Histology of the Spleen
The reticuloendothelial system is an expansive network that encompasses the haematopoietic centres, the formed elements they produce, as well as the conduits that transports them.
One of the key structures of this system is the spleen.
The main focus of this article is the histological arrangement of the spleen. Fortunately, the embryology as well as gross anatomy, anatomical relations, neurovascular supply, lymphatic drainage, and clinically relevant points will also be discussed.
This intraabdominal organ, situated in the left hypochondriac region plays an important role in haematological homeostasis. During foetal development, the major role of the spleen is hematopoiesis. However, as the patient ages, the spleen adapts the role of a secondary lymphoid tissue and also participates more in filtration activities. However, in pathological conditions that result in damage to the bone marrow (e.g. myelofibrosis), the spleen can assume a hematopoietic role in order to meet the body’s demand for blood cells.
The average erythrocyte remains in circulation for 120 days. The cells are typically flexible and able to pass through the microvascular beds within specific tissue. However, as time progresses and the cells make more laps around the body, some of the plasma membrane is lost due to mechanical forces on the cell membrane. Consequently, the red cells become less deformable; which makes it more difficult for them to pass through the fine capillary beds. The spleen is able to detect defective red cells and remove them from the circulatory system with the aid of macrophages. The macrophages are able to phagocytose these cells as they get trapped more easily in the cords of the red pulp.
Under normal physiological circumstances, the spleen harbours about 35% of the total body platelet content and roughly 35 mL of red cells. It also has the capacity to significantly expand these volumes under pathological conditions such as idiopathic thrombocytopenic purpura (platelet sequestration and destruction of unknown cause) and haemoglobinopathies (i.e. sickle cell anaemia, where sickled cells are less flexible and will get trapped in the spleen). Leukocytes can also be trapped in the spleen, resulting in a fall in the white blood cell count (leukopenia).Probably the most important role of the spleen is its ability to mount a response to invading pathogens. Resident dendritic cells are antigen presenting cells (APC) that engulf pathogens and present specific antigens to the lymphocytes in the white pulp. This screening procedure will result in the production of antibodies against any invading organism in the blood stream. It is particularly useful against encapsulated organisms such as Klebsiella pneumoniae and Streptococcus pneumoniae.
Most embryological studies of the spleen utilize the Carnegie staging in describing its development. This is a comparison of the embryo’s gestational age, size, and morphology. Please refer to the information below for the correlation between Carnegie staging and gestational age:
- Carnegie stage 11 - Gestational age of 29 days
- Carnegie stage 12 - Gestational age of 30 days
- Carnegie stage 13 - Gestational age of 32 days
- Carnegie stage 14 - Gestational age of 33-35 days
- Carnegie stage 15 - Gestational age of 36-38 days
- Carnegie stage 16 -Gestational age of 39-40 days
- Carnegie stage 17 - Gestational age of 41-43 days
- Carnegie stage 18 - Gestational age of 44-45 days
- Carnegie stage 19 - Gestational age of 46-48 days
- Carnegie stage 20 - Gestational age of 49-50 days
- Carnegie stage 21 - Gestational age of 51-52 days
- Carnegie stage 22 - Gestational age of 53-55 days
- Carnegie stage 23 - Gestational age of 56 days
Also note that this is not an extensive list of the staging system, but only those relevant to the discussion of the development of the spleen. The spleen is a mesenchymal derivative that appears in the dorsal mesogastrium during the 5th week of gestation (Carnegie stage 14). Here, the primitive spleen appears only as a bulge. The composite pseudostriated mesothelial cells are gradually replaced throughout the 16th Carnegie stage by tall columnar cells. The tall columnar cells progressively get lower and by Carnegie stage 17 a prominent basement membrane underscores these cells. Haematopoietic stem cells originating from the yolk sac infiltrates the spleen during Carnegie stage 18. Further linear growth of the spleen ensues subsequently during the 6th to 8th gestational weeks (Carnegie stage 19 – 23).
