24. The anatomy and histology of the spleen. Differentiation of the ectoderm.

24. The anatomy+ histology of the spleen. Differentiation of the ectoderm.

* Completely Intraperitoneal, except at the hilum, where splenic vessels enter/ exit, part of FOREGUT

Anatomy of Spleen:

General Info:

• Ovoid, usually purplish, pulpy mass about the size+ shape of one’s fist.
• Largest of the lymphatic organs
• Spleen varies in size, weight+ shape, it is usually app. 12 cm long+ 7 cm wide
• It participate in the body defense system as a site of lymphocyte(white blood cell) proliferation+ of immune surveillance+ response.
• PRENATALLY= a hematopoietic organ
• After birth = is involved in identifying, removing + destroying expanded red blood cells+ broken- down platelets, in recycling iron+ globin. S
• serves as a blood reservoir
• storing RBC+ platelets+ to a limited degree
• can provide a sort of”self transfusion” as a response to the stress imposed by hemorrhage.

Location: Located in the left upper abdominal quadrant or hypochondrium, where it receives the protection of the lower thoracsic cage

Surface Projection:

• b/w 8th & lower border of 11th rib,
• Ant = tip of 11th L rib,
• Post = 2 finger medial to L inguinal line,
• Long axis runs along the post part of 10th rib

Topography: 2 main surfaces – Costal & Visceral

• Costal – contact w/ rib cage & diaphragm
  • In the costal region, it rests on the colic flexure
  • associated post w/ 9-11 th ribs+ separated from them by the diaphragm+ the costodiaphragmatic recess
• Visceral – contact w/ abdominal organs = impressions made by them
  • Gastric – ant/med, fundus of stomach
  • Renal – behind, L kidney
  • Colic – below, L Colic flexure of LI
  • Hilum – splenic branches of the splenic artery+ vein enter+ leave, SPLENIC HILUM often in contact with the tail of the PANCREAS, L boundary of the omental bursa.

Function: Hemopoetic function or Immune System function

• Primary Functions:
  • produces lymphocytes & antibodies
  • phagocytosis of damaged blood cells
  • storage of blood cells
  • proliferation of lymphocytes
  • formation of blood cells in fetal life
  • removal of macromolecular antigens from blood
  • retrieval of Fe from RBC Hemoglobin
• Secondary Functions:
  • Antigen presentation by APCs = immune response
  • Activation and proliferation of B & T cells
  • differentiation of B cells and plasma cells
  • secretion of antibodies on white pulp
  • blood storage

Blood Supply:

• The SPLENIC Artery is the largest branch of the celiac trunk+ follows a tortuous course
  • post to the omental bursa
  • ant to the left kidney+ along the sup. border of the pancreas.
• SPLENIC VEIN: is formed by several tributaries that emerge from the hilum.
  • It is joined by the Inf Mesenteric v & runs post. to the body+ tail of the pancreas throughout most of its course.
  • The splenic vein unites with the Sup mesenteric v post. to the neck of the pancreas, to form the PORTAL VEIN.

Lymph Drainage: The splenic lymphatic vessels leave the lymph nodes in the splenic hilum+ pass along the splenic vessels to the pancreaticosplenic lymph nodes.

Innervation: The nerves of the spleen, derived from the celiac nerve plexus, are distributed mainly along branches of the splenic artery+ are vasomotor in function.

Peritoneal relations: Spleen is a soft, vascular(sinusoidal) mass with a FIBROELASTIC CAPSULE, entirely surrounded by peritoneum except at the SPLENIC HILUM. These ligaments, containing splenic vessels, are attached to the hilum of the spleen, on its medial aspect

• Gastrosplenic lig (ant) – connects spleen to greater curve of stomach, spleen contacts the post. wall of the stomach+ is connected to its greater curvature by the GASTROSPLENIC lig
• Phrenicolieanal lig (post)
• Lienorenal lig – * If tail of pancreas is retroperitoneal, then inf gastrophrenic lig forms pancreaticolienal ligament & carries splenic a/v –> spleen, Inf border of phrenico lienal extends –> L colic flxure = phrenicocolic lig, and supports spleen inferiorly = nest of spleen

CLINICAL NOTE – frequently ruptured by fractured ribs & repair difficult, may be removed surgically w/ minimal  effect on body function –> function assumed by other reticuloendthelial organs, like Liver.

Histology of Spleen:

Slide #24 Spleen * H&E

Structures to Identify:

• red pulp
• white pulp
• lymphatic follicles
• germinal center
• central a
• trabecular v
• venous sinuses

General Info:

• largest lymph organ, on L side of Ab cavity
• The spleen contains a large quantity of blood, that is expelled periodically into the circulation by the action of the smooth muscle in its capsule+ trabeculae.
• thin fibrous capsule of the spleen
  • is composed of dense, irregular, fibroelastic CT.
  • fibrous capsule is thickened at the splenic hilum
  • divides spleen into compartments called splenic pulp
  • made of collagen w/ myofibroblasts
• CT trabeculae, arising from the deep aspect of the capsule,
  • carry blood vessels to+ from parenchyma or splenic pulp, the substance of the spleen.
  • round or nodular and may contain a/v = trabecular a/v
    

