Overview
The spleen is a brown ovoid organ that forms part of the
reticuloendothelial system. It is
approximately 12cm x 7cm, roughly the size of a clenched fist. The spleen is mobile with respiration. It is an intraperitoneal structure apart from
at the hilum where blood vessels enter and leave.
Gross Anatomy
Development
The spleen
starts to develop at the 4-5th week of foetal life. It develops between the layers of dorsal
mesogastrium as lobules. As the stomach
rotates the mesogastrium fuses with the peritoneum covering the left
kidney. This is the origin of the
splenorenal ligament. The differentiation
of mesenchymal cells forms the splenic capsule and parenchyma.
Surface Anatomy
The spleen
is a relatively superficial organ and lies from the 9-11th rib in
the left hypochondrium. It is not
palpable except in pathological states.
Macro-Anatomy (e.g. lobes, surfaces,
impressions etc)
The spleen
is an ovoid shaped organ covered by a loose capsule that allows expansion. The
spleen has a smooth diaphragmatic and an irregular visceral surface with
impressions for the stomach, left kidney, left colic flexure and pancreas. The hilum is the site of entry and exit of
the splenic vessels and the splenorenal and gastrosplenic ligaments attach
here. The spleen is attached to the
greater curve of the stomach via the gastrosplenic ligament, and to the left
kidney via the splenorenal ligament.
Micro-Anatomy
The
internal structure of the spleen is very important for its function and has a
number of specialised systems. A
supporting network of fibroelastic tissue forms the capsule and
trabeculae. Trabeculae are fibrous
tissue bands that extend into the parenchyma to provide internal structure. The major histological subtypes of splenic
tissue are:
- White pulp: contains lymphocytes and
macrophages, arranged around the arteries.
The white pulp is responsible for the immune response.
- Red pulp: highly vascular parenchyma, sinusoids
(highly permeable specialised capillary system). It is the site of erythrocyte filtration
and storage of iron, erythrocytes and platelets.
- Marginal zone: this is the area between the
white and red pulp, it is involved in immunity.
The
arterial network of the spleen is uniquely adapted to its function. As the splenic artery enters the hilum it
divides into smaller branches (trabecular arteries) that penetrate the
parenchyma. Arterioles branch from the
trabecular arteries and enter the red pulp to form central arterioles lined by
lymphoid cells.
Blood Supply
The splenic
artery, the largest branch of the coeliac trunk, passes along the posterior
surface of the pancreas, between the layers of the splenorenal ligament to
reach the hilum of the spleen. At the
hilum it divides into five or more branches.
The venous drainage of the spleen is via the splenic vein which unites
with the superior mesenteric vein to form the portal vein.
Lymph
The spleen
only has efferent lymph vessels which leave at the hilum.
Nerve Supply (link to dermatome and
referred pain as appropriate)
The coeliac
plexus provides the sympathetic nerve supply.
Physiology (e.g. functions,
relationships to other body systems)
The major
functions of the spleen are:
- Erythropoiesis: secondary site of red blood cell
production during foetal life, this function ceases at birth.
- Erythrocyte removal: defective red blood cells
are removed from the circulation.
- Storage: neutrophils and one third of all
platelets are stored in the spleen, ready for release when required.
- Immune defence: synthesis of antibodies, removal
of encapsulated bacteria, trapping and processing of antigens
Clinical Anatomy
Splenic
rupture
Splenic rupture is largely secondary to trauma,
although neoplasia, infection and splenic infiltration can cause atraumatic
rupture. Both penetrating and blunt
trauma can cause splenic rupture. The
spleen is vulnerable to rupture due to the thin capsule and high
vascularity. In a hypotensive patient
with left upper quadrant pain, splenic rupture should be high on the
differential list. Treatment is largely
by partial or total splenectomy.
Post-splenectomy, patients are more susceptible to certain infections
but otherwise splenectomy has few long term side effects due to the other
reticuloendothelial organs compensating.
Accessory
spleen
This is a benign embryological variant that occurs
in approximately 10% of the population.
It is caused by failure of the splenic lobules to fuse during
development. It is an incidental finding
and requires no specific treatment.
Quick Anatomy
Key Facts
Development:
dorsal mesoderm in dorsal mesogastrium
Artery: splenic
artery, origin: coeliac trunk
Vein: splenic
vein into portal vein
Lymph: efferent
lymph vessels only
Nerve: coeliac
plexus
Aide-Memoire
Summary
The spleen is the largest lymphoid organ in the body. It is only palpable in pathological states
and due to it’s superficial anatomical location is vulnerable to traumatic
injury.
References
Essential
Clinical Anatomy. 3rd edition. Moore and Agur.
http://www.embryology.ch/anglais/sdigestive/pankreas01.html
http://www.histology.leeds.ac.uk/lymphoid/lymph_spleen.php