The spleen is, like the lymph nodes, a
discriminatory filter. Unlike the lymph nodes, the spleen is
inserted into the blood stream. The spleen clears the blood of aged
blood cells and foreign particles and is the site of immune reactions to blood-borne
antigens. The spleen is not essential to life in the adult
as other organs take over its functions if it is removed.
The spleen is surrounded by a
capsule of dense connective tissue from which branched
trabeculae extend into the parenchyma of the spleen (sounds familiar). The parenchyma of the spleen
is termed the pulp of the spleen.
Most of the pulp of a fresh, unfixed spleen is a soft, dark red
mass, the red pulp. It consists of
large, irregular, thin-walled blood vessels, the splenic sinusoids, interposed between sheets and
strands of reticular connective tissue, the splenic cords (of
Billroth). Within the red pulp small, oval or rounded
greyish white areas, the white pulp,
is formed by lymphoid tissue.
Branches of the splenic artery (Where does it enter
the spleen?) divides into
trabecular arteries (Where are they
found?), which enter the white pulp, where they are
called central arteries. Branches of
the central artery almost all divide into smaller vessels in the
marginal zone, i.e. the border between the red and white pulp. Fine
branches of the central artery - penicillar
arteries (cuboidal epithelium) - branch again to form
arterial capillaries, which, as they exit the white pulp, are
surrounded by a sheath of phagocytotic cells and reticular fibres.
They are now called sheathed
arteries. From here, the blood enters the red pulp. Sheathed
arteries may either empty the blood which they carry directly into
the splenic sinusoids (closed
circulation, about 90% in
cats) or into the reticular connective tissue of the splenic
cords (open circulation).
Macrophages are, in addition to reticular cell, the main resident
cell population of the splenic cords.
Blood cells which are emptied into the splenic cords re-enter
the blood vessels through the endothelium of the sinusoids. The
endothelial cells are elongated (in cross section they may appear
cuboidal) and oriented along the long axis of the sinusoids. The
endothelium of the sinusoids has no junctional complexes and its
basement membrane is incomplete (forming narrow circular bands
around the endothelial cells with large intervening fenestrations).
Macrophages ingest aged erythrocytes, platelets and other
particulate matter as they pass through the splenic cords.
The composition of the plasma membrane of
erythrocytes changes as the cell ages. It is thought that these
changes eventually expose erythrocyte senescence antigens, which
bind blood-borne antibodies and thus tag the erythrocyte for
removal by macrophages. Erythrocyte removal is also one function of
the resident macrophages of the liver, although splenic macrophages
take care of most of the job.
The sinusoids continue into the veins of the pulp, which empty
into thin-walled trabecular veins,
which eventually coalesce to form the splenic vein.
The white pulp surrounds the central arteries as a periarterial lymphoid
sheath (PALS). Lymphocytes of
the PALS are likely to be T-lymphocytes. In addition, we see
macrophages and plasma cells in the PALS. Lymph nodules, formed by
B-lymphocytes, are present along the course of the central
arteries. The central arteries are typically located in the
periphery of the nodule.
Lab: Slide
SPLEEN HUMAN H&E. Find a place
close to the capsule where you can identify trabeculae, white pulp
(possibly a nodule with a peripherally placed central artery), and
red pulp. Good penicillar and sheathed arteries are very hard to
find - try it, but do not despair if you (or the demonstrator)
cannot find them. As usual, it is easiest to identify macrophages
by accumulations of particulate matter in their cytoplasm, which
often will represent disintegrating erythrocytes.
Sketch your observations (the 10x objective
should be fine).
Slide SPLEEN T.S. CAT RETICULIN.
This slide illustrates the distribution of reticular fibres in the
spleen. They often appear coarser in the red pulp, where they have
a distinct, stranded organisation. The reticular fibres of the
white pulp appear somewhat finer and, at times, they are arranged
as concentric rings. The peripheral localisation of the central
arteries in nodules is quite distinct.
Occasionally you may see small rings of reticular fibres in (or
close to) the periphery of the white pulp. These rings are likely
to represent the reticular fibres surrounding sheathed
arteries.
Sketch your observations (the 10x objective is
sufficient).
The tonsils are accumulations of
lymphoid tissue surrounding the openings of the digestive and
respiratory tracts. The tonsils and
smaller accumulations of lymphoid tissue, which may be found
between them, are also called Waldeyer's ring.
Depending on their localisation we distinguish between
- palatine tonsils (THE tonsils),
which are located in the lateral wall of the oropharynx and covered
by a stratified squamous epithelium,
- lingual tonsils which are
situated in the lamina propria at the root of the tongue and also
covered by a stratified squamous epithelium, and
- pharyngeal tonsils (also called nasopharyngeal tonsils or adenoids)
which are located in the upper posterior part of the throat (nasopharynx) and covered by a
pseudostratified ciliated epithelium with goblet cells.
The tonsils do not have afferent lymph
vessels. Efferent lymph vessels are present. Exposure to
antigens relies on the contact of antigens with cells of the immune
system across the epithelium which covers the tonsils. The
epithelium of the palatine and lingual tonsils forms deep crypts
into the lymphoid tissue, and the consequent increase of the
surface area is one way to facilitate the contact of antigens with
the cells of the immune system. In addition, the epithelium may
specialise in places to form an open meshwork of cells with an
incomplete basal lamina (a reticulated
epithelium) which allows the infiltration of the epithelium
by lymphocytes and macrophages.
Tonsillar lymphoid nodules consist mainly of B-lymphocytes. Other
areas are occupied by T-lymphocytes, activated B-lymphocytes and
other cells of the immune system.
The tonsils share some histological features with lymph
nodes:
- cells in the tonsils are supported by a fine network of
reticular fibres and
- postcapillary (high-endothelial-) venules function in the
"homing" of circulating lymphocytes - this is actually a shared
feature of all lymphoid tissues and organs.
The palatine tonsils are surrounded by a thick hemicapsule of
connective tissue, which delimits them from the pharyngeal muscle
and facilitates their removal in
tonsillitis.
The basic structure of the vermiform appendix
has already been described under HA 235 - Histology -
Gastrointestinal tract.
GALT (you should be able to decipher this
acronym) is found throughout the gastrointestinal tract.
However, the most prominent accumulations occur in the ileum in the
form of Peyer's patches and in the appendix.
In the appendix, lymphoid tissue, which consists of B- and
T-lymphocytes, may extend from the lamina propria into the
submucosa and fill a substantial part of it. Some crypts penetrate
deep into the lymphoid tissue. The epithelium in contact with the
lymphoid tissue is specialised to facilitate the contact of
antigens with cells of the immune system. The epithelium appears
low columnar and contains cells with deeply invaginated basal
surfaces (microfold cells or
M-cells). Immune system cells can enter these invaginations
(intraepithelial pockets) where they are exposed to materials which
have been endocytosed by the epithelial cells and then released
into the invaginations.
An extreme proliferation of lymphocytes
(lymphoid hyperplasia) as a consequence of bacterial or viral
stimulation may lead to the obstruction of the lumen of the
appendix and thereby cause appendicitis, but this is only one of
many possible causes.
The term "Peyer's patches" is usually used in
conjunction with the GALT of the ileum but may be applied for GALT
throughout the small and large intestines.