Histology of spleen
2
nd
stag
Dr.Elham majeed
TUOCM
INTRODUCTION
The
spleen
is located in the left upper quadrant region
of the abdomen, is found posterior to the
stomach
and
in relation to the left
kidney
. The hilum of the spleen is
the only part of the spleen that is peritoneum free.
Spleen is an intraperitoneal organ, is covered by a layer
of visceral peritoneum. Underneath the peritoneum is
the capsule of the spleen. The capsule of the spleen
consists of dense irregular fibroelastic tissue.
The
connective tissue
of the capsule contains
contractile
cells
called myofibroblasts. By producing
weak contraction of the capsule, these cells help to
discharge the blood stored within the spleen into
the
circulation
. The capsule also allows the spleen to
significantly increase in size when necessary . At the
level of the hilum, the capsule splits into several septa
called trabeculae which penetrate into the parenchyma
of the spleen and partly divide its tissue.
Histology of spleen
Like every other organ, the spleen consists of stroma and
parenchyma. The stroma of the spleen is composed mainly of a
network of
reticular connective tissue
. This mesh provides
support for
blood cells
and immune cells (lymphocytes,
macrophages, and dendritic cells). The parenchyma of the
spleen is divided into two functionally and morphologically
distinct compartments (red pulp and white pulp) divided by a
tissue layer called the marginal zone. Outside the marginal zone
is the perifollicular
zone which contains sheathed
capillaries
and blood-
filled spaces without endothelial lining.
Red pulp :
The red pulp occupies the majority of the
stromal tissue of the spleen. It consists of the cords
of Billroth and splenic sinusoids. The cords of
Billroth (splenic cords) are the cellular aggregations
Supported by the reticular connective tissue. They
Splenic sinusoids are found between the cords of Billroth. They
are
filled with blood and give the red pulp its
distinguishable red appearance. Blood slowly flows through the
sinusoids where it is exposed to macrophages from the
cords of Billroth, patiently waiting for foreign antigens that can
appear in the blood. In a nutshell, the red pulp functions
as a blood filter for various toxins, destroying them before they
enter
systemic circulation and get the chance to spread
throughout the body and damage other organs
White pulp
: The white pulp of the spleen is made of three different compartments: Periarterial
lymphatic sheath
(PALS), lymphatic follicles and the marginal zone.
The PALS consists of a central artery (a branch of the
splenic artery
) surrounded by a sheath of
lymphoid tissue. Here, the lymphoid tissue organized into two layers: The inner layer and outer
layer. The inner layer is mainly composed of
T
lymphocytes which is why it is also called the T-
zone. The outer layer has a more diverse cellular morphology, containing T and B lymphocytes.
The branches of central arterioles are surrounded by the sharply defined areas of B
lymphocytes, comprising the lymphatic follicles of the spleen. There are two types of lymphatic
follicles depending on the features of the B lymphocytes that comprise them: Primary follicles
and secondary nodules.
A follicle that consists mainly of small, immature lymphocytes is called a primary follicle.
However, most nodules found in the spleen are secondary nodules that arise from primary
follicles as the lymphocytes mature and increase in size. They differ from primary follicles by
featuring a distinctive centrally positioned zone called the germinal center. The germinal centers
are the sites where lymphocytes mature and acquire the ability to produce antibodies. So,
seeing the germinal center is a sign that lymphatic tissue is responding to an antigen. Other
than B lymphocytes, the germinal centers also contain follicular dendritic cells (FDC) which also
increase in number after antigen activation. They support B lymphocytes, initiate and modulate
their immune response.
Blood circulation :-
Since the spleen is a blood filter, one has to assume that it is a highly vascular organ. Blood from
the splenic artery enters the spleen through the hilum. From there, the artery divides into smaller
branches that enter the splenic parenchyma following the course of trabeculae. Together with the
trabeculae, the arteries branch throughout the parenchyma and gradually decrease in diameter.
Eventually, smaller arterioles leave the
dense connective tissue
of trabeculae entering the parenchyma,
where they are surrounded by PALS.
The artery in the center of this region is referred to as the central artery. On a histology slide, PALS has
a similar appearance to a
lymphatic nodule
. The only way to distinguish the two is by the presence of a
central artery. The endothelial cells of the central artery have finger-like extensions that spirally wrap
around the lumen protecting the PALS from a direct antigen invasion.
The central artery s ends branches that enter the marginal zone of the white pulp. It then continues into
the red pulp where it divides further and eventually transforms into arterial capillaries. These capillaries
are surrounded by clusters of macrophages and are called sheathed capillaries. From this point on,
blood travels freely through the venous sinuses of the red pulp. The endothelial cells of the sinusoids
have special histological features; they are elongated, spindle-shaped and don’t have characteristic
cellular junctions to adjacent cells.
Due to the lack of junctions, there are wide gaps between the cells that serve as a mechanical filter
between the blood and splenic cords. When different antigens (e.g. microorganisms, cellular debris and
aged and damaged
erythrocytes
) cross these gaps they can be phagocytosed and destroyed by
macrophages that are waiting on the other side. This type of blood flow is called open circulation which
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