• White Pulp – site of lymphatic nodules
• Red Pulp – site of sinues and cords

NOTE: PALS = T lymphocytes, Nodules = B lymphocytes

White Pulp – lymphatic nodules with germinal center around a central artery

The spleen is characterized by numerous aggregation of lymphatic nodules. These nodules constitute the white pulp of the organ.The lymphatic nodules also contain, GERMINAL CENTERS, that decrease in no. with age. NO distict cortex+ a medulla, as seen in lymph nodes. NO subcapsular or trabecular sinuses. The capsule+ trabeculae in the spleen are thicker than those around the lymph nodes+ contain some smooth muscle cells. The large lymphatic nodules =  white pulp of the spleen. Each nodule normally exhibits a peripheral zone with densely packed small lymphocytes = the peri-arterial lymphatic sheeth = PALS, aka Thymus dependent zone,

The central artery, in the lymphatic nodule, has a peripheral or an eccenteric position. Because the artery occupies the center of the periarterial lymphatic sheath, it is called the central artery. The cells found in the periarterial lymphatic sheath are mainly T cells. A germinal center may not always be present. In the more lightly stained germinal center are found B cells, many medium- sized lymphocytes, some small lymphocytes+ lymphoblasts.

• White pulp is the side of immune response to bloodborne antigens
• T cells surround the central arteries,
• Whereas B cells are mainly in the lymphatic nodules
• Antigen- presenting cells + macrophages are found in the white pulp.
• Passing through each lymphatic nodule is a blood vessel called a CENTRAL ARTERY, that is located in the periphery of the lymphatic nodules.
  • Central arteries are branches of trabecular arteries,
  • become ensheathed with lymphatic tissue
  • between the venous sinuses+ form a spongy meshwork of reticular CT
  • usually obscured by the density of other tissue.

Red Pulp – consists of splenic cords+ splenic(blood) sinusoids

The red pulp contains the SPLENIC CORDS+ VENOUS SINUSES that course between the cords. These represent the branches of the central artery after it leaves the lymphatic nodule. Capillaries+ pulp veins are also present.

• Splenic cords of Billtoth
  
  • Splenic cords are thin aggregation of lymphatic tissue containing small lymphocytes, associated cells+ various blood vessels.
  • loose meshwork of reticular cells & fibers w/ high # of RBCs
  • contain macrophages, lymphocytes, plasma cells, granulocytes, & enterocytes
• Splenic sinuses
  • are dilated vessels lined with modified endothelium of rod shaped cells that appear cuboidal in transverse sections.
  • Endothelium has longitudinal axis parallel to direction of vessels
  • prominent IC spaces
  • no continuous basal lamina & wraps loops around sinuses (perpendicular to long axis)
  • mostly washed out due to increased # of RBCs
• Also present in the red pulp are the pulp arteries.
  • surrounded by PAMS –  Peri-Arteriolar Macrophage sheath – macrophages w/ hemosiderin pigment granules
• CT trabeculae with trabecular artery+ trabecular vein are evident
  • These vessels have endothelial tunica intima+ muscular tunica media
  • tunica adventitia is not apparent because the CT of the trabeculae surrounds the tunica media.

Arterial Flow:

Central a  (white pulp) –> branches –> marginal sinus –> red pulp –> straight arterioles –> pericellular arterioles –> arterial capillaries –> empty directly into the reticular meshwork of spleen –> splenic sinuses & cords of red pulp –> blodd exposed to macrophages (in sheathed arterioles, arterile capilaries) –> squeeze through walls of splenic sinus = Open circulation

Venous Flow:

Blood collected in sinuses –> tributaries of trabecular v –> larger v –> splenic v –> joins sup/inf mesenteric v –> hepatic portal v.

Embryology of Spleen:

• Develops from mesoderm, w/in dorsal mesogastrium
• development starts w/in the 5th week
• spleen appears as proliferation of mesoderm
• with the rotation of mesogastrium, goes into the post side of the abdomen
• remains intraperitoneal
• supported by lienogastric (to greater curve of stomach) and lienorenal lig (to L kidney)
• Mesenchymal cells in the developing spleen differentiate to form the capsule, trabeculae, and reticular framework.
• Development involves establishment of mesenchymal trabeculae within a blood vascular network consisting of a large number of endothelial sinuses.

Embryology- Differentiation of Ectoderm:

• Ectoderm is one of the three layers of the trilaminar germ disk
• Cells remaining in the epiblast after endoderm and mesoderm are produced = ectoderm
• @ beg of 3rd week, the ectoderm layer has shape of disk that is broader towards the head (cranial portion), than the tail end(caudal).
  
• After notochord and prechordal mesoderm form, these cause ectoderm on top of them to thicken and form neural plate
• Cells of neural plate = neuroectoderm — process = neuralation
• From Wikipedia:
  Generally speaking, the ectoderm differentiates to form the nervous system, epidermis, and the outer part of integument.

  In vertebrates, the ectoderm has three parts: external ectoderm (also known as surface ectoderm), the neural crest, and neural tube. The latter two are known as neuroectoderm.