Lymphatic system

Lymphatic system

Lymphatic System

The lymphatic system is formed from the lymphatic vessels Lymph and . The major lymphatic organs

Functions: drain the protein containing fluids from the tissue spaces, such proteins can not be directly reabsorbed by the capillaries of the blood. transport fats from the digestive tract to the blood. produce lymphocytes & antibodies (immunoglobulins) forming part of the immune system of the body).


Lymph: Is an exudates from the capillaries to the tissue space, it is similar to the blood (formed of plasma + cells) but some differences are: No granular leukocytes. No R.B.C"s. Platelets are absent . Albumin and globulin proteins are higher than that of the blood. Fibrinogen is less. Coagulation is slower than blood. Oxygen level is less than blood. Carbonic acid is higher than blood. Fats are seen in the lymphatics of the intestine & appear milky (chyle) due to the presence of fat globules.


Lymphatic vessels: lymphatic capillaries: Irregular in diameter but wider than blood capillaries. Thinner than blood capillaries. Lined by a layer flat endothelial cells with incomplete or no basal lamina. Have no pericytes Lie deeper than the blood capillaries in skin & muscularis mucosa. More permeable than blood capillaries, thus the colloid material can pass through the lymphatic capillaries while only soluble materials can pass through the blood capillaries.

The lymphatic vessels have thinner wall than veins & they have more valves. all lymphatic vessels end in 2 main ducts: right lymphatic duct & thoracic duct. The lymphatics are absent in the sclera & lens of the eye, avascular tissue, uncalcified cartilage, in the C.N.S,, fetal part of the placenta, internal ear and in the bone marrow.

Lymphoid TISSUE: 

Lymphoid Tissue Lymphoid tissue is a type of connective tissue characterized by a rich supply of lymphocytes. It exists free within the regular connective tissue or is surrounded by capsules, forming the lymphoid organs. Because lymphocytes have very little cytoplasm, lymphoid tissue stains dark blue in hematoxylin and eosin-stained sections. Lymphoid tissues are basically made up of free cells; as a result, they typically have a rich network of reticular fibrils that supports the cells In most lymphoid organs, the fibrils are produced by a fibroblastic cell called a reticular cell, whose many processes rest on the reticular fibrils


Cells of the Immune System
The primary cells that participate in the immune response are lymphocytes, plasma cells, mast cells, neutrophils, eosinophils, and cells of the mononuclear phagocyte system The cells of the immune system (1) are distributed throughout the body in the blood, lymph, and epithelial and connective tissues;

(2) are arranged in small spherical nodules called lymphoid nodules found in connective tissues and inside several organs; and (3) are organized as differently sized organs called lymphoid organs(the lymph nodes, the spleen, the thymus, and BM. And tonsil

Types of lymphocytes: T-lymphocytes: responsible for cellular immunity (i.e. sensitized T- lymphocyte itself has the capacity to invade the foreign agent & destroy it) they are circulating in the blood B- lymphocytes: responsible for humeral immunity (i.e. they produce antibodies & these antibodies attack the invading antigen) they are the fixed type.

1-Helper T-lymphocytes : They assist other T- lymphocytes by secreting some immune chemicals. 2-Cytotoxic T- cells ( T- killer): They can recognize foreign cells, binding to the target cells they secret cytotoxic substances.Secreting a substance called ( MAF ) 3-Suppressor T- lymphocyte: they inhibit the function of helper T-lymphocyte & cytotoxic T-lymphocytes & inhibit the development of B- lymphocyte into plasma cells. 4-Memmory cell: are long-lived progeny of T-lymphocytes

Function of B- Lymphocyte: Plasma cells: which produce antibodies specific for the invading Ag then Ag-Ab reactions will occur forming immune complexes. Memory cells: some activated B-lymphocytes not become plasma cells instead they persist as memory B-lymphocytes which produce a more rapid immune response when exposed to the same antigen.

Lymph node

:The L.N are encapsulated bean or kidney shaped organs composed of lymphoid tissue.distributed throughout the body along the coarse of the lymphatic vessels.each L.N characterized by a convex & concave side.the afferent lymphatic vessels enter through the convex side while on the concave side there is identation called “hilum” through which the arteries & nerves penetrate & the veins with the efferent lymphatic vessels leave.

Characters which differentiate the lymph node from other lymphatic organs: The only lymph organ that lies along the coarse of lymphatic vessels. The only organ that contain lymph in the lymphatic sinuses. The only lymphatic organ that have afferent & efferent lymphatic vessels. The only organ deal with filtration of lymph. Usually present in groups or chains ( paraaortic L.N ).