The appearance of reticular cells that contain alpha-smooth muscle actin throughout the spleen during week 17 of development heralds the formation of the typical splenic reticular network. Proliferation of the network continues from the 20th to 23rd gestational weeks, and is associated with an increase in the quantity of T-lymphocytes and B-lymphocytes within the network. Subsequently, an early white pulp area can be seen encompassing the central arterioles. Divergence of the antigenic subtypes within the reticular network during the 24th gestational week results in the formation of periarteriolar lymphoid sheaths as well as definitive lymph follicles. The periarteriolar lymphoid sheaths chiefly contains T-lymphocytes, while the B-lymphocytes form the lymph follicles within segments of the periarteriolar lymph sheaths whose reticular network did not have the alpha-smooth muscle actin molecules. The marginal zone finally forms within alpha-smooth muscle actin positive reticular networks during week 26 of gestation, within the substance of the white pulp.
Originally, the spleen develops in the midline between the dorsal aspects of the mesogastrium. The rotation of the stomach shifts the position of the spleen from the dorsal position to the left hypochondrium.
Histology Architecture of the Spleen
The capsule of the spleen is made up of dense connective tissue that completely engulfs the splenic parenchyma. The trabecular extensions of the capsule penetrate the splenic pulp (parenchyma of the spleen) at the hilum, in order to carry the relevant neurovasculature and lymphatic channels to the parenchyma of the organ.
The most abundant cell type within the spleen is the reticular tissue. These are comprised of:
- antigen presenting cells (APC)
- fibrous material
- reticular cells.
There are two main zones of splenic pulp that can be appreciated grossly as well as histologically. The smaller, less abundant white pulp houses periarteriolar lymphoid sheath (PALS), as well as lymphoid nodules. This collection of lymphocytes makes up only about 20% of the cellular constituents of the organ. The more frequently encountered red pulp is made up of splenic cords and sinusoids, which processes a large volume of erythrocytes. The white and red pulps are separated by the marginal zone.
Unlike white pulp, red pulp is made up of a loose network of fibroblasts, intermingled with reticular fibres that facilitate filtration and decommission of defective erythrocytes. The meshwork is subdivided into the splenic cords of Billroth and splenic sinusoids.
The splenic cords of Billroth are made up of myofibroblastic reticular cells that aid in splenic contraction. Their cell processes ensheath the reticular fibres, which are made up of a mixture of microfibrils, the basal laminae of the reticular cells, elastic and collagenous fibres, as well as unmyelinated adrenergic neurons. There are also supporting macrophages distributed throughout the red pulp, which assist in the removal of defective red blood cells and other formed blood elements.
The potential spaces between the cords of Billroth are populated by both subtypes of lymphocytes, erythrocytes, and other white blood cells. There is also a significant population of haematopoietic cells and plasma cells that migrate to the area following antigenic differentiation within the periarteriolar lymphoid sheath.
The splenic sinusoids intervene between the splenic cords. The sinusoids are lined by stave cells (elongated endothelial cells) that lay along the same axis as the direction of blood flow. The basement membranes within the sinusoids are discontinuous and are incompletely encompassed by the reticular fibres.
The splenic sinusoids are formed by venous dilatation throughout the red pulp. The endothelial cells lining these sinusoids are loosely arranged on the basement membrane that also supports the ensheathing reticular fibres. Importantly, they are also found adjacent to the marginal zone, which serves as the transition point between the red and white pulp. Of note, the spleen also acts as a storage site for several metabolic byproducts. Both ferritin and hemosiderin pigments have been observed in splenic macrophages, following red cell breakdown and haemoglobin metabolism. Lipofuscin and ceroid have also been observed in the spleen following lipid oxidation.
There is a transition point between the red and white pulp known as the marginal zone. It functions as an interface between the splenic sinusoids of the red pulp and the periarteriolar lymphoid sheaths of the white pulp. In this capacity, it processes pathogens and antigens arriving from the systemic circulation.
