758
UNIT 4
Maintenance of the Body
20
of
splenic cords
, regions of reticular connective tissue, that
separate the blood-filled
splenic sinusoids
(venous sinuses).
Te names of the pulp regions reflect their appearance in
fresh spleen tissue rather than their staining properties. Indeed,
as you can see in the photomicrograph in Figure 20.6d, the
white pulp sometimes appears darker than the red pulp due to
the darkly staining nuclei of the densely packed lymphocytes.
Homeostatic Imbalance
20.4
Because the spleen’s capsule is relatively thin, a direct blow or
severe infection may cause it to rupture, spilling blood into the
peritoneal cavity. Once,
splenectomy
(surgical removal of the
ruptured spleen) was the standard treatment and thought nec-
essary to prevent life-threatening hemorrhage and shock. How-
ever, surgeons have discovered that, if le± alone, the spleen can
o±en repair itself and so the frequency of emergency splenecto-
mies has decreased dramatically at major trauma centers. If the
spleen must be removed, the liver and bone marrow take over
most of its functions. In children younger than 12, the spleen
will regenerate if a small part of it is le± in the body.
Thymus
Te bilobed
thymus
(thi
9
mus) has important functions pri-
marily during the early years of life. It is found in the inferior
neck and extends into the superior thorax, where it partially
overlies the heart deep to the sternum (see Figure 20.5 and
Figure 20.7
). In the thymus, ² lymphocyte precursors mature
to become immunocompetent lymphocytes. In other words,
the thymus is where ² lymphocytes become able to defend us
against specific pathogens in the immune response.
Prominent in newborns, the thymus continues to increase in
size during the first year, when it is highly active. A±er puberty,
it starts to atrophy gradually and by old age it has been replaced
almost entirely by fibrous and fatty tissue and is difficult to dis-
tinguish from surrounding connective tissue. Even though it
atrophies, the thymus continues to produce immunocompetent
cells as we age, although at a declining rate.
²o understand thymic histology, it helps to compare the
thymus to a cauliflower head—the flowerets represent
thymic
lobules
, each containing an outer cortex and an inner medulla
(Figure 20.7). Most thymic cells are lymphocytes. In the cortical
regions the rapidly dividing lymphocytes are densely packed,
with a few macrophages scattered among them.
Te lighter-staining medullary areas contain fewer lym-
phocytes plus some bizarre structures called
thymic corpus-
cles
. Consisting of concentric whorls of keratinized epithelial
cells, they were thought to be sites of ² cell destruction. Recent
evidence suggests that thymic corpuscles are involved in the
development of
regulatory T cells
, a class of ² lymphocytes that
are important for preventing autoimmune responses.
Te thymus differs from other lymphoid organs in three im-
portant ways:
Te thymus has no follicles because it lacks B cells.
Te thymus is the only lymphoid organ that does not
di-
rectly
fight antigens. Instead, the thymus functions strictly as
important are its blood-cleansing functions. Besides extract-
ing aged and defective blood cells and platelets from the blood,
its macrophages remove debris and foreign matter from blood
flowing through its sinuses. Te spleen also performs three ad-
ditional, and related, functions. Te spleen:
Stores some of the breakdown products of red blood cells for
later reuse (for example, it salvages iron for making hemo-
globin). It releases other breakdown products to the blood
for processing by the liver.
Stores blood platelets and monocytes for release into the
blood when needed.
May be a site of erythrocyte production in the fetus (a capa-
bility that normally ceases before birth).
Like lymph nodes, the spleen is surrounded by a fibrous cap-
sule and has trabeculae that extend inward. Histologically, the
spleen consists of two components: white pulp and red pulp.
White pulp
is where immune functions take place, so it is
composed mostly of lymphocytes suspended on reticular
fibers. Te white pulp clusters or forms “cuffs” around central
arteries (small branches of the splenic artery) in the organ
and forms what appear to be islands in a sea of red pulp.
Red pulp
is where worn-out red blood cells and bloodborne
pathogens are destroyed, so it contains huge numbers of
erythrocytes and the macrophages that engulf them. It is es-
sentially all splenic tissue that is not white pulp. It consists
Thymic
corpuscles
Medulla
Cortex
Capsule
Figure 20.7
The thymus.
The photomicrograph of a portion
of the thymus shows part of a lobule with cortical and medullary
regions (85
3
).
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