Histological structure:

It is surrounded with a C.T capsule formed of collagen, elastic fibers, & smooth muscle cells, which send C.T trabeculae that divide the lymph nodes into compartments called cortical trabeculae which extend to the medulla & called medullary trabeculae. Each lymph node has 3 regions: 1- Cortical zone: lies at the periphery below the capsule. 2- Medullary zone: lies in the central part of the lymph node. 3- paracortical or juxtamedullary or deep cortex: lies between the outer cortex & inner medulla.



Cortical zone: It is formed of reticular tissue infiltrated with lymphocytes, it contain 3 components: Lymphatic nodules. Marginal sinuses (or subcapsular sinuses). Cortical trabeculae.


"lymphatic nodules" are spherical aggregation of lymphocytes, they could be: Primary nodules containing inactive or naпve B-lymphocytes distributed uniformly throughout the nodule. secondary nodules containing a peripheral dark-stained area crowded with small B-lymphocytes with a central light area called germinal centre containing lymphoblasts which give rise to the plasmoblasts (immunoblasts) this cells divide to give rise to plasma cells which are immunoglobuline-producing cells. The germinal center also contains follicular dendretic cells characterized by long processes with large basophilic nuclei, its function is to trap the antigen – antibody complex on their surface for a long period of time then the antigens are recognized by the B-cell.


The "marginal (subcapsular) sinus“: lies below the capsule is continuous with the cortical (trabecular) sinuses which extend between the trabeculae & the aggregations of the cortical nodules. the cortical sinuses are lined with fenestrated endothelial cells (with gaps in between them) associated with reticular cells & macrophages which are more in the side of the sinus facing the cortical nodule.The "cortical trabecula" are C.T. septa arise from the C.T capsule & run in the cortex surrounded by the cortical trabecular sinus.


Paracortical region (thymus depending zone),(deep cortex) ,(juxtamedullary zone): consist of dense lymphoid tissue, it contain T-lymphocytes& high endothelia venules (HEV) which are lined with tall cubical endothelium having receptors on their surface which can be recognized by the circulating T-lymphocyte to enter the lymph node. Medulla: consist of Medullary cords: contains numerous B-lymphocytes, plasma cells & macrophages. Medullary sinuses. Medullary trabeculae.

Circulation of the lymph: From the afferent lymphatic vessels the lymph pass to the subcapsular sinuses then to the cortical sinuses then to the medullary sinuses then it will be collected by the efferent lymphatics & leave through the hilus. Functions of the lymph node : 1-filteration of lymph. 2-production of lymphocytes. 3-production of antibodies. 4-phagocytosis of foreign bodies.

Tonsils:

Palatine Tonsils:
are paired structures lie in the oropharynx they are surrounded by a connective tissue capsule which separate this lymphatic aggregation from the remaining parts of the oropharynx, connective tissue trabeculae extend from the capsule to the inside of the tonsils in between the lymphatic aggregation called internodular septa. covered by the same epithelium that lines this part of the pharynx which is non keratinized stratified squamous epithelium.

Histologically

formed from lymphatic tissue either diffuse form or arranged in the form of nodules. The surface of the tonsils shows indentations or downward depressions lined by the same surface epithelium called "crypts" (primary crypts) which give rise to secondary crypts, the lymphatic nodules distributed regularly along these crypts, lymphocytes sometimes migrate to lie inside the cavity of the crypts & surrounded by saliva forming salivary corpuscles.

Pharyngeal Tonsil:

single structure lies in the nasopharynx. covered by pseudosratified ciliated columnar epithelium with goblet cells similar to that of the respiratory system. It is formed of randomly distributed lymphatic tissue with no crypts. It may enlarge & interferes with the respiration so called "adenoid", removal of this adenoid is indicated. connective tissue capsule is thin or absent. mucus & serous alveoli are present around the tonsil.

Pharyngeal Tonsil:

Lingual Tonsil: several small aggregations of lymphocytes. lie on the back of the tongue between the circumvallate papillae behind the V- shaped line which divides the tongue into anterior 2/3 & posterior 1/3. lined by non- keratinized stratified squamous epithelium. contain deep crypts. they are rarely infected because the duct of the mucus alveoli open in the bottom of the crypt so continuous washing of the crypt occur.

LINGUAL TONSIL

Thymus:
is lymphoepithelial primary central organs of lymphoid tissue lies in the superior mediastinum. in children it is pinkish grey in color, soft & finely lobulated consist of 2 pyramidal lobes connected by areolar tissue. varies in size with age, at birth it weighs (10-15gram) continues to grow to the age of puberty when it become (30-40g) after mid adult life it's weight is (10g) then it undergo gradual atrophy, decrease in size & replaced by fatty tissue..