There are three important layers of the marginal zone:
- The innermost macrophage layer
- A middle marginal sinusoid layer
- And the outer marginal zone
The thin lining of specialized macrophages, surround the nearby periarteriolar lymphoid sheath of the white pulp. The specialized macrophages are marginal metallophilic macrophages; they are disrupted by marginal zone bridging channels (MZBCs) at points where periarteriolar lymphoid sheaths (and not B-lymphocyte follicles) are directly adjacent to the marginal zone. Specialized stromal cells known as fibroblastic reticular cells within the marginal zone binding channels have been shown to create pathways that facilitate migration of T-lymphocytes into the periarteriolar lymphoid sheath.
The middle layer or marginal sinus layer communicate directly with the capillary beds of the periarteriolar lymphoid sheath and associated follicles. It is circumferentially enclosed by a thick ring of:
- reticular fibroblasts
- dendritic cells
- marginal zone B-lymphocytes
- marginal zone macrophages known as the marginal zone
The marginal zone then blends directly with the surrounding red pulp.
Arborisation of the arteries results in the formation of central arterioles that pierce the splenic pulp. Approaching the end point of the vessel, the surrounding adventitia transitions into periarteriolar lymphoid sheaths that are made up of:
- dendritic cells (a form of antigen presenting cells)
- plasma cells
Adjacent to these periarteriolar lymphoid sheaths are the marginal zones and adjacent follicles rich in B-lymphocytes; referred to as B-cell areas. These three areas form the lymphoid-rich white pulp.
On a freshly dissected spleen, the white pulps are visible to the naked eye. The presence of any foreign pathogen present in the central circulation is likely to trigger aggregation of nearby B-lymphocytes that subsequently leads to proliferation of a lymphoid nodule within the white pulp. Further branching of the central arteriole into capillary branches extends towards sinuses within the peripheral marginal zones. The central artery becomes penicillar arterioles after exiting the white pulp that may or may not have an added level of security screening in the form of adherent antigen presenting cells.
A fully grown spleen on average measures around 12 cm x 3.5 cm x 7 cm in length, width, and thickness, respectively. The weight range depends on the amount of blood within the organ at that particular time. However, the normal weight ranges from 80 g to 300 g (average 150 g). While it is the largest collection of lymphoid tissue within the body, with age there is gradual reduction in all dimensions of the organ. Generally its shape is consistent with a concave wedge or a tetrahedron with the diaphragmatic side being more rounded than the others. Eventually, the final shape is influenced by the surrounding structures during intrauterine life.
It is an encapsulated, lobulated structure whose long axis is parallel with the tenth rib. There are several landmarks of the spleen that should be appreciated:
- The posterior extremity of the spleen corresponds to the superior pole. It is in line with the body of the vertebral column at that level.
- The inferior border traverses the space between the renal impression on the visceral surface and the dorsum (diaphragmatic surface) of the spleen. It is in line with the lower margin of the 11th rib.
- The anterior extremity or inferior pole is the meeting point between the superior and inferior borders. It is larger than the superior pole and is most likely palpated when the spleen is enlarged.
- The superior border is a notched convexity that lies between the gastric impression on the visceral surface and the diaphragmatic surface.
- The hilum is located on the visceral surface of the spleen and runs obliquely from the posterior extremity to the anterior extremity. It is the point of entry for the splenic arteries and nerves. The splenic veins and lymphatics leave the spleen from that point as well.
The spleen develops within the mesogastrium in utero. In postnatal life, there is a peritoneal envelope that is tightly adhered to the splenic capsule and also to the adjacent abdominal walls. There are several ligamentous attachments that keeps it suspended in the abdominal cavity. These are not rue ligaments as seen in the musculoskeletal system, but rather olds of peritoneal tissue that help to hold viscera in place. These include:
- The splenorenal (lienorenal) ligament, which is the most prominent of the splenic attachments. It is made up of two layers of peritoneum. The anterior component is continuous with the peritoneal sheath of the lesser sac of the left kidney and the posterior sheath is continuous with the diaphragm. The splenic neurovasculature and lymphatics as well as the tail of the pancreas, lies in the splenorenal ligament.
