Lymphatic system microanatomy

histologic appearance

small, dark, boring

small cells

dense round nucleus

scant cytoplasm

functions of the lymphatic system

transport of materials via lymphatic vessels

filtration of lymph and blood—phagocytosis of foreign material

production of defensive cells

production of immunoglobulins (antibodies)

primary lymphoid organs

produce and train lymphocytes

marrow, bursa (birds), thymus

secondary lymphoid organs

surveillance of incoming pathogens

lymph nodes, spleen, mucosa associated lymphoid tissue (MALT)

diffuse unencapsulated

GI tract and resp tract (tissue exposed to the outside)

loose collection of leukocytes

-lymphocytes (mostly T cells)

macrophages

plasma cells

diffuse unencapsulated lymphoid tissue

individual cells not forming an aggregate or mass like structure

diffuse unencapsulated

individual cells do not form an aggregate or mass-like structure

subepithelial aggregates of leukocytes (lymphocyte, macrophages, plasma cells)

**tonsils

specialized epithelium has M cells (cannot be seen except electron microscopy)

crypts of tonsils

epithelium invaginates into lymphoid tissue

horses, swine, sheep, goats, humans (carnivores usually lack)

depends for ruminants

african and classical swine fever

hemorrhaging of the tonsils

MALT

mucosal associated lymphoid tissue (MALT)

dense unencapsulated

GLAT (GI)

Peyers patch

dense unencapsulated

BALT

Bronchus associated lymphoid tissue

primary follicle+antigenic stimulation

secondary follicle

spherical accumulations of lymphocytes—found within most lymphoid tissue

primary vs secondary follicle

immunologically inactive vs immunologically activated

small dark resting B cells + dendritic cells vs pale center with larger paler proliferating B cells +mitotic figures (germinal center)

no germinal center vs germinal center with peripheral mantle zone (dark blue resting B cells)

diffuse unencapsulated definition

individual cells do not form an aggregate or mass-like structure

dense unencapsulated definition

individual cells form an aggregate or mass-like structure

dense encapsulated can be both

primary and secondary lymphoid organs

thymus and bursa

lymph nodes, spleen, hemal nodes, hemolymph nodes

lymph nodes

found throughout the body of mammals, absent in most avian species

functions: filter particulate matter and microorganisms, surveillance of incoming antigens, activation o fB cells

lymph node structures

capsule—dense fibrous connective tissue which sends out sub-dividing trabeculae

subscapular sinus

lymph node cortex

outer (superficial) portion—composed primarily of lymphocytes, often lymphoid follicles, B cells

deep portion of cortex is paracortex—primary T cells, no lymphoid follicles

lymphoid follicles composing outer cortex

medulla—cords of leukocytes, b cells, plasma cells, macrophages, separated by sinuses

where in the lymph node do you look for follicles

cortex

arteries enter at the (lymph node blow flow)

hilus

follows trabeculae

capillaries in paracortex

high endothelial venules

vein exits at the hilus

lymph flow from capsule to hilus

1. afferent vessels penetrate the capsule

2. subscapular sinus

3. cortical sinuses

4. medullary sinuses

5. efferent lymphatic vessels

spleen functions

blood filtration

immune response: phagocytosis

blood storage: hemoglobin and iron metabolism

blood cell formation and production

**lifelong risk for overwhelming sepsis in splenectomized patients

spleen capsule

capsule—surface lined by mesothelium

dense irregular fibrous connective tissue

trabeculae—extend into tissue

smooth muscle—contraction

elastic fibers—expansion for storage

spleen does not have a

cortex or medulla

spleen white pulp

periarteriolar lymphoid sheaths (PALS)— T cells surroudning central aterioles

lymphoid follicles—T and B cells

spleen red pulp

blood filled sinuses

sinuses lined by phagocytic cells—remove debris, abnormal cells; macrophages phagocytose

lymph flow has not

afferent lymphatics

efferents arise in the white pulp

exit out of the hilus

B lymphocytes are found where in the spleen

lymphoid follicles

thymus

starts out epithelial—>lymphocytes migrate here to be “educated”—> lymphoepithelial organ

atrophies with age

no afferent lymphatics

efferent lymphatics—exiting lymphocytes populate secondary lymphatic organs

thymus capsule

extends septa into the parenchyma, sub-dividing it into lobules

septa end at the corticomedullary junction

which is a secondary lymphoid organ

spleen