeloid Series
WHITE BLOOD CELLS: The development of myeloid cells from myeloblast takes
NORMAL AND REACTIVE place in the following sequence (Fig. 14.4):
1. MYELOBLAST. The myeloblast is the earliest
The leucocytes of the peripheral blood are of 2 main varieties, recognisable precursor of the granulocytes, normally
distinguished by the presence or absence of granules: comprising about 2% of the total marrow cells. The myelo-
granulocytes and nongranular leucocytes. The granulocytes, blast varies considerably in size (10-18 μm in diameter),
according to the appearance of nuclei, are subdivided into having a large round to oval nucleus nearly filling the cell,
polymorphonuclear leucocytes and monocytes. Further, has fine nuclear chromatin and contains 2-5 well-defined pale
depending upon the colour of granules, polymorphonuclear nucleoli. The thin rim of cytoplasm is deeply basophilic and
leucocytes are of 3 types: neutrophils, eosinophils and devoid of granules. The myeloblasts of acute myeloid CHAPTER 14
basophils. The nongranular leucocytes are 3 types of leukaemia may, however, show the presence of rod-like
lymphocytes: T, B and natural killer (NK) cells.
cytoplasmic inclusions called Auer’s rods which represent
abnormal derivatives of primary azurophilic granules.
GRANULOPOIESIS
The nuclei of successive stages during their development
Site of Formation and Kinetics from myeloblast become progressively coarser and lose their
nucleoli and the cytoplasm loses its blue colour. As the cells
All forms of granulocytes are produced in the bone marrow become mature lysosomal granules appear; firstly non-
and are termed,‘myeloid series’. Myeloid series include specific primary or azurophilic granules appear which are
maturing stages: myeloblast (most primitive precursor), followed by specific or secondary granules that differentiate
promyelocyte, myelocyte, metamyelocyte, band forms and the neutrophils, eosinophils and basophils.
segmented granulocyte (mature form). The myeloblast,
promyelocyte and myelocyte form a ‘proliferative or mitotic 2. PROMYELOCYTE. The promyelocyte is slightly larger
pool’, while the remainder (i.e. metamyelocyte, band forms than the myeloblast (12-18 μm diameter). It possesses a round
and segmented granulocytes) make up a ‘mature or post- to oval nucleus, having fine nuclear chromatin which is
mitotic pool’. It takes about 12 days for formation of mature slightly condensed around the nuclear membrane. The
granulocytes from the myeloblast. Normally the bone nucleoli are present but are less prominent and fewer than
marrow contains more myeloid cells than the erythroid cells those in the myeloblast. The main distinction of promyelocyte Disorders of Leucocytes and Lymphoreticular Tissues
in the ratio of 2:1 to 15:1 (average 3:1), the largest proportion from myeloblast is in the cytoplasm which contains
being that of metamyelocytes, band forms and segmented azurophilic (primary or non-specific) granules.
neutrophils.
Normally, the bone marrow storage compartment 3. MYELOCYTE. The myelocyte is the stage in which
contains about 10-15 times the number of granulocytes found specific or secondary granules appear in the cytoplasm, and
in the peripheral blood. Following their release from the bone accordingly, the cell can be identified at this stage as
marrow, granulocytes spend about 10 hours in the circulation belonging to the neutrophilic, eosinophilic or basophilic
before they move into the tissues, where they perform their myelocyte. Primary granules also persist at this stage but
respective functions. The blood pool of granulocytes consists formation of new primary granules stops. The nucleus of
of 2 components of about equal size—the circulating pool that myelocyte is eccentric, round to oval, having somewhat
is included in the blood count, and the marginating pool that coarse nuclear chromatin and no visible nucleoli. The
is not included in the blood count. Granulocytes spend about myeloid cells up to the myelocyte stage continue to divide
4-5 days in the tissues before they are either destroyed during and, therefore, comprise mitotic or proliferative pool.
phagocytosis or die due to senescence. To control the various 4. METAMYELOCYTE. The metamyelocyte stage is 10-18
compartments of granulocytes, a ‘feed-back system’ exists μm in diameter and is characterised by a clearly indented or
between the circulating and tissue granulocytes on one side, horseshoe-shaped nucleus without nucleoli. The nuclear
and the marrow granulocytes on the other. The presence of chromatin is dense and clumped. The cytoplasm contains
a humoral regulatory substance, ‘granulopoietin’ analogous both primary and secondary granules. The metamyelocytes
to erythropoietin has also been identified by in vitro studies are best distinguished from the monocytes by the clumped
of colony-forming units (CFU) and is characterised as G-CSF nuclear chromatin while the latter have fine chromatin.
(granulocyte colony-stimulating factor) and GM-CSF
(granulocyte-monocyte colony-stimulating factor). 5. BAND FORMS. Band form is juvenile granulocyte, 10-
The kinetics of monocytes is less well understood than 16 μm in diameter, characterised by further condensation of
that of other myeloid cells. Monocytes spend about 20-40 nuclear chromatin and transformation of nuclear shape into
hours in the circulation after which they leave the blood to band configuration of uniform thickness.