- The phrenicocolic ligament projects from the right colic flexure to insert at the level of the 11th rib in the diaphragmatic surface. It takes an inferolateral course toward the lateral region of the transverse mesocolon.
- The gastrosplenic (lienogastric) ligament is also a bi-layered structure that extends from the hilum of the spleen and extends over the posterior aspect of the stomach.
In summary, the features of the spleen can be readily recalled using Harris’s Dictum of Odd Numbers. Considering that the spleen itself is an odd organ, the odd numbers 1, 3, 5, 7, 9, and 11 can be used to recall the dimension, weight, and location of the organ. Using metric units, the spleen is 1 inch x 3 inches x 5 inches in three dimensions, 7 ounces in weight, and located deep to the 9th to 11th ribs.
Arterial Supply & Venous Drainage
Blood supply to the spleen is achieved solely by the splenic artery. It is indisputably, the largest division of the coeliac trunk (celiac artery); travelling along a tortuous route along the superior and posterior borders of the pancreas.
The exact course of the vessel is such that as it branches from the coeliac trunk, it travels a short distance inferiorly. It then takes a brisk left turn in the horizontal plane – superior to the neck of the pancreas – after which it ascends towards the superior border of the pancreas. Here it travels laterally, it moves to the posterior part of the upper border to the gland in a serpentine manner. Its course results in it being anterior to the left adrenal gland and kidney as it traverses the Splenorenal ligament with the tail of the pancreas. Division of the artery prior to entering the hilum of the spleen results in the formation of two or three arterial branches; each of which subsequently divides into four or five other branches that pierce the hilum to perfuse specific splenic segments.
As the branches of the splenic artery enter the hilum, they arborize throughout the substance of the spleen. Arterioles derived from the arteries traverse the white pulp for a very short distance before entering into the marginal zone and red pulp areas. There is an area of intermediate circulation where blood traverses the space between the arteries and the veins. Two schools of thought exist regarding intermediate circulation. In Homo sapiens, researchers have found both anatomical and physiological evidence which supports the concept that blood drains slowly through the splenic cords and re-joins the rest of the blood flow by way of slit perforations in the sinus walls (i.e. an open circulation). The alternative pathway (i.e. a closed circulation) would have blood passing directly from the arterioles into the venules.
Filtered blood then drains from the venous sinusoids into the venules. Of note, the venous sinusoids tend to anastomose, creating vast communication between the venous networks of the spleen. The venules merge within the trabeculae of the spleen and form several tributaries along the hilum of the spleen. These tributaries – which are similar in number to the branches of the splenic artery – merge to form the splenic vein.
The splenic vein then enters the splenorenal ligament behind the tail of the pancreas and inferior to the splenic artery, where it follows the course of the artery. As it travels along the posterior border of the pancreas, the vessel also receives tributaries from the gland along its inferomedial course. Other tributaries to the vessel distally include the left gastroepiploic and short gastric veins. It is anteriorly related to the left kidney and its hilum and the arteries and veins of the same organ. The left sympathetic trunk and left crus of the diaphragm, superior mesenteric artery and abdominal aorta are also posteriorly related to the vessel. At the neck of the pancreas, the splenic vein is joined by the superior mesenteric vein and forms the portal vein that brings blood to the liver for further processing.
The spleen does not have afferent lymphatic tributaries. However, the efferent vessels pass through the splenic trabeculae and into the lymphatic conduits at the hilum. The lymphatic vessels take a similar course to the artery and vein, behind the pancreas. They subsequently drain to the peri-hilar, splenic artery and coeliac lymph nodes.
The right vagus nerve, left celiac (coeliac) ganglion and the celiac plexus all contribute neuronal fibres to the splenic plexus. This is primarily a sympathetic plexus that innervate the splenic smooth musculature, trabeculae and blood vessels of the spleen. Painful stimuli arising from the pulp of the spleen is diffuse and referred to the central epigastrium. However, pain from the capsule is localized to the posterior part of the left upper quadrant.
