C h a p t e r 1 1
Disorders of White Blood Cells and Lymphoid Tissues
245
The granulocytic precursor cells, which are called
myeloblasts,
have round to oval nuclei, with delicate
chromatin and a blue to gray cytoplasm. During their
next stage of development, the myeloblasts are trans-
formed into
promyelocytes
with similar nuclei, but
with a cytoplasm containing many primary granules.
In the subsequent
metamyelocyte
stage, the nuclei dis-
tort and become arclike, producing the band devel-
opmental stage. Maturation from metamyelocyte to
mature neutrophil involves progressive condensation
of nuclear chromatin, increasing nuclear lobulation,
and the appearance of secondary (specific) granules.
Eosinophils and basophils undergo similar develop-
mental stages but develop different secondary gran-
ules. Like granulocytes, monocytes develop from the
granulocyte-monocyte progenitor cell and progress
through monoblast and promonocyte stages.
Lymphocytes derive from lymphoid stem cells and
progress through the lymphoblast and prolymphocyte
stages. The prolymphocytes leave the bone marrow and
travel to the lymphoid tissues, where further differentia-
tion into T and B lymphocytes occurs.
LymphoidTissues
The body’s lymphatic system consists of the lymphatic
vessels, lymphoid tissue and lymph nodes, thymus, and
spleen (see Chapter 15). Although both precursor B
and T lymphocytes begin their development in the bone
marrow, they migrate to peripheral lymphoid structures
to complete the differentiation process. B lymphocytes
mature in the bone marrow, differentiate into plasma
cells, and then move to the lymph nodes, where they
continue to proliferate and produce antibodies. T lym-
phocytes leave the bone marrow as precursor T lympho-
cytes travel to the thymus, where they differentiate into
CD4
+
helper T cells and CD8
+
cytotoxic T cells, after
which many of them move to lymph nodes, where they
undergo further proliferation.
Lymph nodes consist of organized collections of
lymphoid tissue located along the lymphatic vessels.
2–4
Typically grayish white and ovoid or bean shaped, they
range in diameter from 1 mm to 1 to 2 cm. A fibrous
capsule and radiating trabeculae provide a supporting
structure, and a delicate reticular network contributes
to internal support (Fig. 11-4). The parenchyma of the
lymph node is divided into an outer cortex and an inner
medulla. The cortex contains well-defined B-cell and
T-cell domains. The superficial outer cortex contains
aggregates of cells called
follicles
. Follicles are the B-cell
zones of the lymph nodes. There are two types of fol-
licles: immunologically inactive follicles, called
primary
follicles,
and active follicles that contain germinal cen-
ters, called
secondary follicles.
Germinal centers contain
large lymphocytes (centroblasts) and small lymphocytes
with cleaved nuclei (centrocytes). The mantle zone is the
small layer of B cells surrounding the germinal centers.
The cortex around the follicles is called the
paracortex
.
This region contains most of the T cells in the lymph
nodes. Like normal lymphocytes, malignant B and T cells
tend to home to particular nodal sites, leading to char-
acteristic patterns of involvement. For example, follicu-
lar lymphomas develop in the B-cell areas of the lymph
node, whereas T-cell lymphomas typically grow in the
paracortical T-cell zones.
Wandering
macrophages
Myeloid stem cell
Lymphoid stem cell
Monoblast
Myeloblast
Promonocyte Prolymphocyte
Promyelocyte
Bone
marrow
Blood and
lymphoid
tissue
Neutrophilic myelocyte
Neutrophilic metamyelocyte
Neutrophilic band cells
Granular leukocytes Monocytes Lymphocytes
(some
become)
(some
become)
Plasma cells
Committed
cells
Lymphoblast
FIGURE 11-3.
Leukocytes originate from multipotential stem
cells in the bone marrow. Granular leukocytes (neutrophils,
eosinophils, basophils) have their origin in the myeloid stem
cells and develop through a sequence involving myeloblasts.
Monocytes, like granulocytes, are progeny of the myeloid stem
cell line, but develop along a pathway involving monoblasts.
Only lymphocytes originate from the lymphoid stem cell line.
They develop through a sequence involving lymphoblasts and
are released from the bone marrow as prolymphocytes, which
undergo further differentiation in the lymphoid organs.
