The red pulp of the
spleen
The spleen is an organ found in almost all vertebrates. Similar in structure to a large lymph node, it acts primarily as a blood filter. The word spleen comes . is composed of connective tissue known also as the
cords of Billroth and many splenic
sinusoids
A capillary is a small blood vessel from 5 to 10 micrometres (μm) in diameter. Capillaries are composed of only the tunica intima, consisting of a thin wall of simple squamous endothelial cells. They are the smallest blood vessels in the body: ...
that are engorged with blood, giving it a red color.
Its primary function is to filter the blood of
antigens, microorganisms, and defective or worn-out red blood cells.
The spleen is made of red pulp and
white pulp
White pulp is a histological designation for regions of the spleen (named because it appears whiter than the surrounding red pulp on gross section), that encompasses approximately 25% of splenic tissue. White pulp consists entirely of lymphoid ti ...
, separated by the
marginal zone
The marginal zone is the region at the interface between the non-lymphoid red pulp and the lymphoid white-pulp of the spleen. (Some sources consider it to be the part of red pulp which borders on the white pulp, while other sources consider it to ...
; 76-79% of a normal spleen is red pulp.
Unlike white pulp, which mainly contains
lymphocytes such as
T cell
A T cell is a type of lymphocyte. T cells are one of the important white blood cells of the immune system and play a central role in the adaptive immune response. T cells can be distinguished from other lymphocytes by the presence of a T-cell r ...
s, red pulp is made up of several different types of blood cells, including
platelet
Platelets, also called thrombocytes (from Greek θρόμβος, "clot" and κύτος, "cell"), are a component of blood whose function (along with the coagulation factors) is to react to bleeding from blood vessel injury by clumping, thereby i ...
s,
granulocytes,
red blood cell
Red blood cells (RBCs), also referred to as red cells, red blood corpuscles (in humans or other animals not having nucleus in red blood cells), haematids, erythroid cells or erythrocytes (from Greek ''erythros'' for "red" and ''kytos'' for "holl ...
s, and
plasma.
The red pulp also acts as a large reservoir for
monocyte
Monocytes are a type of leukocyte or white blood cell. They are the largest type of leukocyte in blood and can differentiate into macrophages and conventional dendritic cells. As a part of the vertebrate innate immune system monocytes also ...
s. These monocytes are found in clusters in the Billroth's cords (red pulp cords). The population of monocytes in this reservoir is greater than the total number of monocytes present in circulation. They can be rapidly mobilised to leave the spleen and assist in tackling ongoing infections.
Sinusoids
The splenic sinusoids, are wide vessels that drain into pulp veins which themselves drain into
trabecular veins. Gaps in the
endothelium
The endothelium is a single layer of squamous endothelial cells that line the interior surface of blood vessels and lymphatic vessels. The endothelium forms an interface between circulating blood or lymph in the lumen and the rest of the vesse ...
lining the sinusoids mechanically filter blood cells as they enter the spleen. Worn-out or abnormal red cells attempting to squeeze through the narrow intercellular spaces become badly damaged, and are subsequently devoured by
macrophages in the red pulp. In addition to clearing aged red blood cells, the sinusoids also filter out cellular debris, particles that could clutter up the bloodstream.
Cells found in red pulp
Red pulp consists of a dense network of fine
reticular fiber
Reticular fibers, reticular fibres or reticulin is a type of fiber in connective tissue composed of type III collagen secreted by reticular cells. Reticular fibers crosslink to form a fine meshwork (reticulin). This network acts as a supportin ...
, continuous with those of the
splenic trabeculae, to which are applied flat, branching cells. The meshes of the reticulum are filled with
blood
Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood in the cir ...
:
*
White blood cells are found to be in larger proportion than they are in ordinary blood.
* Large rounded cells, termed splenic cells, are also seen; these are capable of ameboid movement, and often contain pigment and red-blood corpuscles in their interior.
* The cells of the reticulum each possess a round or oval nucleus, and like the splenic cells, they may contain pigment granules in their cytoplasm; they do not stain deeply with carmine, and in this respect differ from the cells of the
Malpighian corpuscles.
* In the young spleen,
macrophages may also be found, each containing numerous
nuclei or one compound nucleus.
* Nucleated
red blood cell
Red blood cells (RBCs), also referred to as red cells, red blood corpuscles (in humans or other animals not having nucleus in red blood cells), haematids, erythroid cells or erythrocytes (from Greek ''erythros'' for "red" and ''kytos'' for "holl ...
s have also been found in the spleen of young animals.
Red pulp macrophages
Macrophages are highly diverse mononuclear phagocytes that are present throughout the body, including the spleen. Those located in the red pulp are known as red pulp macrophages (RPMs). They are necessary for maintaining blood homeostasis by performing phagocytosis upon injured and senescent erythrocytes and blood-borne particulates. Evidence suggests that RPMs are mainly produced during embryogenesis and are maintained during adult life.
Additionally, there are a number of cell-intrinsic and cell-extrinsic factors that regulate the development and survival of RPMs, these factors being: Spi-C, IRF8/4, heme oxygenase-1 and M-CSF.
RPM's are capable of inducing the differentiation of regulatory T cells by the expression of transforming growth factor-β. They can also secrete type-1 interferons during parasitic infections.
Blood in the arteries end in the Billroth's cords (red pulp cords). These cords are made up of fibroblasts and reticular fibres which form an open blood system without an endothelial lining, and it is within these cords that F4/80+ macrophages are found, which are associated with the reticular cells of these areas and are collectively known as red pulp macrophages. From the Billroth's cords, the blood passes into the venous sinuses of the red pulp, which are lined with discontinuous endothelium as well as stress fibres extending under the basal plasma membrane, parallel to the cellular axis. This arrangement of the stress fibres combined with the parallel arrangement of the sinus endothelial cells forces the blood in the red pulp through slits that are formed by the stress fibres. However, this passage can become difficult for ageing red blood cells due to their less flexible membranes and therefore they get stuck in the cords and they will be subsequently phagocytosed by the red pulp macrophages. This process is known as erythrophagocytosis, which is important for the turnover of red blood cells and the recycling of iron, which is a major function of the red pulp macrophages and is made possible by this special structure of the red pulp.
The iron from the red blood cells is either released by the red pulp macrophages or stored in the erythrocyte itself in the form of ferritin. Also, the erythrocyte can store larger amounts of iron in the form of hemosiderin (an insoluble complex of partially degraded ferritin), and large deposits of this can be seen in the red pulp macrophages. The red pulp macrophages also obtain iron by scavenging a complex of haemoglobin (released from erythrocytes destroyed intravascularly throughout the body) and haptoglobin, via endocytosis through CD163. The iron stored in the splenic macrophages is released in accordance with the requirements of the bone marrow.
Diseases
In
lymphoid leukemia
Lymphoid leukemias are a group of leukemias affecting circulating lymphocytes, a type of white blood cell. The lymphocytic leukemias are closely related to lymphomas of the lymphocytes, to the point that some of them are unitary disease entities t ...
, the white pulp of the spleen hypertrophies and the red pulp shrinks.
[ In some cases the white pulp can swell to 50% of the total volume of the spleen.] In myeloid leukemia
Myeloid leukemia is a type of leukemia affecting myeloid tissue.
Types include:
* Acute myeloid leukemia
* Chronic myelogenous leukemia
* Acute megakaryoblastic leukemia
* Blastic plasmacytoid dendritic cell neoplasm
See also
* Hematological m ...
, the white pulp atrophies and the red pulp expands.[
]
References
External links
* - "Spleen: Red Pulp"
{{Authority control
Spleen (anatomy)