The spleen is situated in the left upper quadrant (more specifically, the left hypochondriac region). Superolaterally it is related to the abdominal surface of the left hemi diaphragm; while inferomedially it is related to abdominal viscera. The convexed diaphragmatic surface of the spleen is separated from the lower left lobe of the lung, the pleura of the base of the lung, as well as the ribs 9, 10, and 11.The pleural sheath occupying the left costodiaphragmatic space can reach as far as the inferior border of the spleen.
The visceral surface of the spleen bares the impression of adjacent organs. The inferior pole of the spleen has a flattened surface of the splenic flexure of the colon (going from transverse to descending colon). The gastric impression is seen near the posterior extremity, posterior to the insertion of the vasculature in the hilum. At this point the spleen is in contact with the proximal part of the body, proximal greater curvature and fundus of the stomach.
Adjacent to the inferior border is the renal impression of the left kidney and suprarenal gland. Adjacent to this region is the superolateral part of the anterior surface of the kidney. Occasionally there is a pancreatic impression at the hilum of the spleen. This impression is created by the tail of the pancreas that lies in the Splenorenal ligament.
The spleen has a remarkable ability to increase many times its normal size. Irrespective of the cause, an enlarged spleen is always pathological. A quick acronym to remember the broad categories of splenomegaly is SLIM CSI. This translates to:
- Storage diseases such as Gaucher disease or mucopolysaccharidosis
- Lymphohematogenous diseases such as the lymphomas (Hodgkin and Non-Hodgkin, haemolytic anaemias, multiple myeloma)
- Immunologic or Inflammatory conditions including systemic lupus erythematosus or rheumatoid arthritis
- Miscellaneous disorders including amyloidosis, metastatic disease
- Congestive States including (but not limited to) cardiac failure or hepatocirrhosis
- Infections, including (but not limited to) malaria, syphilis, cytomegalovirus and toxoplasmosis.
Examination for an enlarged spleen is done during the abdominal exam. Adequate exposure of the patient involves removal of clothing covering the neck to the mid-thigh. With the patient supine, inspect for any obvious signs of abdominal distension or asymmetry. Also, keep an eye out for any stigmata of diseases that can result in splenomegaly (i.e. jaundice, pallor, reduced BMI and leg ulcers in a patient with sickle cell disease). Palpation of the abdomen for the spleen should begin in the right lower quadrant, peradventure there is significant splenomegaly and the organ has expanded across the midline. Palpate in a diagonal fashion towards the right hypochondriac region. If the spleen is not felt, then try the following techniques:
- Roll the patient in the right lateral decubitus position and rest your left hand above the left hypochondrium. If the spleen is mildly enlarged, then gravity will pull it forward and it can be palpated with the right hand moving diagonally from the umbilicus upwards.
- With the patient supine, palpate from the left lower quadrant toward the left hypochondrium, as some spleens also enlarge in this direction.
Once an enlarged spleen is felt, it is important to measure it. It should always be measured in reference to the midclavicular line, either vertically or obliquely. There are several key features on clinical exam that confirms that the mass being palpated is actually the spleen and not some other organ. The spleen is most likely to enlarge diagonally, you are unable to reach above it (on deep palpation), it is not ballotable, a notch can be palpated and it is dull on percussion.
Unlike most organ systems, the spleen does not have any collateral vascular supply. Furthermore, each lobule is supplied by individual segmental branches of the splenic artery. Consequently, if one of those branches is occluded, then that segment will start the downward spiral of ischaemia, infarction, followed by tissue necrosis.
This series of events is not uncommon in patients with sickle cell disease (particularly those with the SC genotype). The sickled cells are not as pliable as normal red blood cells. Consequently, they become trapped and occlude blood flow, resulting in the above mentioned series of unfortunate events. The recurrent infarctions results in regression and reduced functionality of the spleen; a condition referred to as asplenia. The process described above is also known as an auto-splenectomy, since the reduce function of the spleen is equivalent to having the spleen removed. Surgical resection of infarcted splenic segments is impractical owing to the vast communications between the venous sinusoids.