Histological structure:

It is formed of 2 lobes, each lobe is surrounded by a delicate fibrous capsule from which septa penetrate each lobe dividing it into lobules. each of 0.5-2 millimeters in diameter consist of darkly stained densely cellular cortex & light stained less cellular medulla.



Lymphocytes: they lie between the epithelial reticular cells, they are mainly T-lymphocytes migrate from the bone marrow &enter thymus to continue their maturation then leave the thymus & circulate in the blood to reach other lymphatic organs. Other potential immature T-lymphocytes remain for a certain time 3-5 days then undergo degeneration & phagocytosed by macrophages.

the histological structure of the thymus consists of 3 components: Epithelial reticular cells: which are endodermal in origin. not phagocytic & not associated with reticular fibers. stellate shape cells having numerous processes linked with the processes of other cells by desmosomes forming a meshwork lies in the cortex & medulla, lines the capsule & surround the blood vessels. their cytoplasm contains lysosomes, vacuoles & dense secretory granules which secrete "Thymic hormone" which stimulate the development of other lymphatic organs.

Blood- thymic barrier

Hassal's corpuscles ( thymic corpuscles ):
are spherical lamellated structures formed of central acidophilic hyalanized area surrounded by multiple layers of epithelial reticular cells. lie in the medulla of each thymic lobule. function of Hassal's corpuscle is unknown, but suggested to be phagocytosis of the gama globulins & antigens.

Functions of thymus: important in immunity. The thymus produces several proteins that act as growth factors to stimulate proliferation and differentiation of T lymphocytes. T-lymphocyte continue their processing & maturation in the thymus.

Spleen:

is the largest accumulation of lymphoid tissue in the body, lies in the left hypochondrium beneath the diaphragm extending in the area between 9th & 11th ribs. ".

Histological structure: 

The spleen is surrounded by connective capsule containing few smooth muscle fibers from which connective tissue trabeculae extend even to reach the red pulp containing nerves & "trabecular vessels



The spleen is formed of 2 areas: white pulp & red pulp. The white pulp also called "splenic pulp" or "Malpighian corpuscle" is formed of aggregations of lymphocytes which are mainly B-lymphocytes arranged as multiple lymphatic nodules differ from that of the lymph node in that they contain "central artery" surrounded by a sheath of T-lymphocytes, it could be eccentric, sometimes two sections of the central artery is seen in one lymphatic nodule, or may be the nodule shows no central artery.

The red pulp is formed of two elements: Splenic cords: which are formed of lymphocytes (mainly B-lymphocytes) with all types of blood cells (RBCs, granular leukocytes, platelets, plasma cells & macrophages) arranged in the form of thin cords. venous sinusoids: are irregular dilated blood spaces containing blood lie between the splenic cords, they are lined with cubical epithelium with gaps in between the cells lie on an incomplete basal lamina & they are surrounded by reticular fibers& macrophages.

Circulation of the spleen

splenic artery enters through the hilus.it branches into trabecular arteries run side by side with the veins called trabecular vessels.then the arteries immediately enveloped by a sheath of T-lymphocytes these vessels called "central arteries" the lymphocytic sheath thickened till they form spherical nodules (white pulp) in which the arteries may occupy eccentric position. The central artery extends to the periphery of the white pulp & subdivides into many straight, symmetrical branches which are similar to the branches of a teethbrush, these are called pencilli or "pencillar arteries”, each of which terminates as 3 divisions:

Pulp arteries: true arteries that are the wall is thick formed of circularly arranged smooth muscle fibers. Sheathed artery or (elipsoial arteries): near termination some of the penicillar arteries become surrounded by a sheath of reticular fibers with macrophages (so they are not true arteries because their walls contain no smooth muscle fibers). Arterial capillaries: are normal capillaries that carry the blood to the venous sinusoids


The manner in which blood flows from the arterial capillaries to the venous sinusoids has not been completely explained but 3 suggested theories: Closed theory: the arterial capillaries open directly into the venous sinusoids. Open theory: the blood from the arterial capillaries is rushed into the spaces between the red pulp cords then moves on to be collected by the venous sinusoid. Combined theory: that the connection between the arterial & venous side is closed when the spleen is contracted,& it is open when the spleen is relaxed.


Function of Spleen: formation of lymphocyte. immunity for the body. development of new RBCs which are stored in the spleen thus it act as a reservoir for the blood. It can give blood whenever it is needed such as after car accident. filtration of blood. destruction of old RBCs. which are destroyed in the spleen & the iron which is left is used for the formation of new RBCs.