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Lymphatic system
Dr. Samia Farrara. MBS University of Denver. USA
introduction
• The lymphatic system is a network of tissues and organs that
primarily consists of lymph vessels, lymph nodes and lymph. The
tonsils, adenoids, spleen and thymus are all part of the lymphatic
system.
• The human circulatory system processes an average of 20 liters
of blood per day through the capillary filtrations, which remove
plasma while leaving the blood cells. Roughly 17 liters of filtered
plasma get reabsorbed directly into blood vessels, while the
remaining 3 liters are left behind in interstitial fluid.
• One of the main functions of the lymph system is to provide an
accessory route for these excess 3 liters per day to get returned to
the blood.
• The other main function is that of defense in the Immune system.
It involved with lymphocyte production and immune response.
Lymph is very similar to blood plasma but contain lymphocytes
and other white blood cells. It also contain waste products and
debris of cells together with bacteria and protein.
• Lymphoid organs are:
1.Primary or central: Thymus and bone marrow.
• 2.Secondary or peripheral: Lymph node, spleen, tonsils, solitary
lymph nodules, appendix, Peyer ‘s patches of ileum.
Immunity
• Innate Immunity: non specific, Immediate, including physical
barriers such as the skin, mucous membranes of GIT, Respiratory
and urogenital tracts that prevent penetration of host body. Cell
involved are neutrophil, natural killer cells.
• Adaptive immunity: acquired, specific, gradual, slower in
response, involve B and T lymphocytes. Antigen presenting cells.
• T lymphocytes : it form 65-‐75 % of circulating lymphocytes
Originate in bone marrow. Migrate to Thymus where they mature
and proliferate. and acquire surface markers CD4( helper T),CD8(
cytotoxic), and T cell receptors.
• It recognize the antigen when it is presented as apart of MHC ( MHC
restricted
B lymphocytes
Thymus
• Lymphoid organ present in superior mediastinum behind the upper
part of sternum.
• Active at young people (puberty). And involutes in old age but
continues to produce lymphocytes.
• Originate from mesoderm (lymphocyte ) and endoderm (epithelial
reticular cells).
• Main function is central tolerance which prevent autoimmunity
Structure
• Capsule: vascularized connective tissue send septa to parenchyma
divided the organ to incomplete lobules and forming short
interlobular septa end at cortico-‐medullary junction. These septa
carry blood vessels branch at cortico-‐ medulary junction.
• Parenchyma: composed of lobules consist of of dark stained
zone Cortex and lightly-‐stained medulla.
• Cortex: composed mostly of T-‐lymphocytes( Thymocytes). Few
macrophages and epithelial reticular cell
Types of TECs
• Squamous TECs: line the connective tissue capsule and septa
form continues layer join by desmosomes and occluding
junction. form blood barrier prevent unregulated exposure of
thymocytes to antigens.
• Stellate TECs: deeper to first layer, has processes contain
keratin tonofilaments join by desmosomes. These cells are
antigen presenting cells express MHCI, MHCII.
• Other Squamous cortical TECs: express MHCII. From sheetlike
structure contributing to cortico-‐medullary barrier between
the two regions of each lobule
Figure 14-9
Copyright © McGraw-Hill Companies
• T Cell: precursors with no surface receptor migrate from bone
marrow and reside in cortex where they presented to self antigen
bound to class I and class II MHC molecules on APCs surface.
• 95% of cortical T-‐lymphocytes that react to self antigen are
eliminated by process of apoptosis.
• The rest of T cells mature and attain T cell receptor and migrate
to medulla and enter blood stream through the venules and go to
secondary lymphoid organs.
medulla
• contain more epithelial reticular cells and few lymphocytes.
• It contain characteristic acidophilic staining Hassall’ s corpuscles:
they consist of concentrically arranged epithelial reticular cells
with keratin Filaments in the cytoplasm. Epithelial reticular cells
degenerate and calcify. Function unknown. in young thymus
corpuscles are small but in old thymus its large.
• ACTH and adrenal cortex hormone and male and female
hormones accelerate thymus involution in experimental animals.
Hassall’s)corpuscle)
• Arteries enter through the capsule: arterial branches follow the
septa into the gland.
• Arteries leave the septa to enter parenchyma at cortico-‐
medullary junction, break to capillaries.
• Capillaries which supply the cortex are impermeable to proteins
and prevent the circulating antigens to reach the site of
lymphocytes maturation, forming blood-‐thymus barrier.
• BLOOD THYMUS BARRIER
• Thymus cortical capillaries have non-‐fenestrated endothelial with
occluding junction. very thick basal lamina.
• Reticular epithelial cells.
• Post capillary venules in cortico-‐medullary region with simple
cuboidal cells allow lymphocytes in and out the thymus.
• Medulla supply by capillaries branch of arterioles at medulla-‐cortical border. Medullary capillaries are fenestrated and highly
permeable.
• Medulla drained by venules which joint to form veins
• Medullary vein s pass through the septa and leave the gland
through the capsule
Unique feature
• it consist of reticular epithelial cells derived from endoderm while
in other lymphoid organ is from mesoderm.
• Thymus has not lymphatic nodules instead it is divided into
incomplete lobules .
• Thymus gland just has efferent lymphatic , no lymph sinuses.
Thymus does not filter lymph.
• T thymoblast are thought to enter the thymic stroma through the
large venules at the corticomedullary junction, and re-‐ enter the
circulation through the vascular lining of post-‐ capilliary venules
Lymphatic nodules
• Found in all lymphatic organs except thymus.
• • Found in lamina propria of digestive tract, upper respiratory and
urinary
• passages.
• • Spherical ( diameter 0.2 -1mm) without connective tissue
capsule.
• • Composed of dense aggregation of small B-lymphocytes.
• • Cell at center are larger with more cytoplasm. Light stain central
area is known as a Germinal center.
• Germinal center is the site of proliferation and aggregation of B
lymphocytes. It include B lymphoblast, plasma cells, follicular
dendritic cells,and macrophages.
• Primary lymphatic nodules aggregation of B lymphocytes with
help of T helper these B cells became activated and form much
larger and more prominent Secondary lymphatic nodules which is
characterized by light stain germinal center filled with lymphoblast.
Non proliferating cells pushed aside produce dark zone Mantle