624
UNIT FOUR
During fetal development, pulp cells of the spleen produce blood
cells, much as red bone marrow cells do after birth. As the time of
birth approaches, this splenic function ceases. However, in certain
diseases, such as
erythroblastosis fetalis
(see chapter 14, page 547), in
which many red blood cells are destroyed, the splenic pulp cells may
resume their hematopoietic activity.
The tissues in the lobules of the spleen are of two types.
White pulp
is distributed throughout the spleen in tiny
islands. This tissue is composed of nodules (splenic nod-
ules), similar to those in lymph nodes and are packed with
lymphocytes. The
red pulp,
which F lls the remaining spaces
of the lobules, surrounds the venous sinuses. This pulp con-
tains abundant red blood cells, which impart its color, plus
many lymphocytes and macrophages
(f
g. 16.14)
.
Larynx
Thymus
Lung
Liver
(a)
Diaphragm
Thyroid gland
Trachea
Heart
Stomach
Spleen
Connec-
tive
tissue
Lobule
(b)
FIGURE 16.12
Thymus and spleen. (
a
) The thymus is bilobed,
between the lungs, and superior to the heart. The spleen is inferior to
the diaphragm, posterior and lateral to the stomach. (
b
) A cross section
of the thymus (15×). Note how the thymus is subdivided into lobules.
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